Checking nuts on top of head studs: What is factory recomendation...??

[911 tweaks]

just curious what is the factory recommendation of when to check the snugnes a/o torque value of the nuts on the ends of the head studs ie when you have the valve covers off for what ever reason...
I am of the belief, "if it aint broke, leave it alone...within reason of course", thus, I am leary of applying torque to a nut that "MAY" be ok if left alone.

However, when you have the valve covers off, it is real tempting to go and check the headstud top nuts for being snug = to check if headstud is not broken, to check that head stud is not pulling from case a/o top nut is at proper torque...

Anyway, I know that it is a good idea on a fresh rebuild where the heqadstuds were worked on to re torque them 1x after about 1k miles and leave them alone there after unless of course there is something glaringly telling you to check them...

Anyone know what the factory says re checking the end nuts for proper torque = is this a check item every x# of miles...??
thx, bob

[Eagledriver]

The factory doesn't do it. I do, and have found some that have loosened up over the first year. I don't see how it can do any harm to check them.

-Andy

[KTL]

I think it's good to check the head stud nuts shortly after assembly/run-in. After that, leave them alone.

Reason I say that is because right after assembly & break-in period, the nuts still can turn freely on the studs w/out too much resistance. That's because they were installed with antiseize paste on the threads and nut/washer contact surface.

I think it can be asking for trouble checking tightness on aged studs. Certainly dilavar studs could break. Steel not likely. That said, when I did the top end on my '87 in 2007 the dilavars did not break upon loosening. All of the nuts came free with some considerable noise. But no breaks. Reinstalled the heads just fine and no broken studs 20K miles later knock on wood.

Rest of the engine work i've done has been complete overhauls so I can't really speak to how frequently the studs could break if tightened when aged for many years.

[911 tweaks]

thx for the replies...the subject engine is a 1977' 911S 2.7 w/ dil studs...I do not see anything of a suspicious nature, so I will leave well enough alone since they are dil studs...IF they were steel, I would put an socket in there to check...engine has never been apart & has 34k on it...

interesting that the factory did not publish anything for down the road on this subject...especially since it is such a hot topic still for debbate = head studs in general...

true if a new rebuild or rework that if they would mechanize smoothly I would test them whether dils or steel...

thx for the input...bob

[Walt Fricke]

Years ago a mechanic who owned a shop specializing in Porsches told me how to deal with the 2.7 and with original steel studs: When convenient, torque to the factory torque specification. If they hold torque, whether with or without moving, just keep enjoying the engine.

When one no longer holds torque, it is time to pull the motor, install Timecerts or the like, and move on.

That was, of course, to find a stud which was pulling out of the case, so that could be fixed before the lack of clamping pressure on the head could start to cause other problems. A sort of "if it ain't broke, etc." approach, but with respect to pulling the engine, not just crossing your fingers and hoping if (most might say when) one or more studs pulled, you'd catch that some other way and intervene soon enough.

How does a 1977 2.7 which has never been apart come to have Dilivar studs? I don't believe they came that way from the factory. My Euro 2.7 didn't have Dilivars. 34K miles is pretty low for a 35+year old motor. Is that figure from a rebuild? Which would explain the Dilivars, but still leave the question of whether the threads in the case were reinforced with inserts or not.

You ought to be able to see if the case has been inserted. If it hasn't, I don't think swapping in Dilivars is any guarantee that the studs aren't pulling.

I have trouble seeing how retorquing to the stock specification could cause something which is in good shape to break, though. The head studs have to resist the forces trying to lift the heads up. That would be peak combustion pressure. The torque's purpose is to stretch the studs enough so that they act like a spring whose seat pressure is higher than the combustion pressure. If torqueing to that value causes the stud either to break, or to pull, that's going to happen pretty soon from normal running, isn't it?

I don't know enough about materials failures to know if a stress riser (these seem to be the culprits with the Dilivars) on a cyclically axially stressed fastener is more likely to cause premature failure if also torqued (twisted radially) below the nominal failure value, or if it is just as likely to fail due to the axial stresses of normal running.

I think I see Kevin's point. Torquing a lubricated fastener should produce the needed stretch at the specified torque. If the lubrication disappears (and even more so, if the threads corrode or gall or otherwise have a much higher friction coefficient), more of that torque would be applying a twisting force than it would otherwise, and less would go toward stretching. I had an object lesson in that when installing aftermarket super steel rod bolts using the stretch method. When I got up to around 90 pounds/foot to get the needed stretch, I called the manufacturer. He told me this was because I hadn't properly lubricated the threads and nut to washer surfaces. But not to worry about the ones I had stretched, just that I might consider better lubing of those I still had to do. Of course, those were 220,000 psi steels or thereabouts. And none subsequently failed.

However, I am not sure that engines under normal operating conditions are going to have that large a change in friction. Normal rocker shaft leakage tends to mean an oily film in that whole area (hence the grunge we have to clean off when doing top end work), and might protect things. Is moisture going to get in there (it could, coming in with the cooling air blown over the head nuts) enough to cause trouble? This area isn't quite like the exhaust studs, which are notorious for having the nuts rust on, is it?

Anyway, I agree that checking torque at some point after a rebuild is a good idea, even if not necessarily required. Thermal cycles could cause things to settle in and reduce the stretch on a fastener or two. No one does this on the case 8mm studs (unless vainly trying to stop a leak of perhaps unknown origin), and you can't get to the case through bolts, which are subjected to significant cyclical tensions.

If this were my motor, and I'd never checked (or re-checked) the torque, I think I'd do it just for the peace of mind. But not routinely thereafter if the case has been inserted. Looking is good enough - when a head stud snaps you could tell by sticking a magnet down onto the nut and seeing if it pulls up if you didn't find something sticking out.

I've got a 2.7 race motor with Dilivar studs (some real, some the fake or not quite pure kind which are slightly more attracted to a magnet) that ran for 15 years with no head stud problems (in a sand cast case, so pulling not likely). Not subject to the harsher environment of the streets and highways and rain and snow and de-icers, though.

[911 tweaks]

thx walt for your many good points as always... :-)

I double checkd the studs w/ a magnet & they are "fairly" magnetic so I conclude dils...

As for putting a wrench on the top nuts, I am not going to do such as much as I would like to know if they are ok as the barrel nuts are fairly rusty & I forsee more twisting of the stud vs being able to check tightness/torque of the nut on the headstud...oh well...

there are no glaring issues ie leaking base(s) of cyls, no barrel nuts in the valve covers when removed, no strange engine noises, ect so I will leave well enough alone and NOT "tweeks" it on this particular item...:-)

thx for the replies guys...interesting that the factory never said at lets say 50k miles, to retorque the nuts or apply a penetrating type of material to allow the nut to free up & be able to spin w/o twisting the headstud to thus be able to retorque the nuts to factory spec...I guess it is a set & forget in the factory engrs minds unless one has obvious reason to investigate further...fyi, i did do a valve adj & all rocker shafts were fine & in proper position & no oil leaks on the inside of the valve covers either...

maybe this will be a winter project to pull the motor and investigate further what studs for sure it has & if the nuts are tight a/o apply liquid wrench or kriol or the like to free up the nuts to retorque them...but, I keep coming back to the adage, "if it aint broke...sorry for the horrible english...dont fix it"...

thx again & carry on guys...bob

[chris_seven]

Originally Posted by Walt Fricke

I don't know enough about materials failures to know if a stress riser (these seem to be the culprits with the Dilivars) on a cyclically axially stressed fastener is more likely to cause premature failure if also torqued (twisted radially) below the nominal failure value, or if it is just as likely to fail due to the axial stresses of normal running.

It is true to say that torqueing a bolt will introduce a combined tension and torsional stress and the effects of this can be calculated using a Von Mises Equivalent Stress method but as the axial force can only be guessed the result is unlikely to be very accurate without significant testing.

Typically, however, if a bolt were tightened to failure (assuming threads didn't strip) the load would be around 85% of that measured if the bolt were simply tensile tested.

The torsional stress developed is normally referred to as a 'parasitic' stress and is usually only developed during the tightening procedure and dissipates quickly once the tightening operation is completed.

It is possible that if thread quality or lubrication of the thread and under the nut could result is some residual torque which could relax due to vibration and thermal cycling and may cause the nut to loosen which would clearly be an issue.

If the nuts are tightened correctly then torsional stresses should be very low and are very unlikely to affect fatigue life beyond the effects of any cyclic axial loading.

As the preload in the stud needs to be higher than any of the combustion generated forces then fatigue shouldn't be an issue.

I am convinced that the majority of Dilavar failures are corrosion related and there is some evidence from Power Plant studies about Intergranular Stress Corrosion Cracking with this basic family of alloys.

Dilavar is very similar to A286 in basic chemistry although it is higher in Mo and should be less susceptible to Hydrogen embrittlement. Here is a brief extract from a recent paper:

A286, an austenitic, precipitation hardened, stainless steel is strengthened by(Ti,Al), formed during aging at 1330°F (720°C). Its use is favoured where the expansion coefficient relative to other austenitic stainless steels is an important design factor.

Unfortunately, it is susceptible to IGSCC in cooling water when loaded at or above the room temperature yield stress, typically 100 ksi (700 MPa).
5-10 Cold work prior to aging in combination with the lower of two commonly used solution annealing temperatures of 900 and 980°C has a particularly adverse effect on IGSCC resistance.

Hot heading of bolts, which can create a heat-affected zone between the head and shank, is another known adverse factor. Nevertheless, even if these metallurgical factors are optimized, immunity from cracking cannot be assured unless the stresses are maintained below the room temperature yield stress, which necessitates strictly controlled bolt-loading procedures.

There is also strong circumstantial evidence that superimposed fatigue stresses can lower the mean threshold stress for IGSCC even further.

Once initiated, cracks grow relatively easily even in well-controlled primary cooling water.

[911 tweaks]

thx chris for that very informative explanation & info...

as for my concern, I feel that there is enough rust on the barrel nuts & I am sure on the head stud threads underneath the nuts, that if I give em' a torque, I will probably be doing more harm than good to headstuds that are fully operational at the moment...so, I am going to let sleeping dogs lay as the expression goes...

maybe this winter if the motor is out for something else, I will give em' a more proper inspection and possible torque...

[tom1394racing]

Bob

Dilivar studs are non-magnetic. On a stock 2.7, you should have magnetic steel studs.

[911 tweaks]

thx tom...thats correct...why i am getting a little bit of magnetism when placing a magnet on makes one a bit curious...either my eyes a/o hands are going bad or maybe...just maybe, this motor has been opened and they are steel...
they definatley are not 100% steel as i put the magnet on a steel stud and it was like white on rice and not so w/ the studs on the engine...
something else to make not of to check out later...thx

[Walt Fricke]

Tweeks - has the case been inserted for the head studs? You can easily enough check that by looking, though you might need to pry one of the air diverters up a bit.

[chris_seven]

Originally Posted by 911 tweaks

thx tom...thats correct...why i am getting a little bit of magnetism when placing a magnet on makes one a bit curious...either my eyes a/o hands are going bad or maybe...just maybe, this motor has been opened and they are steel...
they definatley are not 100% steel as i put the magnet on a steel stud and it was like white on rice and not so w/ the studs on the engine...
something else to make not of to check out later...thx

Dilavar is basically a precipitation hardening Austenitic Stainless Steel which has been heat treated and then threaded on the ends.

A fully Austenitic material should, in theory, not exhibit any ferro-magnetism at all and should not display any coercivity in the presence of a magnet.

I practice, however, there are a couple of mechanisms that can result in small amounts of residual magnetism being produced.

The first is that in just the same way that Ferritic Steels can retain Austenite after heat treatment, Austenitic alloys can retain Ferrite.

The measurement of residual magnetism is often used as a Quality Control measurement for some grades of Stainless Steel.

Cold Working of Austenitic Stainless Steels can also cause a small level of magnetism to be produced and the thread rolling operation could cause this to occur.

If ferro-magnetic behaviour was a fundamental requirement clearly the Dilavar studs would not be satisfactory but small amounts of retained ferrite and residual cold work are, IMHO, unlikely to affect the stud's basic performance.

It may be interesting to try to carry out a small poll to see if there is much subjective variation it the residual magnetism of Dilavar studs and to see if this could correlate to mechanical performance. I would be surprised if this were the case.

If the studs were fully magnetic then it would be clear that the material was not Dilavar.

[911 tweaks]

car will be back here in the next week or 3 & i will check further to let us all know more then...i asked for the car to return w/in such time so i could recheck the valve covers & other areas to hopefully not be leaking oil...stand by for a little while guys...thx

[tom1394racing]

Chris and Bob

FWIW......In my experience, I have never seen a Dilivar stud attract a magnet. I use the magnet as the test to see if a stud is Dilivar or steel.

[Eagledriver]

I think I read (in Anderson's book) that the 77 engines had dilivar studs. This was after they found out about the stud pulling problem and had started using dilivar on the Turbo's. I don't know if it was the bottom row only like the later cars or both rows. If it were my engine I'd leave the studs alone until I had symptoms.

-Andy