Specific Question - ARP Head Studs on 2.7 Magnesium case - Insane?

[Lapkritis]

I searched and read.... Wondering what the experts say - I have the new studs already but have scared myself by reading. I plan to load to ARP spec. Case already has timecerts and front oil cooler in cold climate (New England) for entire life. 1974 so not a case subjected to the thermal reactors. It has been suggested I go with the generic Canyon studs sold by the host here instead... I'm not sure I see the benefit over ARP though because the expansion rates are about the same with only lower loaded torque. if I'm going to have an issue with a cert pulling then I expect I would whether it's with ARP or Canon. Anyone?

[chris_seven]

As I discussed in a recent post I find the use of Aero Engine grade material a bit over the top.

ARP in the UK tell me that their 911 Head Studs are made from Inconel 718.

Its room temperature CoE is about 12.5 ppm/degC compared to 8720 Steel which is about 11.6ppm/degC.

Not a great deal different and not enough to worry about.

The difference in the forces due to expansion will be about 7% higher and not something I would worry about.

I am not sure there is any benefit to using the Canyon Studs compared to the ARP except cost and if you already have ARP why buy more?

It is interesting to note that ARP say on their Technical page that Inconel 718 is 'completely immuse' to Hydrogen Embrittlement.

Whilst I don't think that HE is an issue for 911 head studs - so please don't worry - I seem to remember that Inconel 718 does have some issues in this area.

Vol.35 No.10 pp.689-694

[Lapkritis]

Thanks Chris - I appreciate the feedback. I've been harassing the ME's that I know for their insight and one who works for Lockheed is going to run the numbers this week. Your line of thinking jives with what I had originally researched. I'll likely stick with the ARP and thoroughly clean the threads of the case before install.

As far as HE with the inconel 718 - I have SharkHide that I planned to coat the aluminum of the engine with. This is also compatible with steels and other metals... probably no harm in coating the shaft of the ARP as well once they're installed and torqued. Any feedback there?

[GeorgeRud]

I just had some Timeserts pull on my 2.7 that was rebuilt about 10 years ago. I had used all Dilivar studs at that time, and the engine has not been abused or overheated since the rebuild. It is now at the machine shop having case savers installed, so even the Timeserts are not bulletproof.

[Lapkritis]

Originally Posted by GeorgeRud

I just had some Timeserts pull on my 2.7 that was rebuilt about 10 years ago. I had used all Dilivar studs at that time, and the engine has not been abused or overheated since the rebuild. It is now at the machine shop having case savers installed, so even the Timeserts are not bulletproof.

Timesert and Dilavar and still pulled... yikes.

-Andrew

[chris_seven]

Originally Posted by Lapkritis

As far as HE with the inconel 718 - I have SharkHide that I planned to coat the aluminum of the engine with. This is also compatible with steels and other metals... probably no harm in coating the shaft of the ARP as well once they're installed and torqued. Any feedback there?

I just can't see a source or externally applied hydrogen being an issue with a 911 engine as there is nothing associated with the environment to produce nascent hydrogen.

Inconel 718 is very unlikely to corrode even if you don't apply a coating.

Originally Posted by GeorgeRud

I just had some Timeserts pull on my 2.7 that was rebuilt about 10 years ago. I had used all Dilivar studs at that time, and the engine has not been abused or overheated since the rebuild. It is now at the machine shop having case savers installed, so even the Timeserts are not bulletproof.

I think that a standard Time sert uses the same size tap as a Helicoil Insert and this could mean that you still have space to fit a 'Big Sert'

These have been designed to use when helicoil or other inserts pull out and if there is enough case material they may provide a second shot at a solution.

http://www.timesert.com/html/bigsert.html

I think case Savers are a similar idea.

[Lapkritis]

Would it make sense to go incrementally higher with the expectation that they all pull eventually and the longest life for the case would be refitting the next largest saver each repair cycle? It doesn't sound like there is a bullet proof solution for these.

[Henry Schmidt]

CaseSavers with Supertec head studs. Hundreds of sets sold and personally installed to date, not one complaint about studs pulling.

[Lapkritis]

Henry, what material are your studs made of?

[Henry Schmidt]

Originally Posted by Lapkritis

Henry, what material are your studs made of?

Aged 17-4PH

The design of the stud is equally if not more important than the material.

[Lapkritis]

I see there are cast iron cylinder options which would maybe negate the materials expansion issue that are present with aluminum cylinders. Would installing iron jugs be part of a reliable fix?

[Henry Schmidt]

Here are some shots of a mag case engine with 20K easy miles.
Timecerts and Dilivar. Only a few studs were loose but every head was leaking.



3.5 liter race engine with 60hrs. Every head was leaking. 993TT Dilivar stud. Anyway you slice it, they are just plain crap.

[Lapkritis]

What about the QSC iron cylinders? That would be less stress on the case than aluminum jugs I imagine. Are they junk?

[chris_seven]

How about JB Racing Aluminium Fin Steel Cylinders

JB Racing - Porsche Engine Components

I didn't like to cut and paste their photograph due to Copyright Issues.

[Lapkritis]

Great link - The billet finned steel cylinders are a work of art. I would love to have those in my budget but I'm not sure I could reason $3000 into this area. I'm not sure quite the benefit of the cast irons... if they are not within spec 95% of the time due to cheap manufacturing then I wouldn't want to waste the time on experimenting.

[Lapkritis]

Originally Posted by chris_seven

How about JB Racing Aluminium Fin Steel Cylinders

JB Racing - Porsche Engine Components

I didn't like to cut and paste their photograph due to Copyright Issues.

I looked closer at JB and I see he previously sleeved the cylinders which meant the shoulder was still aluminum that tensioned on the gasket surface of the case. The new design with billet sink appears to be a larger diameter sleeve that is turned so the steel sleeve is now tensioned on the gasket surface of the case. Working on the sloppy math still for CoE of the iron replacing the aluminum under the head nut. 35mm of the material is aluminum in the head, the rest is cylinder. Should have something to share later tonight once the day job let's me out. If there is much gain reducing pressures on the tensioner/case then I may have to save for another month to have less aluminum under the head nut.

Has anyone measured the temperature of the cylinder while on a dyno yet? I've only been able to find some examples of cylinder head temperature (CHT) but not cylinder temp. This would be useful for determining maximum expansion of the jugs.

We're looking at the impact of conduction/convection of the replacement materials. Fun exercise to explore anyway.

[Lapkritis]

Okay so did some rough math/calcs for CTE based upon certain assumptions but also hard measurements. In a nutshell, you'll have far less tension on the studs with steel cylinders under the fastener. Here's how we arrived at that conclusion:

Coefficients
ARP Fastener (Inconel 718) - 7.2Min/inF
Cast iron - 5.9 to 6.2 Min/inF
Aluminum - 12.8-13.1 Min/inF
(we're working on dilavar numbers to compare now as well).

Here we're using one of my Mahle aluminum cylinders from my RS spec setup for measurements. The fastener is clamping 1.322" of aluminum in the head and 3.367" of aluminum in the cylinder at room temperature of 70F.










Assumption 1 - 400F material temperature of the cylinder during heavy operation giving a delta for our calculation of 330F (400F minus the room temperature of 70F for an increase of 330F).

First combination is what I have in hand now, all aluminum under the head nut for the fastener to hold:

3.367" (cylinder) + 1.322" (head) = 4.689"
13.1 x 4.689 x 330F = 20270.547 Min/inF or .020270547" of growth once the cylinder reaches 400F from room temperature.

Second combination is with cast iron cylinders in place of the aluminum.
13.1 x 1.322 x 330 = 5715.006 or .005715006"
6.2 x 3.367 x 330 = 6888.82 or .00688882"

Combined stack expansion .00688882 + .005715006 = .012603826"

Third item for consideration is the ARP Inconel 718 material to be used:
7.2 x 4.689 x 300F (assuming this runs slightly cooler than the cylinder because there isn't flame/friction inside it) = 10128.24 or .01012824"

At this point it's a fair conclusion that cast iron cylinders with the ARP fastener would be a wise choice to relieve pressure on the case threads due to thermal expansion of the materials above.

Growth of fastener: .01 inches (engineer says he would bet closer to .008 in his experience)

Growth of all aluminum stack: .02 inches
Growth of iron/aluminum stack: .012 inches
Growth of fastener: .010 inches

With the cast iron cylinders we would be reducing the growth of the stack under operation at 400F by roughly 60%. We need a text book to go further with this hypothetical for determining the pressure of the expansion of the stack relative to the sheer strength of the magnesium threads in the case. It would also be nice if someone with an infrared temperature gun could confirm temperatures of the items during or immediately following heavy operation. I believe we'll be able to dive further and review these basic calculations above in the next couple days. I won't be putting my engine back together with aluminum cylinders and ARP fasteners however. Feedback is welcome - we understand the assumptions leave some wiggle in the conclusion.

[chris_seven]

Andrew,

I am not sure that I can agree with all of the assumptions that you have made otherwise any Nikasil Barrel used with a Magnesium case would fail quite quickly.

The main area that I disagree is the Temperatures you have considered.

I believe that 400 degF as a running temperature is too high and most Elektron Alloys would suffer serious problems at this level.

It is fair to say that it is now possible to find Elektron alloys such as WE43 which are rated for use at up to 300degC but this is a relatively modern alloy.

It also loses about 35% of its yield and 50% tensile strength at this temperature so would need to be down rated

I believe that operating temperature of the engine case is more likely to be around 125 degC and this should be the cylinder bulk temperature as the numbers all become more meaningful. It would be great to obtain good temperature information.

Assuming a good high strength Mag Alloy was used the pull out strength of an individual stud should be about 12000 lbs. Yield would start to occur at around 9500 lbs so load would be lost at around this level.

There is some guesswork involved in this as I don't have accurate material properties.

The 24lbsft of torque will produce a preload of about 3600lbs (mean value) in the stud.

If we assume a Delta T of 100 degC the differential expansion between the stud and the cylinder will result in a compressive strain in the cylinder of around 8.3x10^-4 and using the simple relationship for Young's Modulus this will result in additional force of about 7500 lbs per stud.

The Total force on the stud would then be around 11000lbs which suggests a problem.

I think that the numbers I have used a conservative as many engines survive and my rough calcs indicate that they should fail - but only just.

If you substitue a good quality AluSil material the Pull Out force increases to around 16000lbs with yield not occuring until 12000lbs and as Aluminium Cases don't seem to suffer I can again assume my rough calcs are conservative.

If you increase Delta T significanlty then the pull out forces generated by expansion are directly proportional to temperature and failure is almost assured for both types of case.

A typical Grey Cast iron has a Coefficient of Expansion of about 9.5ppm per degK and this says that the stud should be losing preload with increasing temeprature.

I think the one thing that we can see is that Nikasil Barrels and Steel Studs of any Grade or type of material that is not Austenitic (hence high expansion) used with Mag cases runs close to pull out and the variations in torque/preload may be an issue.

Clearly using a Timesert or Casesaver increases the Pullout force needed and improves the situation.

[Lapkritis]

Great feedback Chris - thank you for sharing. The percentage of aluminum in the head (28% of total compressed by fastener) that would remain and expand would potentially offset the low expansion of the cast iron cylinder relative to the ARP fastener. We are shooting from the hip without real data which will mean we're only working in the general neighborhood. I promise to put the infrared thermometer on my engine once assembled to get a better idea of the true temperatures. To get a better idea of the case strength it was suggested that I contact AMEC for a materials analysis/test and then to consult matweb.com in order to get decent ultimate, yield, and CTE values. I imagine someone has done this already and maybe I just haven't been able to find it on the vast internet yet.

On another note, the ARP fastener calls for a torque of 36 ft-lbs. Would this in itself exceed the case thread capability? Another question for us to answer once we know for certain what the case is made of... we could assume weakest case scenario for a certain margin of safety. How much torque is transmitted through the nut/thread and washer to the root/case with the ARP lube? If the fastener wasn't loaded properly then a host of issues may present.

[Henry Schmidt]

Although it's fun to watch the metal masturbation, I think you guys are investing way too much in this exercise.
We've been dealing with this issue (stud pull out) since the mid seventies (even earlier in type I VWs) and there really is nothing new to learn.

Too much heat kills mag cases.
Timecerts have a thread pitch that is too fine for head stud repair.
Dilivar studs offer too little clamping force.
Supertec studs with professionally installed CaseSavers have never had a reported pull out.
We have built in excess of 400 air cooled Porsche engines (mostly mag case, race and street) and find this combination to exceed our warranty requirements.