Headstuds - would you use these?

[Henry Schmidt]

Originally Posted by Carrerax

I want to know if anyone has ever heard of a 993tt head stud failing? Just curious. I have a motor going back together soon and have to make some decisions.

Too much emphasis is placed on breakage alone.
We have seen a total of 3 broken Late Dilavar studs. Keep in mind that we work primarily on early, 89 and earlier engines by choice.

What we do see more often than not (that means most) is seeping at the head and evidence of cylinder to head movement. I assume that this movement is caused by an insufficient clamping pressure offered by properly torqued Dilavar.
Simply put, the head studs allow the head to lift off the cylinders.
This is not the case with our studs.

[johnnywishbone]

henry, pm sent. johnny in alpine.

[K24madness]

If we accept for argument sake that dilavar has similar expansion rates as the cylinders and steel less so then in theory the steel studs will have a higher clamping force when at operating temp than dilavar. Instead of trying to prove or disprove it with math why not look at the cylinders from each after being run for a long period of time. I would imagine a large bore 100mm cylinder would show uneven wear patterns if it were being over torqued from the differences in expansion rates on the steel studs. It would also show in the leakdown test.

I am just offering another way to look at things.

[Henry Schmidt]

Originally Posted by K24madness

If we accept for argument sake that dilavar has similar expansion rates as the cylinders and steel less so then in theory the steel studs will have a higher clamping force when at operating temp than dilavar. Instead of trying to prove or disprove it with math why not look at the cylinders from each after being run for a long period of time. I would imagine a large bore 100mm cylinder would show uneven wear patterns if it were being over torqued from the differences in expansion rates on the steel studs. It would also show in the leakdown test.

I am just offering another way to look at things.

WE do and we have. The results are that the Dilavar studs show signs of extensive head movement where the Supertec studs do not.

We see the same movement with Racewear and we see ARP head nuts coming loose so those heads are definitely moving.

Of course we see most of this phenomenon with race / high performance engines.

[m42racer]

I am sure the Supertech studs are very well made and good. However, the issue of studs breaking is something rare.

I am sure if you asked ARP or racewaer thay may say the same about the Supertech studs.

Most often, heads moving and leaking is the result of some other issue. Pistons hitting, Heads softening from use and the washer load bearing surface collasping, the case threads pulling or stretched in the case. ARP and racewear studs stretching under load I find hard to believe as the tensile ability of the materials used is well above the tensile values exerted upon use.

Seems to me studs are sold and promoted for the sake od making sales and not finding the real cause and solution.

JMHO.

[Henry Schmidt]

Originally Posted by m42racer

..... edit....
Seems to me studs are sold and promoted for the sake od making sales and not finding the real cause and solution.

JMHO.

If you're going to post a veiled accusation that I'm lying about my observations just to make a sale, the least you can do is use your name.
JMHO

I spend a great deal of time here assisting DIY engine builders in their pursuit of excellence. I'm not going to be called a lyre by some Gysmo who lacks the stones to use his own name.

[John TArsicwitz]

I am sorry, I did not hear anyone accuse you of lying here. The underlying problem is that the obvious self promotion and quiet sales has no place in a forum like this. In my last post I stated the same thing, I have nothing against you or your products but I am not fond of your self promotion or the fact that you obviously get angry when someone calls you on it.

[m42racer]

Ouch!!!

I suppose that puts me in my place.

There are many company's who build engines with some very talented experienced personnel that do not behave the way you do we questioned about their products or ways.

They are either knowledgeable from experience or have an engineering background where they can answer questions with emperical data, unlike yours.

[Henry Schmidt]

Originally Posted by m42racer

Ouch!!!

I suppose that puts me in my place.

There are many company's who build engines with some very talented experienced personnel that do not behave the way you do we questioned about their products or ways.

They are either knowledgeable from experience or have an engineering background where they can answer questions with emperical data, unlike yours.

Gizmo is kind of cuddly but do feed him after midnight or he'll gum up the works.

No, I didn't put you in your place. I just asked your name so that we could put an appropriate weight behind your words.
If we knew who you were, we could understand the experience and engineering behind your claims.

I put my name behind my posts and yes I make a nice product that I'm proud of.
Americans used to be proud of what we made and weren't afraid to say so. I guess I'm a throw back.

And you've done what to contribute here?

[m42racer]

No one has attacked you sir, or your product.

If you feel your answers have in some way contributed here, I stand in humble silence.

However, I read posts by Rennsport, Burns Bros and others and learn something.

[ix0ifan]

I hve been trying to follow all of these posts with a detached and objective attitude as some of the questions raised seem quite important and valid but some of the staments being made concern fundamental engineering and don't seem to make sense.

Dilavar seems to be condemed out of hand for all sorts of reaons but lets look at the basic case for using this material.

The basic alloy is a realtively high Nickel content steel with a controlled expanison of 20 x 10^-6m/m/degK - very similar to an Aluminium Alloy barrel but higher than a Biral or cast iron Barrel and maybe its not a good idea to use this type of stud on an early engine as they may lose clamping force with expansion.

There doen't seem much other information about Dilavar other than its tensile strength which is quoted at 170000psi.

We also know it is non-magnetic and this would indicate that it is an Austenitic Stainless Steel which, I believe, has been precipitation hardened to produce the high level of strength.

It would also be good to know the Young's Modulus for this material but as it is around 80% iron a figure of around 195GPa would be a reasonable guess.

I have asked the manufacturer for data on several ocassions but without success.

If a Dilavar stud is correctly tightened I can see no reason why clamping force would be reduced when the engine was running unless it had Biral/cast iron barrels (see above)

I would agree that the clamping force would not increase as much as a conventional steel stud as the engine warmed but there would be no other differences.

Even if we consider ignition forces on a Turbo engine there should be little effect.

Assuming a BMEP of around 15MPa the force on a single head would be about 165 lbs. This would increase the stress in each stud by about 500psi.

Assuming a Supertec stud has a similar Young's Modulus to Dilavar there would be little or no difference.

As we don't have any information about Supertec studs again the Modulus Value is an estimate but all alloy steels tend to be between 193 and 202 GPa so errors would be very small an well within the normal scatter caused by manufacturing tolerance and torque wrench uncertainty.

The only real problem I can see with Dilavar is that if it is an Austenitic Stainless Steel it will be prone to Stress Corrosion Cracking particulalry in the presence of Chlorides and this must be the cause of failures.

The phenolic coating on the new studs would help this problem.

I was interested by the example of the Dilavar studs which seem to break after a few years of being tightened in a engine which has never run nor been outside.

I am not really able to find a failure mechanism that can account for sudden brittle facture in these circumstances. The only mecanism tht could cause this type of failure is Hydrogen embrittlement but I don't think Dilavar studs are electroplated and I can't see another source of hydrogen and Austenitic Stainless Steels are are commonly used in Hydrogen environments as being safer than more conventional engineering steels.

I would be confident that using Dilavar would be a good idea on early mag case fitted with Nikasil barrels.

The relatively low Young's Modulus of magnesuim alloys would allow the thermally induced stresses to concentrate around the studs and any reduction in strength of the Elektron caused my overaging would account for failure.

This is much less of a problem with a Biral or cast iron cylinder and with an Aluminium engine case.

Looking at the ARP studs, there is reasonable information about all of the alloys the supply and with the exception of mag cases/Nikasil barrels I can't see any problems.

If the nuts used on these studs are coming loose then they were either not tightened correctly or they haven't been machined correctly so the thread from is wrong. It is fairly easy to measure the thread form. I can't see any meatllurgical issue with ARP studs.

My conlusion is that in terms of stud selection there isn't really a one size fits all.

I would certainly use the latest spec Dilavar without too much concern, especially on ealry mag enines with Aluminium cylinders but not with Biral or cast iron (as previously stated)

I would be quite happy with the majority of ARP fasteners on Aluminium engines as there is sound data available.

With regard to Supertec Studs, I am sure that they do just what is promised but I would be nervous about the lack of technical data and probably not use them for this reason.

[Henry Schmidt]

Originally Posted by m42racer

No one has attacked you sir, or your product.

If you feel your answers have in some way contributed here, I stand in humble silence.

However, I read posts by Rennsport, Burns Bros and others and learn something.

Wow, I've posted 3,300+ times and you haven't learned anything from them? Here's something to learn.

Originally Posted by BURN-BROS

I perfer the Supertec studs over any of the others. More thread engagement into the case and a nice fine thread for the head nut. The head nuts are nice as they provide clearance for the spark plug socket in twin plug applications.

Originally Posted by Steve@Rennsport

......edit.........
To be fair, we've not used the Supertech head studs as of yet simply due to our successes with the late 993TT studs (same as RSR/GT-2). There is no question that these Dilavars cannot be compared to the early Dilavars which were prone to corrosion and soon after, failures. Since we cannot afford a problem with a customer's engine, we tend to use solutions that we know from experience will be durable and reliable over the long term. R&D and new product evaluations are normally done on our own personal equipment or when a customer consents to being the "guinea pig" in such rare circumstances.

We've had similair problems using ARP head studs due to their inability to remain torqued under high heat and rates of expansion. Having discovered far too many loose head nuts using these, we stick to either steel ones for the early small motors and the late Dilavars for the larger bore and turbo engines.

.......edit........)

Please note that I did not participate in these threads: Of course this is a very small sampling.

which head studs?

The Ultimate Head Stud Poll

Originally Posted by ix0ifan

......edit.....

With regard to Supertec Studs, I am sure that they do just what is promised but I would be nervous about the lack of technical data and probably not use them for this reason.

The technical data / material spec sheet has been posted more than a couple of times. I will post it again when I return to the shop.

[WERK I]

Originally Posted by ix0ifan

...........................edited to save space.......

With regard to Supertec Studs, I am sure that they do just what is promised but I would be nervous about the lack of technical data and probably not use them for this reason.

Oo-oo-oo-ooh boy, that sucking sound you just heard is Henry running to the Supertec Technical Reference Library.

ix0ifan, nice writeup, btw.

For me, I'm outta here. Two active threads on the same subject is just a bit much, don't you think?(The other Dilivar thread)

[K24madness]

I used the 993tt head studs because....

Strong endorsement by Steve Warner.
Previous 993tt never leaked at 600tq x 600hp
2 other builders (not going to name drop because they are not part of this discussion and I don't want to be quoting them) of BIG 3.8 turbo motors uses them. Both of these guys have done tons of these builds without head leaking issues.

As a previous poster said there is no one right answer. It all comes down to making a choice based on what you know and who's opinions you trust the most. While I truly respect and value Henry's opinion the other side of the table has a longer list of people who I also respect and trust.

[Kemo]

this is an interesting read... I read thru most of it just to understand why my Dilivar studs broke in my SC. I just recently had to pull my engine to address the issue. of the 12 Dilivars, I had 3 missing and a 4th simply break off in my hand. All of them seemed to be broken at or near the same place and all of the non-broken studs had corrosion at the same spot on the stud. For me, the problem was simply corrosion and time. So.... do the new 993 Dilivars have a new metalurgy or is it simply the same ole Dilivar with a better coating? If the metalurgy hasnt changed, only time will tell if the special coating will last. Considering all of this, I cant make an argument for Dilivar. So to answer the original question, "I would not use the 993 studs unless money is an issue." its all a calculated risk.

FWIW, I went with OEM steel studs, top and bottom when I put my SC back together, simply because of costs and the fact that its just a stocker. I replaced all of the exhaust studs and re-used the allen hex bolts and washers when I could.

At what point does an engine builder start to be concerned about the strength of the case? I know for a fact that I had several head studs that I was not able to "bottom out" into the case because of the limitations of the allen head stud nut threads(wayne's book). So basically, im not using all of the available threads on the bottom so I can use all the threads on the top. This might not make much difference on a stock motor. On a much higher HP motor or a Turbo, I would want as much thread in the case as possible. Supertec studs seem to address this issue due to the open end nut.

[BURN-BROS]

As controvertial as head studs apparently are, Henry made a very good stud. He did his homework and did not compromise. The material specs have been posted numerous times and IIRC they are similar to steel. The studs do not loosen up nor have I seen issues at the sealing surface.

The last 993 dilivar studded engine that I was around had to have one head welded and resurfaced due to excessive head movement. 4 out of the six had movement. This was a 3.0 liter turbo.

Hundreds of engnes have Henry's studs in them with zero problems reported. That's pretty good claim to be able to make.

[ix0ifan]

[QUOTE=Kemo;5173207]
On a much higher HP motor or a Turbo, I would want as much thread in the case as possible. QUOTE]


I don't think that the horsepower has much influence on the studs. If we consider the barrel/head as a closed hydraulic cylinder the ignition forces involved are relatively small compared to the preload.

The forces involved due to thermal expansion are much more significant.

I think high horsepower turbo engines must have higher case temperatures and would almost cceratainly have higher head temperatures and this is the most damaging effect. The very high power Turbo engines seem to have water cooled heads to help with this problem and the engines with EB welded head/barrels eliminate the problem of the thermally induced stresses by removing the expansion/seal problem completely.

I would think that when using Nikasil cylinders the hotter the engine the more Dilavar is the best solution.

The length of thread engaged is also probably a bit of a 'red herring' and I think that the improvements offered are not too significant.

With a simple nut and bolt the stress distribuiton within the nut is very poor.

The first engaged thread carries a disproportionate amount of load.

The other threads only carry the amount of load transmitted to them by the deformation of the threads. About 35% of the load is carried by the first thread.

The last thread carries zero load and making nuts longer does little to increase their strength.

A similar situation exists with studs and the first few threads carry the majority of the load so I don't think adding a couple more threads would do much for the overall load bearing capacity.

[ix0ifan]

Just searched for the Supertec Stud spec and found the following:

17-4PH 38-42 HCR 4hrs @1025

I think this translates as: (I hope I interpreted this correctly)

17-4 PH is a precipitation hardening maternsitic stainless steel

38-42 Rockwell C is the hardness condition and equates to values between 175000 and 195000 psi.

H1025 is the aging condition and relates to 4 hours at 552degC with a typical hardness of 38HRC (175000 psi). This is also known as an overaged condition.

It's Young's Modulus is typically 196 GPa and Coefficient of expansion
11 x 10^-6 m/m/degK

It is rated as having excellent corrosion resistance.

As with all of this type of alloy heat treatment is very significant to ist performance.

In a solution treated condition this material will have a tensile strength of around 160000psi but its corrosion resistance and resistance to SCC will be poor.

After precipitation hardening the corrosion resistance increases significantly but is resistance to SCC isn't great. At a strength of 195000psi it would be rated as 'poor' in this respect.

Following a heat treatment complying with H1025 resistance to SCC is said to be excellent and this still relates to s strength of 170000psi (38HRC)

As ageing temperature increases resistance to SCC continues to improve and H1150 is the optimum heat treatment if this is an overiding factor. (33HRC, 150000psi)

In fact in sensitive application areas such as oil fields or petrochemical plants a maximum harndness os 33HRC is specified as in the presence of sulphides and chlorides some failures have still occured, but these are very 'ugly' environments.

I have used 17-4PH for transducer applications in the past (force measurement in helicopter engine dynos) and I am sure it is a very good choice, the only problem that could occur would be due to incorrect heat treatment.

[lindy 911]

I would agree with your thread theory if the threads were not loaded in the first place. Supertec studs allow you to install the stud to the bottom of the hole and tighten them some, which loads all the threads making the entire tapped hole part of the load bearing structure. Without pre-load in the threaded hole, like most other studs, the first few threads indeed carry most of the load.

Lindy

[BURN-BROS]

Originally Posted by ix0ifan

I don't think that the horsepower has much influence on the studs. If we consider the barrel/head as a closed hydraulic cylinder the ignition forces involved are relatively small compared to the preload..

The stud material Henry chose WAS directly from the 935 program.
They dumped the dilivar studs due to the heads lifting at higher boost levels.

Originally Posted by ix0ifan

The forces involved due to thermal expansion are much more significant.
I think high horsepower turbo engines must have higher case temperatures and would almost cceratainly have higher head temperatures and this is the most damaging effect. The very high power Turbo engines seem to have water cooled heads to help with this problem and the engines with EB welded head/barrels eliminate the problem of the thermally induced stresses by removing the expansion/seal problem completely.

I would think that when using Nikasil cylinders the hotter the engine the more Dilavar is the best solution.

So Porsche eliminated studs completely from their prototype turbo programs... and eliminated Dilivars fro the 935 program. To go back to a Dilivar after they eliminated it does not seem prudent.....

Originally Posted by ix0ifan

The first engaged thread carries a disproportionate amount of load.

The other threads only carry the amount of load transmitted to them by the deformation of the threads. About 35% of the load is carried by the first thread.

The last thread carries zero load and making nuts longer does little to increase their strength.

A similar situation exists with studs and the first few threads carry the majority of the load so I don't think adding a couple more threads would do much for the overall load bearing capacity.

You will have to clarify, this seems more appropriate in sheer/or bending loads(at the nut)....not tension. Truthfully there is too much mental masterbation over this, no offence.

Just because Porsche continued the Dilivars into the 993 turbos does not mean they are right....They thought running 9mm rod bolts was an appropriate choice as well.