3.2 Head Stud Project Thread

[tmaull]

Starting a thread to document my progress as I go about fixing my 911. Could surely use some advice, and if nothing else I can record it for myself.
In yesterday’s thread (Top End Advice Needed - What is this thing?) I learned during a valve adjustment that one of the head studs in the #2 cyl intake was broken.
I also wasn't able to complete the valve adjustment because the fan belt is too loose, and the wrong number of washers are on it, so it won't tighten up enough. Pretty bad day. One interesting thing is that it’s the intake stud (the steel one) not the dilivar one that broke.

My plan for the weekend is to button the car back up with the reusable gaskets, and just park it until I know what to do next. I don’t have a garage or paved driveway yet, but I’d really like to rebuild the engine myself, but the garage is probably 6 months away. It looks like I can either wait until then, and then get to it, or I can pay to have it done. Bruce (flatsixpac) has already offered, and I’ll admit it’s tempting. I’m ordering Wayne’s book today, so at least I’m an informed customer. I don’t want to drive the car in the meantime, as driving a 911 “easily” just seems wrong to me, and I tend to push pretty hard. Any advice going forward? What would you do?

[darrin]

re your fan belt comment -- My understanding is that the more washers you have between the pulley halves, the LOOSER the fan belt gets -- put another way, if you had too few washers, your fan belt should be too tight. I recall reading here that there are some fan belts being sold as compatible with our cars that are slightly too big (and would therefore be slightly too loose) and wouldn't get completely tight regardless of washer placement. This might be your fan belt issue? Do you have a spare in the porsche toolkit in your frunk? If so, you might want to swap out

[tmaull]

No spare pulley unfortunately. The car only had four of the five washers. They are supposed to stack inside and outside the pulley to affect where on the pulley the belt rides. The car had three inside the pulley and one outside. Without two on the outside (five total) the bolt can't really be tightened down all the way.

[Flat6pac]

I have extras.
Bruce

[Jcslocum]

THere are quite a few top end rebuild threads on here already for you to read up on.

1st drop & top end observations & questions

Home 3.2 top end rebuild: Costs, Thoughts & Lessons Learned

Those are 2 of the later ones.

I would drive the car as is until you do the rebuild. There is really not that much that can go wrong or be damaged.

[Jcslocum]

Originally Posted by tmaull

No spare pulley unfortunately. The car only had four of the five washers. They are supposed to stack inside and outside the pulley to affect where on the pulley the belt rides. The car had three inside the pulley and one outside. Without two on the outside (five total) the bolt can't really be tightened down all the way.

It takes some turning of the nut to squeeze and stretch the belt. So swap out one washer and as you tighten the nut allow the engine to turn and then when the nut will not tighten any more, use your hand as a brake on the belt to allow the nut to tighten some more. The belt needs to ride up in the pulley to reach full tension.

[Dick Shift]

FYI: In 30+ years of repairing Porsche's I have never seen a broken STEEL head stud. Even very rusty one's. $10 bucks say's your broken stud is Dilivar.
In my opinion you can safely drive [easy] with one or few studs that aren't side by side if need-be, while saving money for repairs. Bottom line if you heir an exhaust leak sounding noise when accelerating especially up hill, the leak has progressed to the point that cly. head damage will now be a factor.
I also don't feel it necessary to replace all the head studs. If money is a issue just replace the Dilivar, non magnetic, head studs. I use regular [cheapest] STEEL head studs. Good used STEAL head studs when available to save money still get my life time "NO BRAKE" warranty. Yes, life time!

Once the alt belt is tight but not too tight. It is important to have enough shims between the outer pulley and the retaining nut washer to keep the nut from bottoming out on the fan before applying force on the outer pulley half. Generally 3-4 washers should between the outer pulley half and retaining washer.

[pmax]

Isn't it possible the intake stud was replaced with a Dilavar one at some point ?

[cabmandone]

I just did my head studs this spring. The job isn't bad as long as you don't break off a thread chaser in the block like I did. Then things get ugly. I'd get Wayne's book it's great for referencing things when you start tear down and then assembly.

[KTL]

Originally Posted by pmax

Isn't it possible the intake stud was replaced with a Dilavar one at some point ?

Yep. Awhile back when head stud breakage was starting to happen, some chose to do all 24 studs with new Dilavar. I found all Dilavar on my racecar engine. So it sort of made sense that the whole engine was weepy.....

[john walker's workshop]

Put a magnet on that broken stud. I also have never seen a broken steel stud.

[Flat6pac]

That is where I was going on the other posting asking if the broken black stud was magnetic.
Bruce

[wndsnd]

So is it best practice to re-use steel head studs on rebuilds, or replace them anyway with new steel or upgraded studs?

John

[cabmandone]

Originally Posted by wndsnd

So is it best practice to re-use steel head studs on rebuilds, or replace them anyway with new steel or upgraded studs?
John

When I did mine, several guys told me to just buy the lower 12 and be done. I went with all new for peace of mind. I liked that the new didn't use the barrel nuts.

[theiceman]

i am in the middle of doing mine ... i had 4 broken in total , two were broken and two broke while undoing the head stud nuts .. all 4 were lower Dilivars.. and they were a nightmare to get out . needed tons of heat and a collet. i was releaved when i got 12 out. I am not touching the upper 12 steel ones.

anyone report of steel studs pulling out of aluminum case ? i am assuming most of the pulling studs issue was fixed with the aluminum case.

Looks like Porsche did a two pronged attack back in the day , went to aluminum case and the dilivar, but all they really needed to do was go to the aluminum case and stay with steel studs , and everything would have been fine.

[Reiver]

Chris_Seven's comments in the build thread on studs...

I have contributed to many of these discussions over the years and will make an attempt to summarise the issues that need to be considered in order to make a sound judgement.

The first issue is that it is never wise to consider a stud in splendid isolation - the joint design is really the core issue and the stud should be designed/selected to suit the clamping force needed to maintain the integrity of the joint for long periods.

If we consider the cylinder head joint and its interaction with the studs it is easy to realise that the stud generally behaves as a spring and in order to resist peak cylinder forces must be capable of developing a sufficiently high preload.

It is also clear that the stud is much more elastic then the cylinder head and barrel.

The quality and 'fit' of the thread of the stud and associated nut/washer will affect the correlation between torque and preload and the characteristics of these components and the manner in which threads are lubricated all have an impact on the integrity of the cylinder head joint.

The design of the 911 cylinder and head is, however, relatively forgiving compared to heads on water cooled engines.

Once the head is torqued down the ability of the stud/nut to maintain preload preferably without the need to re-torque is also quite important.

The preload requirement of the stud is relatively straightforward to determine and clearly all of the studs available for use with a 911 engine will meet this basic need. The basic tensile strength of all of the studs available is well within the design requirements of the 911 engine so buying ultra high strength studs such as H11 Tool Steels (260ksi) is of no merit.

One we run the engine and expansion occurs the situation becomes more complex.

The use on Nikasil cylinders obviously increased the expansion that occurs as the engine warms up.

The difference in the coefficient of expansion (CoE) between the materials used in the components of the joint and the studs has a direct influence on the force applied to the thread in the engine case.

In very simple terms the greater the difference the greater the increase in the applied force.

As most 'ferritic' steels have very similar CoE then the choice of stud using these types of material has little influence on reliability.

High Strength steels, Martensitic Stainless Steels and even Titanium Alloys are all very similar in this respect and if CoE is the only consideration there is little to chose.

The Nimonic family of alloys does have a slightly increased CoE so will reduce force due to expansion by around 5% but this is not very significant.

Dilavar, which is clearly, an Austenitic Stainless Steel, has a CoE which is 50% greater than ferritic alloys and this does have a significant impact on the 'pull out' forces produced by expansion of the joint.

There are, however, two other characteristics that need to be considered.

The first is Young's Modulus (E) of the stud material. The E value of the stud material will affect the force applied to the screw threads as the 'stiffer' the material the greater the force that will be required.

With the exception of Titanium all of the materials used for studs will have an E value that will not vary by much more than 2% so we can ignore this effect.

The modulus of an Alpha-Beta Titanium Alloy is about 50% lower than Steel so force increase due to expansion will be greatly reduced by the use of this material.

The next consideration is a diameter of the shank of the stud. The smaller the diameter of the shank the lower the force produced by expansion.

In high temperature design it is custom and practice to reduce the shank of a bolt to attempt to control these forces.

The standard steel head stud has a shank diameter of approximately 7.6mm.

Some of the aftermarket studs have diameters of up to 9.5mm and the increase in pull out force will be directly proportional to the difference in the square of these diameters.

I would always try to select a stud with a relatively low increase in pull out force as this will give the thread in the case the best chances of long term survival although SC cases do seem to be totally immune to this type of problem.

There is one other engineering detail that should be considered and that is the use of steel studs mixed with Dilavar studs on a single head joint.

I find the concept of using materials with a significantly different CoE on either side of a single joint to be 'bizarre'.

To suggest that there is a 50% difference in the temperature of the studs on either side of the head so it balances the forces seems a bit optimistic and is something I would consider risky at best.

To justify this on the basis that if Porsche used this approach it must be correct and then to say Dilavar is a poor choice of material is a bit contradictory. I would never advise mixing materials in this manner as the uneven load distribution will inevitably cause must result in problems.

There are also some other metallurgical considerations that should be considered.

It is quite clear that Dilavar suffers from chloride induced Stress Corrosion Cracking (SCC) and for engines used on roads that are heavily salted is not sensible.

I am sure that this is the reason that later Dilavar studs are resin coated.

I would always select studs manufactured from a material that is not sensitive to SCC.

For example ARP advise against the use of L19 in this type of environment. H11 Tool Steel is also poor in this respect.

Inconel 718 does not suffer from this type of problem and is commonly used for the manufacture of high strength fasteners in marine environments.

Some of the Grade 5 Titanium Alloys can suffer from SCC in salt water environments but there is an ELI version that eliminates this specific issue and is used where there are concerns.

Precipitation Hardened Martensitic Stainless Steels can also exhibit sensitivity to SCC in a H960 and H1150 tempers but in HH150 form would not be a cause for concern.

There was some evidence that Dilavar also suffered from Hydrogen Embrittlement (HE) but I find this unlikely due to the high Molybdenum content of this material but there is a rumour that a large number of spurious Dilavar studs did get into the supply chain and this could offer an explanation for this observation.

My overall conclusion is that for SC and older sand cast cases the standard Steel stud is quite fit for purpose and provides a good solution at a good price.

For magnesium cases I would consider using Dilavar as I would like to use studs with a reduced pull out force due to expansion.

We have recently been manufacturing Titanium Studs for this application using a 6AL4VELI alloy which has been aged to 170ksi tensile strength. We have fitted around 10 engines with these studs but so far only have around 8 months of use. They are much more cost effective than Dilavar.

There was also a suggestion made above concerning 'creep' of head studs and I would like to make a comment.

It is vey unlikely that there is a sufficiently high temperature present to activate any of the damaging creep mechanisms that could occur during the life of an engine - say 5000 hours ate running temperature.

There has recently been some work being carried out to determine the effect of room temperature creep on high strength steels but from what I have seen so far I am not very persuaded that the work is very well established.

I would, however, be more than happy to accept that Magnesium Engine cases form 2.7 litre engines used with Thermal Reactors could well have suffered from Stress Relaxation and have lost up to 50% of their strength in the threaded area surrounding the cylinder head studs and again this would seem to justify the use of studs which create lower pull out forces.

[tmaull]

Thanks everyone for the comments. I met yesterday with Bruce Abbott, aka Flat6pac, who is going to rebuild the engine using the steel head studs. I also ordered and received Wayne’s book on rebuilding. I’m a little bummed I’m not going to be able to rebuild the engine myself, but I don’t have a garage or a flat driveway until late spring, and I don’t think my girlfriend would appreciate me rebuilding it in the dining room. So that will have to wait. Now I have the while you’re in there disease – this may have come from reading Wayne’s book. I've only had it for two days so I haven't finished it yet. Currently, the engine in there is a Euro 3.2, with 10.3 compression. It was rebuilt in 1992, so it probably has the Mahle pistons and cylinders. The upgrade I am considering is going to a 9.5 compression 3.4. I would use JE pistons and use cylinders from a Nikasil car as core. Bruce suggested I could probably sell my Mahle pistons and cylinders for $1600ish, which should cover a good bit of the 3.4 cylinders and pistons. Is this a good road to go down? How much more power/torque can I expect (after a Steve Wong chip)? Right now the exhaust is stock, but that could also change.
Thanks,
-Tom

[KTL]

I know of guys who have gone the 3.2 ---> 3.4 route and say the bang for the buck was pretty low. One guy who comes to mind is member 89911 here on the forums. Both he and his brother track their cars and I recall him saying that his built 3.4 twin plug doesn't feel all that much stronger than his brother's 3.2 Euro 10.3:1 single plug build, and if he had it to do all over again he would go the 3.2 Euro route. So, since you've already got the higher compression Euro 3.2 pistons, I don't think you'll see a big gain. Yes, there's "no substitute for displacement" but the gain is going to be somewhat small.


The stud thing is weird. Porsche has been all over the map with the stud material on these engines. 930 had all dilavar, 911 had a combo of steel and dilavar, 964 comes along and uses all dilavar, then the 993 uses all steel, then the 993 twin turbo comes along and uses fully threaded all dilavar. I did some crude research on this a few years ago Broken headstuds. Inevitable?

The dilavar discussion can get pretty crazy sometimes 993 Head studs, two types?

[tmaull]

I've found a ton of good info in this thread: 1984 911 3.2 - The beginning of the end

[larrym]

check this -
Advice on Head Studs for 930