The benefits of the Supertec studs and their design has been discussed ad nauseam.
Your attempt to characterize my statements as a claim of "perfection" strains credulity.
The Supertec stud is an option that has been installed without failure in hundreds of engines without complaint.
Use them or don't, it's your choice.

What about toughness and yield strength (not elastic modulus as previously typed; whoops!)? It would seem like significant amount of the loading on a Porsche head stud could be wildly oscillating loads caused by the cantilevered mass of the jug/head/cam/tower responding to road imperfections whizzing by sideways at frightening speed....

In Henry's picture (post #25) are those heads intake port up?

No, have to be exhausts. Notice that the valves which are up look smaller in diameter, and the other ones have a dish in them, as only intakes do. Plus this motor must still have been running exhaust air injection, with the injectors in place with colored plastic caps.

The yield strength of this type of stud shouldn't really be an issue.

If the stresses produced by the applied preload and the expansion of the head and the casing caused the studs to yield then there would be obvious signs and virtually every engine would fail.

Precipitation Hardened Austenitic Stainless Steels would have a typical 0.2% Proof Stress of around 120ksi at room temperature and this would probably only reduce by 2% at 400 degF

The Fracture Toughness of these alloys are exceptional and the KIC is around
170MPa√m


17-4PH will almost certainly have a higher yield strength or around 140 ksi, the KIC will still be very good but is a lower at around 100MPa√m

The elastic modulus is important as it determines the increase in the preload caused by the thermal expansion of the cylinder/head.

As both Dilavar and 17-4PH have a Modulus within 1% there won't, however be much of a difference due to materials.

I have to say that I just don't believe that the effects of gyroscopic procession will have any influence on bolting :)

Not trying to be argumentative, but just a question for Henry. The heads that you pictured with the leakage, is it fair to assume that any time that you have removed heads that were retained by your studs that there was no leakage of that type?

Just wondering if the characteristic of a high performance head, balanced on an individual cylinder, retained to an aluminium case by 4 studs might have a degree of leakage regardless of the stud type? I guess I get suspicious about transient conditions such as very cold starts or such.

Steve, also curious from your side as you seem to inhabit the Dilavar perspective....has Henry's experience been borne out in your work as well?

D.

Bruce Anderson put this together a few years ago

http://forums.pelicanparts.com/uploa...1324398408.gif

There are indeed at least 2 different 993 head studs
993.101.172.02 used on all n/s 993 except RS
993.101.170.51 used on all turbos and RS

JMHO,

Leakage that I see has been due to other things and it depends a lot on the engine in question: 3.6's and larger are more prone due to the length of the span between the upper & lower row of studs.
Those heads are susceptible to bending along that span so we employ several measures to deal with that issue.

We use the 993TT/RSR Dilavars in all of the aluminum-cased motors since that's what we've had the most success with. I've not seen a broken one to date, even in 13.5:1 race engines.

The heads Henry posted look to me to be 930 heads not 3.6 so head deformation is not the issue. In an earlier post, it says the heads are from a street driven low mileage 930.
I have seen broken 993TT studs in my limited exposure to the Porsche engine building world so I am surprised to hear Steve make this claim. I think I've even seen broken 993TT studs posted on Pelican forums.

Hi Jim,

I didn't say that they never break; I simply wrote that I've not experienced that, as of yet,....:)

Ultimately, we are all simply the collective result of our own experiences and that is what makes us individuals. Its what makes life both interesting and fun,....:)

I am surprised by the low coefficient of expansion shown for both the the Dilavar Studs and the Nikasil Cylinder.

Most Aluminium Alloys have a coefficient of expansion between 20 and 25ppm/degC and 14-15 is very low.

Even NASA398 which has been specirfically developed for Low Expansion is 18.4ppm/degC.

I am reasonably confident that Dilavar is 18ppm/degC.

Wow, thank you for your cordial response. Hello to you. Life is fun and individual expression adds a great deal. It was not my intention to put words in your mouth but merely clarify your statement.
Since you're stating that you don't see 993TT studs breaking, I guess my next question is "are you currently building engines ?"

What about mag-cased motors (w/Nikasil cyls)?

I'm glad you did,.....:) :)

Yessir,..been doing same for 35+ years.

Weissach911[QUOTE]It is very unlikely that peak cylinder pressures cause the head to lift if it had been correctly fitted as this would have a very damaging influence on performance and reliability.

If the stud, however, has too much preload and peak cylinder pressures are too high then there is the likelihood that it can and will pull [QUOTE]

My experience has been that on a twin turbo 2.5 built on a early aluminum case, dilivar studs allowed the heads to blow off the cylinders far enough to blow the CE head gasket out of the groove and pinch it between the cylinder and the head. That problem was solved by installing steel studs and ni-resisting the cylinders and heads. The threads in the aluminum case were undamaged so my assumption at the time was that the studs were allowing the heads to lift at maximum cylinder pressure. My assumption on pulled steel heads studs on Mag case engines was the expansion rate causing excess torque due to thermal expansion and inability of the magnesium to deal with that increased torque. We have never had problems with the aluminum cases pulling head studs even at increased operating temperatures. After refreshing numerous racing engines and never having any head sealing problems, I have never considered anything but steel studs---and in the end that is what counts--experience---NOT theory.
aws
PS: Henry you have shown remarkable restraint with the engineering types.

[QUOTE=aws;6445636]Weissach911 What I don't understand about this description is why a steel stud will survive when a Dilavar stud will allow this problem to occur.

Both studs would have a similar yield strength (Within 20%) and a similar preload so why does one stud fail so badly and the other survive so well?

If I understand correctly to allow the CE ring to escape from the groove the head would need to lift by several millimetres.

If we assume that 4mm would be a 'good' value then the stud would need to elongate by significantly more than its yield point and therefore the stresses would need to be in excess of 120ksi for the Dilavar stud.

A peak cylinder pressure of 2000psi would generate a force of about 62ksi in each stud. If we add this to the existing preload of around 50ksi then I would agree that this is far too close for comfort. Add a bit more load for expansion and you could well be in trouble.

The steel stud, assumimg you are using a higher strength material will see exactly the same loadings but the additional stress casued by differences in expansion will also result in a situation where the studs are very, very close to yield.

Add a detonation to the equation and I could well understand problems occuring.

It is also interesting that Porsche EB welded the heads onto some of their Turbo race engines but I am not sure how this would reduce stud loading.

If we now consider the 4mm displacement needed to allow the ring out of the groove then the stud would have to extend by this amount.

The yield extension of both steel and Dilavar studs will be about 0.3mm so the studs would have to stretch significantly.

Peak cylinder pressure has a duration of about 25 degrees of crank angle and at say 6000rpm then the peak force is maintained for about 42 milliseconds

If the material had to extend by 4mm during this time the 'impact rate' would be about around 7 metres per second.

I don't know of any steels that will transmit plastic waves at this velocity and the only conclusion is that the studs would have to snap.

I am afraid to say I find that unless explanations of problems of this type are backed up by some engineering justification they are just too big a leap of faith.

I am sad to say that the 'argument' about practical experience versus theory has been going on since the start of Scientific Experiment.

All of the real engineering progress we have made in the last two hundred years has been as a reult of improved understanding and better mathematical modelling so please lets not throw it all away over a few studs.

I think it is always important to try to justify claims that one product is good and another is bad and i have no vested interest in either type of stud.

[QUOTE=Weissach911;6445875] I agree with you. I am happy to see that somebody really wants to understand the problem.

Although understanding the problem from a engineering point of view is a noble endeavor, in the real world we fix the problem not the blame. What we seek is a solution. Heads move, how can we make them more stable?
Over the years there have been a myriad of attempts to remedy the head to cylinder connection in the air cooled Porsche engine. Even Porsche has danced around the problem with many different "solutions" all from their engineering department all with limited success.
We can only control certain aspects of the process.
Replacing heads is expensive but doable (Bill @ extreme Cylinder Head) , although cylinders come in a plethora of materials, most function pretty similarly and the case is what it is. We can control heat to certain extent but basically we're stuck with a specific design. The only reasonable variable is head studs. We used the factory studs for years and weren't satisfied. We along with countless others, sought a more stable platform.
After seven different stud configurations we came up with a stud that performs better under most situations than the all thread Dilavar. Notice I say "most situations" because the product is still exposed to an imperfect environment. I use the word "performs" because we and many other engine builders have personal experience with the product and can see the results. Don't believe it, don't use it.
But to claim that the product does not perform better without ever trying it or because you can't explain why it works better seems just a touch squirrelly.
Break out your calculators and soldier on with your projects with my best wishes.

Merry Christmas form the guys at Supertec.

http://forums.pelicanparts.com/uploa...1324482807.gif

Henry,

I believe that the basis of Ivath's original question was only to ask for some basic data and modelling to augment the comments which merely say Stud A - Good, Stud B - Bad. This type of statement relies on faith and it is hard for some of us to accept.

I am quite sure your studs work reasonably well and have never criticised them and do understand the thinking behind the material selection but in the engineering world I have always lived in engineering problems have first had to be researched, documented and analysed. Solutions have had to be justified in terms of data and some basic model to show how the 'fix' resolves the issues and their causes has had to be provided. Unless this work has been done 'fixes' can never be signed off and work has to continue until a solution is found.

As a Chartered Engineer I have worked all my life in the design and development of Power Station plant including steam boilers, pressure vessels and large (650MW) Steam Turbines and to suggest that I don't live in the real world is a bit of a kick in the teeth but maybe I do lack practical experience of high temperature machinery and materials :)

Have an excellent Christmas and a Happy New Year. ;)

Are the numbers right on the spreadsheet?


If they are, the Dilivars result in a net LOSS of clamping force @ temp.....that can't be right....or is it?

This is the most astute and nuanced/implicit argument I have ever read.:)

Aaron, I agree with this observation and I have serious doubts about the validity of this data.

A typical engineering steel would have an expansion of about 13 ppm/degC rather than 10.

All Auminium alloys I have knowledge of have a figure of at least 19ppm/degC and 21 would be a more realistic estimate for an alloy with relatively low Silicon.

Elektron (Magnesium) Alloys are more likely to be around 25ppm/degC

I belive Dilavar is about 18 ppm/degC

17-4PH (Precipitation Hardened Martensitic Stainless Steel) is about 10.8 pp/degC

All of this expansion data is based on measurements taken at 20 degC and there will be some minor variations at 200 degC but similar trends would be observed.

I beleive that clamping force increases with temperature in all cases.

Clearly in the case of Dilavar this increase is relativley low and if you only use Dilavar on the lower studs there will be an uneven distribution of force on the studs and barrels. I am not sure this is entirely christian and not something I would generally recommend.

The 'standard' steel stud would give a moderate increase in clamping force and the 17-4PH the highest increase.

It is interesting that ARP's 'best' material is Age 625 and this an 'Inconel' type alloy which is Nickel based and offers extremely good high temperature properties but has a typican expansion of 13ppm/degC and is very similar to a standard steel stud.

Just for interest Titanium 6AL4V a typical alloy used in race cars has an expansion of about 9.5ppm/degC.

Just a quick review: Please bear with me as I intend to make some assumptions and all of this is from a rather marginal memory.

Porsche engineers starting with a clean sheet of paper and all the material data sheets designed an engine with specific torque specifications.
I assume they selected the head torque specs with the understanding that the steel studs would not expand as much as the aluminum case, heads and Biral cylinders. The torque value selected would (by thermal expansion) increase the clamping pressure as the engine heat increased. They knew this so they either wanted an increased clamping pressure or felt that an increase would not be detrimental. (I’m of the opinion that they wanted the increase). These engines had few if any head sealing issues

Next, they redesigned the case and in that design they change the case material from sand cast aluminum to die cast magnesium. No change in head torque value and no stud issues. As the case specifications progressed so did the cylinder material. Case spigots got larger and the cylinder changed to Nikasil coated aluminum and high silicon Alusil an aluminum alloy with 17% silicon. If high silicon aluminum expands differently that "standard aluminum" wouldn't you expect to see a different stud or at least a different torque specification? Once again, no torque specification changes from the world class engineers. It wasn’t until customers super-heated the engine that stud issues appeared. It was assumed that most head sealing issues were attributed to a loss of clamping force because the head studs were pulling from the case.

At that point, Porsche redesigned the case to accommodate a larger cylinder and specified die cast, high silicon aluminum as the case material. Early die cast aluminum case came with steel studs and of course no stud torque modification. This new case seemed to resolve the pulling stud issue.

The later aluminum case engine is where we started seeing Dilavar (24 in the turbos and 12 on the NA cars). It was around this time that we started seeing head sealing issues. Yes, your right, no stud torque change even on the engines with differing studs. I wonder if they had the same engineering back ground as the experts in the thread. Didn’t they know about clamping pressure and thermal expansion? It was in these engine that we started to see head sealing issue that weren't related to loose nuts (pulling studs).
Within a few months, we started to see stud failure in new street cars. Warranty departments were beside themselves. I even had a brother in-law with a 78 SC who needed me to repair his 16,000 mile one year old car.
The engineers figured the issue was corrosion so they epoxy-coated the studs and decided that all 24 studs should be Dilavar. Did I mention that the "new/coated" Dilavar still broke, sometimes while the engine was on the stand? It was at this point that I started having my doubts about Dilavar. Did I mention there was no head torque specification change? What about clamping pressure?

Porsche stuck to its failed stud for a decade or so until they abandoned the Dilavar for a steel stud. Still the same torque spec.
Shortly thereafter, they came up with the all-thread (looks like a hardware store remedy and how do you engineers feel about a stress raiser in the center of a stud?) that would presumably alleviate the breaking issue. I will admit that the all-thread is less likely to break, although we do see breakage and we continue to see head sealing issues. The torque specifications were changed somewhere along the way but if I remember correctly they went from ft/lb or NM to a torque angle which produces, you guessed it, the same pre-load as the ft/lb method. Wait , wait, wait, what about clamping pressure?

So tell me again, why shouldn’t we try something different? And after testing that “something different”, if it passes the real world trials why shouldn’t we use it ?

Thanks for the history lesson :)

I don't think anyone here is saying that you ought not try something new or continue to use it if it works. The real question being asked is "Why?! Why does it work better?"

I wouldn't hesitate to use your studs in a rebuild; in fact, I'm certain I would. At the same time a good engineer wants to know the why of it, and, if his (her) hypotheses don't match reality to figure out what he (she) has missed. The answer in this case *should* be knowable from a technical standpoint; there's no theoretical physics here- it's all well documented old stuff. Something's been missed is all; the facts are the facts and if we can't match hypothesis and reality the failing is our own and not with the immutable laws of nature!

Seriously, there has to be an explanation. To some, this is germain; to others, less so. I'd love to know, but I'm that way. At the same time if the bigger brains can't figure out the why I'll be happy with the 'what' of "These are currently the optimum choice if you need new studs."

The search for enlightenment is noble and discovery is rewarding but to continue to use a product that has a failure rate that is unacceptable because you don't know why it fails may be boarding on OCD.

Insanity is "Doing the same thing over and over again and expecting different results" Albert Einstein

Henry,

There are of course several interesting issues that come out of this history and it is worth looking at some of these issues on a point by point basis.

It would be standard practice for any designer to consider the influence of expansion on clamping forces and the effect that this would have on both studs, pull out loads and the potential of yielding to occur in compression.

If I were designing a joint of this type I wouldn't look for an increase in clamping due to expansion as I would want enough preload to eliminate the fatigue loadings produced by the peak cylinder pressures but I would want to be sure that I didn't cause any failures due to increases in the forces.

The sand cast cases were likell to have been a 'Eutectic' alloy which would be about 12% Silicon and this would have expansions in the order of 21ppm/degC.

The load bearing capability of the Elektron casing must also have been considered to be adequate for these loads.

I am not sure about the material used for the Nikasil barrels but if it were a typical wrought alloy its expansion could be as much as 24ppm/degC.

The 17% Hypereutectic Alloy developed by KS can be used without liners by etching away the surface aluminium and leaving a small surface of Silicon.

These alloys expand about 15% less than wrought alloys - about 20ppm/degC.

There is a potential issue with the cast aluminium/silicon engine cases and that there is the possibility of silicon migration within the solid phase at temperatures around 200 degC.

This means that silicon particles within the component can grow which depletes the silicon in neighbouring regions and causes a local weakening of the matrix.

(This can be one of the causes of failure of Eutectic and Hypereutectic Pistons and why 2618 is preffered for high end Turbo engines.)

I am sure there must be some detailed information about the behaviour of these alloys but it has not been extensively published.

The die cast aluminium cases would not be too different from the sand cast cases in terms of material expect that the Silicon Morphology is likely to be more even in terms of both size and distribution.

There is, of course some porosity issues with die castings compared to sand castings.

Gravity die castings and even low pressure die castings tend to have low levels of porosity but they can exhibit porosity evenly throughout a structure (many die cast wheels leak air and now the trend is to powder coat to seal them.)

The composition of the case is interesting and in general die castings would be made from a Eutectic Alloy.

This is a Silicon content of about 12.6%. This composition has two adavatages.

The eutectic composition gives the lowest melting point of any Al/Si alloy and hence heating cost is minimised. The second is that Eutectic alloys don't have a freezing range. They remain liquid to a specific temperature and the solidfy evenly and uniformly with a change of only a couple of degrees.

This means that components can be reaily knocked out a die as soon as they ahve cooled, you don't ahve to wait the hundred or so degrees (freezing range) of alloys such as 2618.

These two charcteristics significantly reduce casting costs.

I am sure the stud pulling issue was a feature of magnesium combined witj Nikasil and thermal reactors increasing engine temp.

There is no doubt that Dilavar suffers significant sensitivity to Stress Corrosion Cracking in the presence of Chlorides (salt on the road is a great way to introduce chlorides to the studs).

Stress Corrosion Cracking is a very specific form of corrosion. the chlorides attack the grain boundaries of the material and effectively form a very sharp crack.

When the crack has elongated sufficiently to reduce the remaining area of the stud it can no longer support the load and it will suffer from a brittle failure.

There is a whole science (Fracture Mechanics) devoted to this type of failure but the crack length, crack tip radius and the stress intensity all contribute.

I have to say that apart from manufacturing defects there is no failure mechanism that would result in the brittle fracture of a new stud which had been loaded a left in an engine stand.

It would be good to look at the fracture surface of such a failure - probably with a Scanning Electron Microscope as there is bound to evidence of a defect.

Porviding the clamping loads have been correctly established there is no real reason why Dilavar should give problems due and why heads should lift.

I am suspicious about casing stability and stress relaxation where studs would pull in magnesium cases. Increase in Silicon content is likely to make this worse.

I would agree that the 'all thread' stud looks terrible but the tip root radius of the thread doesn't really cause any significant stress riser in this application.

i different geometries tip root radii of about 0.2mm would be an issue but not in studs or bolts in the type of materials we are disussing.

In fact the probable reason for the all thread is to ensure that any deformation is evenly distributed along the length of the stud - something which the thread will help to achieve.

If you take a bar with a single notch and put into a tensile test machine it will clearly fail at the notch. You would also need to measure the elongation at failure.

If you take a length of the same bar (cut adjacent to the first test length) and machine a series of multiple notches the failure load will be identical - within experimental scatter - but the elongation will be significanltly higher.

This does, of course, also need a material to have some ductility so ceramic bars wouldn't behave this way.

The all thread also have a slightly larger minor diameter than the plian stud so for a given prload will be slightly less stressed.

I am not sure why you would change the way you the nut on a stud from simple torque to a torque + angle and this needs some thinking about.

It may be to try to have a more even preload .

Torque to axial force using torque wrenches tend to give axial force variation of +/- 25% and this could be part of the issue with increased capacity and hence combustion loads.

If 12% is eutectic, then wouldn't 10% Si be hypoeutectic, not hypereutectic?

The KS "Alusil" barrels were meant to be the wear surface and were run with hard, iron coated pistons. The MAHLE "Nikasil" barrels were meant to be the hard surface with their thin Nickel-Silicon-Carbide coating on the Aluminum barrel and were run with a normal Aluminum piston and relatively soft rings.

The MAHLE alloy for pistons and cylinders is a higher Silicon content than the JE alloy, which is the one more stable at high temperature but a little softer and with more thermal expansion. I do not remember the numbers but there was a thread on here about materials for making new, CNC machined heads that discussed alloys. chris_seven was the main contributor.

And a mean tensile stress reduces the fatigue endurance limit. So I would want to have the minimum preload (at any temperature) to be just enough to overcome the cylinder pressures and keep everything from fretting.

Or were you referring to the Aluminum fatigue? Well, Aluminums do not exhibit a fatigue endurance limit and even if they are in compression the polycrystalline nature will mean that there will always be some grains oriented in a bad direction that will be loaded in shear or tension which can start a crack even with cyclical compressive loads.

Found this information about Dilavar:

Dilavar (Ni 13 (Nb NiMnCrV X 68 12 5) - Material No. 1.3937)
It is a nickel alloy.

I also found this text on the Lnengineering website:

On 911 models, certain engines came fitted with Dilivar head studs, which were designed to match the expansion of the cylinders, but also suffered from hydrogen embrittlement, which is a condition that results from the accumulation of hydrogen gas in the atomic structure of the metal.

If hydrogen embrittlement is the cause, and not SCC, it can explain that the bolts fail at the engine stand. In my previous job the same thing happened with new bolts on oil equipment, even before it left the factory.

Speaking for myself, this has been a fascinating and educational thread. Learning new things is the main reason I go to work every day and this one has been all of that.

This discussion has certainly shed renewed light on this subject and offered the variables that may help explain why people have varying experiences with all the head stud options.

My heartfelt thanks to all the participants who have contributed their engineering and metallurgical expertise and hopefully made all of the readers a little smarter. :) :)

Sorry 10% was a typo - should have been 17% - which is hyper -- 10% would indeed be hypo.

4032-T6 is very similar to the Mahle Alloy and the Silicon does reduce the expansion by a significant amount but also reduces the melting point and hence - in general- the softening point.

One important property of materials used at elevated temperatures - an aluminium at piston crown temperatures is just such an application - is the -equi-cohesive temperature. This the point at which grain boundaries in the material start to slide over each other and allow deformation.

2618 is better than 4032 in this respect. The pistons that used to be used in Mercedes F1 engines had a Beryllium content - since banned and this alloy is not only stronger, stiffer has better thermal conductivity but is also more resistant to creep - sorry to drift off the subject.


I agree that preload should be a minimum and I think I said in an early post that anything that increased the pull out force was undesirable.

I don't think fatigue in the studs us a major concern but studs will generally be designed to have enough preload to avoid fatigue loading.

I am not sure what you mean about the 'bad' orientation of grains in a Aluminium based material. Aluminium is face centred cubic which means it has 12 slip systems available to allow deformation so its its behaviour is generally quite isotropic.

Most metals that exhibit fatigue endurance limits have 'interstitial' alloying elements. These interstitial atoms block the movement of dislocations and inhibit the formation of the slip band intrusions and extrusions that usually initiate fatigue cracks. Interstitial atoms such as carbon are quite small.

Aluminium atoms are already quite small and the vast majority of alloying elements are subsitutional and these don't much change fatigue behaviour other than to increase strength.

The precipitation hardening alloys also don't really produce fatigue endurance limits as precipitate particles are again relatively large.

Stell at room temperature is Body Centred Cubic and there is a large octahedral interstital site which can be occupied by a Carbon atom and this is generally responsible for the endurance limit.

If a 'bulk' material is loaded in compression the orientation of individual grains will not lead to the production of fatigue cracks. As you say you will always need a traction vector to generate a crack. If you load a material in hydrostatic compression it will not fail even if it is polycrystalline as are the majority of day to day metals (Gas Turbine blades are single crystal to eliminate grain boundary sliding). I think you will only see resolved shear stresses at surfaces where buckling or other geometric instabilities occur.

I do think, however, that at around 200degC Aluminium/Silicon alloys can exhibit fatigue softening due to variations in tensile loads. The more highly loaded the stud the more likely this is to occur. This mechanism would almost certainly cause a loss of preload.



I don't think you can call this a Nickel Alloy. The convention is to name an aaloy after the main alloy constituent and 13% Nickel is almost ceratinly described as a Stainless Steel. The fact the studs are virtually non-magnetic is a good guide to the fact that the crystal structure is Austenitic in nature. Stainless Cutlery Steels, which are ferritic are about 13% Nickel. The other elements present help to stabalise the Austenite phase to remain at room temperature.

Materials such as Inconel 718 which are similar to ARP's Age625 are around 55% Nickel and are traditinally known as Nickel Alloys.

The Hydrogen embrittlement theory is interesting.

Hydrogen embrittlement is a reasonably common phenomenon but there must be a souce of hyrogen for this to be a problem.

In oil/gas environments knows as 'sweet and sour' gas there is always hydrogen present in pipelines and this hydrogen can and does diffuse into plant and equipment and casues significant problems. The hydrogen normally needs to be in an atomic form to diffuse into a metal and is normally the result of a reaction rather than in gaseous from.

Electroplating high strength bolts can also produce atomic surface hydrogen if plating is not extremely well controlled and it is common practice to de-embrittle plated bolts particulalry in a areospace environment.

If Dilavar is a precipitation hardening Nickel Alloy then its susptibility would be much reduced compared to conventional high strength steels.

Nickel based alloys of this type are the material of choice for most 'Hydrogen' applications.

It is also true to say that many of the high strength precipitation hardened stainless steels can be suseptible to Hydrogen embrittlement but would need to operate in an environment wher hydrogen was present.

A salt water environment could help to induce this type of problem but parts would need to immered for significant amounts of time for this to become an issue.

There are two ways to reduce the sensitvity of Preciptitation Hardened Stainless Steels to hydrogen embrittlement.

The first is to heat treat them to an over-aged condition which lightly reduces strength. but is beneficial.

The second is to add Niobium to the alloy composition.

It is interesting that Niobium is present in Dilavar.

I think it is unlikely that the studs pick up hydrogen in their operating environment and if they did then conventional steel would suffer in the same manenr as would 17-4PH which could be worse.

It may be possible for Hydrogen to have been introduced into Dilavar at some manufacturing stage - such as during acid pickling but it does seem unlikely and would have been relatively easy to resolve and eliminate.

If a bolt or fastener has a prior defect such as a hydrogen crack the loading it to a specific conditon that does not cause failure will mean that it is in a stable condition and providing the stress intensity at the crack tip doen't increase there will be no mechanism to cause a brittle failure.

The propogate a crack in an unstable manner the crack tip has to be provided with additional energy - the stress intensity must increase.

The only explanation can be that the fastener has a prior defect and that it has been initially loaded to a level just below the critical stress and then left.

If there is then some expansion or contraction taking place due to temperature change the KIC of the material could be exceeded and fialure would occur in a brittle manner.

This would only be true for very high strength materials and the 'cracks' would have to quite large.

My conclusions in general are that I would tend to use standard steel studs in standard engines. I would be tempted to use Dilavar or A286 in Magnesium cases with Nikasil barrels and would use Supertec Studs in high performance motors.

During the development stages of the Supertec stud, the engineer considered hydrogen embrittlement a possible challenge and in response to his concern, he called out an aging process as part of the preparation for thread rolling. The studs are cut to length, ground twice (each end is different), aged, threaded, ground again (tips and body) and inspected.
Last year we considered cryogenics as a possible enhancement but after Cryo treating and laboratory testing we discerned no appreciable benefit.
The stud is in a constant state of evolution with this years development being a second stud with a length change for better fit in water cooled GT3 and we're currently testing a flanged, 12 point, titanium nut.

What I was trying to say was that this is not a case of uniform, triaxial normal stress, at least the far field stress. The compression is in one direction only and so Mohr's circle tells us that there will be orientations that will see shear stress. Combine that with the pressure in the cylinder which sets up a tensile hoop stress and there will be tension in the material. Aluminum's many slip planes combined with the polycrystalline nature should mean that there will always be a stress to cause fatigue. And since Aluminum alloys do not show an endurance limit there is nothing that can be done to the stud torque that will cause them not to fatigue, though their lives can be extended by using minimal pre-load.

I agree that materials will fail in uniaxial compression, and there will be shear stresses developed in this case. The point I was making was that grain orientation in ductile materials does not influence this failure as there are plenty of slip systems available. If it were a brittle material the presence of cleavage planes would make this a different situation.

I don't think stress has anything to do with polycrystaline materials. The crystal structure will clearly influence failure stress and failure modes but unless yielding occurs I am not sure there is much influence.

It is possible that elastic modulus could be slightly anisotropic but this is a bit too deep for the macroscopic behaviour we are considering and I would model for elastic behaviour that is entierly isotropic.

If you know the principal stresses than you could contruct a Mohr's Circle and predict failure stress. By drawing the Circles for Uniaxial Compression and Uniaxial Tension on the same axes you can derive an Circle for intermediate conditions and when stresses exceed this envelope failure will occur.

I think that the Mohr's Circle approach is, however, a little too conservative and think a simple maximum Normal Stress Approach is bit more appropriate.

I am not sure that the stud location of the case sees much of a hoop stress as the peak cylinder pressure only occurs within a few degrees of TDC and there will be a steep gradient along the liner. I am failry sure that the tensile loads produced will be the most significant.

I think that the most appropriate model is to consider the threads in the case and look at the stress distribution in this area.

It would be interesting to carefully model this aspect of the design with a good FEA package and try to evaluate the influence pf preload, temperature and peak cylinder pressure.

Part of the confusion surrounding this discussion is the questionable voracity of the data we assume is accurate.
Does all aluminum perform the same under changing environments?
Has the material that we laughingly call Dilavar been accurately identified?
Could Porsche engineers have had an ulterior motive or agenda beyond performance or cost savings when "Dilavar" and stud design was selected?
When Aaron asks "Are the numbers right" any impartial observer would be forced to admit "who knows".
This discussion has gone to a place that honestly leaves me feeling a little inadequate, so I think I'll be leaving.
Thanks for letting me play, I'll collect my parting gifts at the door. SmileWavy

studs
 

I would like to ask a specific set of questions, with emphises on cost and end result.

1 In a Mag case 2.8 engine with Nikasil cylinders putting out 300 h/p which studs are recommended?

2 In an aluminum case 3.0, or 3.2 engine with Nikasil cylinders putting out 315 h/p which studs are recommend?

Please take cost into consideration, I have no need to brag how expensive my rebuild was, or that my studs will support the Titanic. I just want something that will not fail, and costs the least.


Mike

Hi Mike,

I think you'll get varied opinions to your questions here so I'll simply offer mine, FWIW. :) :)

1) Personally, I'd use the late 993TT Dilavars in a mag-cased, high-HP 2.8. Mahle cylinders must be modified for these as they are slightly larger OD than the steel ones. Naturally, case-savers are always installed to properly anchor the studs and strict oil temp control is mandatory for maximum durability.

2) We like the same 993TT studs in these engines to maintain consistent head clamping forces through the range of operating temperatures.

I've had 4 engines rebuilt within the last 9 years. Two mag case and two aluminum case engines. The first one was a 450hp turbo followed by a 2.5 SS, a twin plug MFI 3.2 and a 2.45 twin plug with 40 Webers making 180 RWHP. Three had Dilivar studs in them when disassembled. They were all built with Supertec studs and none show any signs of head to cylinder leaking that was present in my Dilivar engines.


Posted by an engineer who offered us such a thorough analysis of the material properties and their function in this every changing environment.
During this discussion there was little or no mention or debate about the other features of the Supertec studs. Why is that?

Below are prices on some studs offered by our host.

Pelican Racing Head Stud Hardware Super Kit - 24 RaceWare Head Studs, 24 RaceWare Head Nuts, 24 RaceWare Head Washers $909.85 no guaranteeI stand corrected they also have a lifetime guarantee.

Dilavar Cylinder Head Stud (sold per each, 24 required), 911 Turbo (1976-89)
Brand: OEM $41.50 $996.00 without nuts & washers no guarantee

Supertec Performance Cylinder Head Stud Kit (sold as a complete set, hardware included), 24 studs, 24 washers, 24 flanged nuts $660.00 Life time guarantee

It is difficult to consider some of the other features of the Supertec studs without sounding critical and as if I don't like these parts so I would start by saying that in general they appear to be well made and good quality and offer reasonably good value.

As is the case with the material selection, the 'other features' would also bear looking at from an engineering point of view.

1. Longer thread engagement into the case

It is well understood that load distribution along a thread is not uniform and the majority of the load being supported by the first engaged thread.

As a general guide once a thread engagement of more than 5 pitches the thread pitch tolerance accumulation pretty much guarantees that there will be unequal load sharing in the thread engagement.

Also the longer the thread the more difficult it can be to install and the thread as pitch errors can cause seizing.

As a rule of thumb it is not usual to design studs with a thread engagement beyond 1.5 to 2.0D.

If you need to ensure that the root shear stresses are lower than the tensile stresses in the stud so failure always occurs in the stud you should reduce the diameter of the body of the stud hence giving an effectively larger diameter threaded end portion.

It seems that the standard stud meets the criteria of 2.0D stud engagement so it is difficult to see why a longer thread will provide much benefit.

2. Fine Pitch Thread

Fine pitch threads have a larger pitch diameter than a coarse thread and tend to have a higher thread friction torque than a coarser pitch but this is genrally counteracted by the pitch torque change.

This is governed by a simple relationship:

While a fine pitch thread does have a slightly larger pitch diameter, the higher thread friction torque is negated by the pitch torque change. This is a simple relationship:

F = 2•π•M/p

Where

F = Force
π = 3.142
M = pitch torque
p = pitch

Whilst more engaged threads may increase prevailing torque this component is small compared to the total applied torque and for free running nuts there is essentially no difference between a coarse and fine pitch.

Coarse pitch threads have a higher lead angle than fine pitch threads and as the helix angle is a complement to the lead angle the coarse pitcg thread has a have a smaller helix angle than the fine pitch thread.

This means the fine pitch fastener will develop more axial force for a given torque than a coarse pitch fastener. The coarse pitch, however, develops a a more linear displacment for a given angular displacement.

Fine pitches do provide finer adjustment as they advance less per rotation than coarse threads but the higher prevailing friction can make them more prone to allow variation in axial force.

In general the differences in either torque or axial force for a single pitch step is less than 5% so I am not sure there is much practical differences.

3. 12 point nuts with integral washes

I would agree with the reduced space requirement for Twin Plug Motors compared to the standard nuts.

The intergral flange on the nut will increase the friction and hence the required torque for a given load.

I also looks as if the nuts supplied are coated with a dry film lubricant to ensure consistent tightening.

These aspects of the design, along with the fine pitch thread leads me to ask about the difference between the 'Nut Factor' K of this fastener compared to the standard part and the impact this may have on the preload comapred to using a standard nut.

For example an increase of 35% in the bearing area of a nut due to the presence of a flange would typically reduce tha axial force provided for a given torque by around 8%.

I am not sure if there is any difference in the torque recommended for tightening when using these nuts and it would be interesting to strain gauge some of these studs and compare them to the standard components to see if there is a difference in axial force.

I would imagine that carefully ground washers are very helpful in eliminateing any bending moments in the stud and helping to ensure even tension.

galling of nuts onto Dilavar studs is likely to be a function of the coating but also martensitic steels are less likely to gall than Ausenitic Steels due to basic crystallography and surface energy considerations and this observation is a benefit.

Using a Silver Plated Aircraft style fastener or using a suitable dag such as Boron Nitride would eliminate this problem.


4. Guarantee

Is the guarantee against failure of the stud or loss of preload or against pulling out of the case?

My general conclusion is that by simplifying the design of threaded fastener assemblies to a torque-tension relationship, with no mention of rotation angle, displacements, or strains does not fully address all of the relevant issues.

I still believe that the best 'fix' is to install case savers/timeserts and to install the studs in a well controlled environment in a consistent manner.

I am sure that many of the high quality aftermarket studs are manufactured to better tolerances and standards than stock parts and may help eliminate some of the issues.

I believe one of the main drivers behind this was to eliminate the need for careful
control of the axial position of the studs on installation. Instead of "Install to an exposed height of xxxx mm," you run them in until they bottom; the install height is automatically controlled.

I find that appealing from a DIY perspective; one less thing to worry about.

$0.02

Mmm!

Tricky - if you bottom out a stud into an aluminium cylinder block the differential expansion that occurs can make it easier for the thread to pull out particularly if the preload is relatively high.

There are also issues to do with any remaining cleaning liquids causing a hydraulic lock and causing varying preload/torque relationships.

It is not really good practice to bottom a stud - it is much, much better to install them correctly and not worry about them pulling out later.