The shank size will affect the force that is developed due to expansion.

Dilavar will result in a relatively low force increase of around 400lbs per stud.

For the majority of steel and other studs the CoE is about the same level (11-12 ppm per degC) with Inconel being around 5% greater which for the sake of this argument we can neglect.

The diameter of the shank will have a very significant impact on the force developed by the expansion of the cylinder and it is convention to use reduced shank fasteners when expansion is significant.

The chances that any of the materials used for head studs would be prone to the effects of creep at the temperatures involved is extremely unlikely as the activation energy needed to allow the mechanisms that cause creep to occur would tend to be much higher than those experienced by 911 head studs.

ARP studs for 911 engines are made from Inconel 718 which is commonly used for Jet Engine components and is routinely used at temperatures up to 0.6 x Melting Point which is around 2400 degF so not much to worry about in a 911 motor.

I think ARP suggest a torque of 36 ftlbs and this is a relatively high preload compared to Porsche's recommendation and may be when combined with the force created by expansion does allow relaxation to occur in the case threads.

I can't imagine there is much else that can be of concern with regard to the metallurgy of the studs as Dilavar failures always seem to be brittle elastic in nature.

Here's a crazy thought.
After all the head nuts are torqued down good remove one nut at a time and spray all the thread lubricant off the stud and nut threads with brake clean. Then reinstall the nut with low yield loctite (not high yield red loctite) and torque it back down.

Maybe not the most kosher way of doing it but the nuts should stay put after that.

I have considered using an aerotight K Nut and I need to buy a pack to see what the variation in running torque is so that this could be added to the torque to tighten.

I will ask an old contact at Simmonds and see if there is a good method.

It seems that the determination of the 'running torque' to use a Kaynar prevailing torque nut is quite straightforward.

The only additional torque needed is the torque to cause the deformed part of the nut to expand and run along the thread.

Simmonds don't quote any values and always recommend that the user carries out their own testing to determine both the torque to run the nut and the scatter and then determine the torque to add to the value required to obtain the correct preload.

I have just ordered 50 x HM20 Kaynar nuts to match some new studs we have just designed.

These nuts are 12 point prevailing torque and are CAD plated and dry film lubricated with Moly disulphide.

We will also dry film lube the relevant and of the stud .

The plan is to determine the running torque and then to measure the preload in a test fixture which has a relatively precise load cell and to see if we can learn a little more.


http://i197.photobucket.com/albums/a...pspczsg1gx.jpg

We have been making some Ti studs for Mag cases for a while now and we have always made them with a socket head to allow ease of installation and we will continue with this feature on the new design.

We have also added a 'dog point' on the base of the stud to allow the stud to be tightened into the case without 'bottoming' into the last few threads which are never correctly formed. This feature is being used now on the majority of Indy and F1 engines and studs with this feature are also available for 996/997 Turbo engines.

This feature will allow all of the threads in the case to be uniformly loaded and should reduce any tendency to stress relaxation to occur.

It does also provide a second benefit and this concerns use of Loctite.

ARP suggest that studs are installed without thread locker or that they are torqued before the thread locker fully sets.

This is fairly tricky on a 911, although we have made a simple fixture to try to achieve this result.

By using a socket head on one end of the stud and a dog point on the other it will be relatively easy to install the stud to a modest torque of say 12 ftlbs and ensure that the thread locker is correctly applied and that the joint is stable.

Interesting.... So Loctite may not be completely stable over time?

I think is concerned with the fact that there if there is no load it could sit in the clearance and allow some movement but I am not really sure and I haven't found any good published data.

Never had a stud come loose, and never taken apart a motor with a loose stud... Dilivar, steel, ARP or otherwise...

Cheers

I am not sure anyone has suggested that studs loosen in terms of the case to stud interface but that doesn't mean that if thread locker is used incorrectly the resulting assembly is stable.

As any thread locker works by filling the gaps that occur when there is clearance it is possible that a thin film of thread locker could sit between threads. ARP's suggestion to torque any fasteners before the theadlocker has set could be to ensure that there is a metal to metal contact which will be more stable in an axial direction.

If thread clearance has been increased by poor cleaning techniques it is possible that some deformation of any thread locker that is present between the threads once they are loaded could occur.

This axial deformation may allow axial movement which could reduce preload. Only a very small and in the context of a stud virtually immeasurable axial displacement is needed.

The thread locker sitting in the gaps at the ends of the truncated threads will still operate to prevent loosening due to vibration or thermal cycling.

Installing studs that are torqued before the thread locker sets can only be a positive action and would eliminate another variable.

Sorry, wasn't clear. I meant, never seen a loose head stud NUT, ever.

We had a 3.0 SC with 100K miles about 3 months ago with significant oil leaks at the cylinder bases on 2 cylinders.

There were no broken studs but head nuts on both cylinders with the leaks were loose.

Both cylinders showed compression ratios of about 50psi. One had the complete CE ring missing and the other had a section of about 30% remaining.

The damned thing didn't even demonstrate a misfire and the only giveaway was the oil leak.

On an old engine I would suspect some relaxation of the threads in the engine case.

My concern is that although this issue may be rare it does occur and is generally unexplained, particularly on new builds.

Reducing variables seems to be a sensible idea.

Chris,

Was this a rebuilt motor, a motor that had never been apart or?? Was/is the issue related to assembly?

All 4 nuts loose, top or bottom or both?

Stock studs with Dilivar on bottom row? Which ones had loose nuts?

What sort of thermal stresses had it been exposed to? I realize we never really get the true story from the client... :)

Which failure mode occurred first, the CE ring or the nuts coming loose?

Reducing the variables is important, but understanding what causes them is equally important.

I realize our sample size is small. We see these motors daily and have rebuilt around 40 and inspected another 60 or so. But, every time we see significant leakage between cylinder and case, cylinder and head or a damaged mating surface (either cylinder or head) there is a broken stud to go with it.

I am not saying it doesn't happen (because clearly it does), just saying we are yet to see it.

Would love to know/understand a method or parts selection which would help improve or eliminate the issue.

Cheers

The engine was a 1982 (UK Spec) 3.0 SC.

It had never been apart but had been fitted with a pair of 40IDA3C carbs about 5 years before we first looked at the car.

The car is driven regularly including TrackDays when it is driven exceptionally hard but is has regular oil changes, is never thrashed until it is warm etc, etc.

We had the car in our workshop for routine maintenance and to fit a lightened flywheel and a set of headers.

We carried out a compression test as a matter of routine and found 2 cylinders which were well down.

Once we had dropped the engine and removed the fan shroud the oil leaks at thee cylinder bases were obvious and had clearly been leaking for some time.

We suspected broken Dilavar Studs.

It was surprising to find that there weren't any broken studs just loose nuts.

The upper nuts were 'finger tight' on the two cylinders but the nuts on the Dilavar studs seemed tighter but I couldn't state a torque figure.

The remnant of the CE Ring seemed to show that it had been burnt by combustion gases but the head and its interface were not too bad , again surprising.

We did refurb the sealing face on all the heads but didn't change any studs (No budget available!)

We cleaned all the leaks re-fitted the cylinders and torqued it all up and away it went.

For the last 2 years it has performed as well as ever, its compressions have been checked twice and are consistent.

I think the engine needs an overhaul and we have it booked in this winter and will see if we can find any other issues.

I can't say for sure but I think that the rings failed due to the heads loosening but have no proof.

I don't like the difference on CoE between the upper and lower studs as this must allow some odd forces to be applied to the interface of the joint and not something that IMHO is a sound practice.