It's OK to check that the nuts aren't loose but you don't want to be retorquing them. (Please, someone correct me if I'm wrong.)

People get hung up on the accuracy of the torque wrench but what they really should be worrying about is duplicating the conditions for which the torque spec was specified. I've read that as much as 80% of the torque on a fastener is due to friction.

With that in mind, lets take the case of head stud nuts. When assembed the nut was antisiezed on a (hopefully) smooth washer. A year later that nut, stud and washer are rusty and have a totally different coefficient of friction.

-Chris

Steve: This is a very timely discussion for me. I put ARP studs in about 5 years ago. They still look great - perfect threads. I pulled two studs on No. 1. All others look fine, though I intend to timesert the whole case to be sure.

I see that you have had great success with the 993 Dilavars, so I certainly understand your commitment to them. If I didn't already have the expensive little ARPs, I'd probably jump at buying the 993 studs. But, I am wondering, though, what is your opinion based on that the ARPs "do not accurately match the expansion rates of these engines?" Would my pulled studs be an example? Have you seen other problems? I still haven't found anyone who will say that they've seen pulled studs on a 964. Have you? Could the ARP be my problem?

Thanks!

I you are certain that no tread damaged was caused during the removal of the original studs, ARPs torqued to 38 lb on a street engine would be my guess.
We have never seen studs pulling in an aluminum case unless tremendous horse power out puts were achieved.

Steve:

Like Henry, I've never seen a pulled stud on an aluminum-cased engine unless they were improperly installed. In the case of high HP Turbocharged race motors like 935's, other things happen first such as head lifting. :)

The only issues I've seen to date with ARP are the occasional loose head nuts but not a pulled stud. Either they were improperly installed or this is the first instance of pulled studs from expansion-related stresses.

I'm not one to speculate but it seems that if this was due to stresses, several of these would have pulled their threads out,......OTOH, the combination of too much torque and maybe a loose one might have been the culprit. At this juncture, any accurate forensics will be almost impossible to do.

If it were my engine, I would sell those ARP ones and install the 993TT Dilavars.

My .02…this certainly isn’t directed at anyone, I ‘compete’ with no one on this forum.

Using a torque wrench is just guesswork, it gives a pre determined number to shoot for but doesn’t come close (unless you just get lucky) to the tension elastic limit which will yield the fastener’s maximum inherent strength. There are computerized ‘torque wrenches’ that will signal you of this condition but are not accurate enough (despite their boasts) to be used for much other then a fail safe…i.e. bringing the fastener to a particular torque number and warning you of a possible fastener problem, burr, thread degradation etc. I use one of these for that purpose, to slowly bring a fastener towards the correct stretch without going over…expensive at well over $10,000 but mandatory in my opinion. For non-critical fasteners this wrench is enough, but I would never dream of stopping there for cylinder heads and other critical fastening. For that I use an extensometer, I personally use a BoltMike III, great tool, but also over $15,000. A few engine builds using one of these and you will quickly realize just how inexact a ‘torque wrench’ is.

Meticulous engine building isn’t cheap, that is for certain. It is science as much as art.

Why should you screw the studs all the way in and then back them out 2 turns?

Lube on studs
 

Hi all, I am somewhat confused here.
Tomorrow, I am going to rebuild the heads and cam boxes etc on to my std 3.0 Sc engine.
I have bought a full set of steel head studs....You mention not to use the lube on head nuts on your type of studs. Is this true of the steel studs, or should I lube the washers and threads as suggested in waynes book.

Thamks

The subject of expansion is regularly discussed and I would just like to rpeat the numbers:

Typical Aluminium Silicon Alloy: 20ppm/degC

Typical Cast Iron: 9.5ppm/degC

Typical Steel Stud (8740 or similar) : 12ppm/degC

Dilavar Stud: 18ppm/degC

ARP Inconel 718 Stud: 13ppm/degC

17-4PH Martensitic Stainless Steel: 11 ppm/degC

Clearly the preload in the studs will increase as the cylinder expands and I am not sure why this would cause the nut to loosen.

If the studs were to yield then there could be an issue but this is unlikely.

I come back to my argument that excessive preload in the studs will cause the Alusil to suffer from local stress relaxation which will cause this problem and reduce the clamping force.

Alusil alloy are known to exhibit this type of behaviour even at modest temperatures.

I think the message is to torque them carefully, glue them in well with Green Loctite.

If you are going to use either Inconel or 17-4PH I would use a thread lube.

Silver Goop made by Swagelock is good and prevents any galling during tightening and has good anti-sieze.

As Dilavar is an Austenitic Material it is also one of the stainless steel family and can have a tendancy to gall unless they have a surface treatment to prevent this.

Having never used Dilavar I don't know if the threads are coated.

To minimize imparting stress into the engine case.

If the engine is assembled with the studs bottomed out and If the stud for whatever reason ever gets hotter than the engine case, then the stud will potentially elongate (thermal expansion) and then push against the case, impart stresses into the engine case.

There are at least three states,
1. start-up transition (warm up)
2. steady state - run (normal operating temperature)
3. shut-down transition (cool down)

Yeah, I know it's over-simplified as what exactly constitutes steady state, I'm sure the surface temp for the cylinder and for the studs is all over the place, but, I believe that the loosening has to happen on a cool down, where the stud is hotter than the cylinder. What do you think?

If the stud does hold more heat than the cylinder, which I think is unlikely, then surely all types of stud would be affected regardless of their material. In this situation Dilavar would be the most likely to cause a problem as it will be longer than all of the other materials during the cool down transition.

The heating and cooling cycles do change the stress/strain conditions in both the studs and the cases and the focus of problems is always blamed on stud quality.

At the temperatures involved it is extremely unlikely that there are any creep mechansims operating in any of the stud/nut materials we are concerned with.

Inconel 718 and Dilavar (similar to A286) are materials that are commonly used at elevated temperatures and both have excellent stress/rupture characteristics.

17-4PH is not quite as resistant to creep as the the above materials but can still be used succesfully at around 900 degF.

Even the most basic stud material 8740 has more than adequate properties to show that creep is an unlikely mechanism at the temperatures being reached in a 911 motor.

Well it IS a complicated device. Who could blame you? All that turning and looking at numbers.. Makes the head spin. :rolleyes:

*cough, 5yr old thread revival... *cough

Metal expands when heated and fasteners hold best when drawing against one side of the thread. Imagination should be able to take it from here.

I think if you bottom the stud into the hole when they expand they will tend to push the threads out of the case and cause a serious failure. The really do need a clearance.:)

Old thread and for what it's worth I called ARP customer tech service and asked why they mention not useing loctite on their head studs when installing them in the case in the instructions if it might not be a permanent installation.

They said using loctite when doing a permanent installation or race motor installation is fine and good but if the studs are ever removed in the future using a stud remover tool that has the 3 rollers inside that clamp the threads as you turn it they could squeeze them too hard and damage them if the studs are glued in with loctite and require a lot of torque to remove. That could make them nonreusable.
That's the only reason.

If double nutting the threads to remove the studs gets them loose that shouldn't damage the threads.

I used red loctite when installing ARP head studs in my aluminum case. It gives me more confidence in them staying put.

MAP gas in the yellow bottle works good for heating the case around the stud to soften the loctite if you ever need to remove them.

In response to what JFairman found out from ARP customer tech service:

They said using loctite when doing a permanent installation or race motor installation is fine and good but if the studs are ever removed in the future using a stud remover tool that has the 3 rollers inside that clamp the threads as you turn it they could squeeze them too hard and damage them if the studs are glued in with loctite and require a lot of torque to remove. That could make them nonreusable.
That's the only reason.

There are tools for removing studs that don't damage the threads. I have a couple of them. I believe they are called "collet style" stud removers. The kind I have aren't available anymore but Snap on makes some.

Double nutting can also damage/distort the threads if you tighten them too much so you need to be careful.