Reusing Head Studs?

[BMB]

hi. i have a 76 911s targa in many pieces. i am sending the cases out to be-- timeserted ? keencerted ? or helicoiled ?--- which one??? i have read the archives, but they leave me more undecided/confused than when i started reading. there seems to be lots of advice on what not to use, but little advice on what really works. any guidance would be appreciated(hand holding not required) . also, i removed the 24 headstuds so i can ship the cases to the machine shop. can these headstuds be reused if they look to be in good shape or is there fatigue and stress on studs after removal. do they break after reassembly or is that just an SC problem? thanks

[yelcab1]

With a magnesium case, the OLD preferred method has been Timeserted and Dilavar studs. This is NO LONGER the preferred one after much debate. The preferred method now is have the case drilled and installed "case savers" (not timeserts, not helicoils) and use either Raceware studs or factory studs.

The old studs COULD possibly be reused, but they are 26 years old, and pretty hard to tell what kind of fatigue they have faced. So, since it sounds like a big rebuild job, I WOULD get new head studs either from Raceware (Bulletproof) or the factory new style head studs with the threads almost all the way through (and with the loctite material already on them).

When I did my upper end job, I had several studs that were stretched (not broken) on a 84 911. I just replaced those with the factory ones right into the same holes. Never had any problems since.

Your job is different, it is a total rebuild top to bottom, and you are spending lots of money installing "case savers" so spend another $400 to get the 20 new studs.

[Superman]

Ask Wayne. He's been doing quite a bit of research, it seems to me. Here's my input. I am told, though some disagree, that factory steel studs very rarely fail. I reused mine. In fact, I replaced the Dilivar exhaust-side studs with factory steel. New ones, but I would have been just as happy with used ones.

For a magnesium-case engine, even with case savers or whatever, I wonder if new Dilivar studs might still be the right choice for the exhaust side. They take many years to corrode and break, if that's what they're going to do, and they have a thermal expansion rate very similar to aluminum, like your cylinders.

[tobluforu]

I used the stock studs on my 72. Over 45,000 hard mile and still going strong. God I hope I didn't just jinks myself.

[BMB]

anyone else?

[john walker's workshop]

i have no qualms about reusing steel studs. never seen one break. the new dealer all-thread studs are just the latest dilivar version. raceware studs are overkill for a street engine.

[Wayne 962]

You can reuse the steel studs - not the Dilavar. The steel ones are magnetic, Dilavar is not. Here is a section from the new book:

Case-Savers - Perhaps the most common upgrade/repair that you can perform on your case is the addition of case-savers. The soft magnesium material used to manufacture the early cases has a tendency to let the studs pull out of the case when the case is stressed. This is most prevalent with the cylinder head studs, although other studs in the case also have a tendency to pull out. As mentioned in Chapter One, if you have a case with pulled head studs, it basically requires a full rebuild. Aluminum cases are strong enough that they don’t suffer from the same head-stud pullout problem. However, the magnesium cases manufactured from 1969 through 1977 will encounter these problems at one time or another. Particularly as the case ages and is exposed to an increased quantity of temperature cycles, the magnesium will weaken and the studs will pull.

The most widely accepted solution for repairing pulled studs, is to insert what are known as Time-Certs into the case. These are threaded inserts that are larger than the diameter of the original studs, and are manufactured out of steel. The hole in the case is first tapped with a tap, and the insert is installed into the case using a special insert tool. The tool helps to expand and press the Time-Cert into the case so that it will not back out if you try to remove a stud from a Time-Certed hole in the case.

However, there is an inherent problem using Time-Certs with a magnesium case. The method in which the Time-Cert is actually installed can damage and warp the case. Since the Time-Cert creates an interference fit, it actually pushes out into the case to maintain its ‘grip.’ Since the magnesium material is weak, there is a tendency to deform the material around the case. This actually makes the case weaker, and increasingly prone to having the studs pull out. As shown in Figure X, you can see that a properly installed Time-Cert has been pulled out of the case.

Instead of using Time-Certs to repair your case, I recommend that you use what are known as case-savers. Competition Engineering uses these steel inserts that are specially manufactured for installation into 911 engine cases. The inserts are not an interference fit, and install into the case without damaging the material. They have a larger outer diameter than the Time-Certs, so they can even be installed on a case that has had Time-Certs pull out of the case.

The case-savers have a very coarse, thread pitch on their outer threads. As most engineering books will tell you, a fine thread will give you greater strength than a coarse thread. However, this is not the case with weaker materials like magnesium. The coarser threads actually maintain a stronger grip in the magnesium because there is more material to grab onto. A fine threaded case-saver would pull out very easily, whereas the coarse thread ones don’t. The opposite is true with aluminum, because the material is much stronger.

Every cylinder head stud should have the case-savers installed, along with the long stud that is close to the intermediate shaft on the inside of the case. In addition, it’s sometimes wise to have the studs that hold the engine to the transmission reinforced as well. If you really want to treat your case right, you can have all the small outer case studs removed, all of the holes replaced with helicoil inserts, and have brand new studs installed. It’s very difficult to pull a case-saver out of the magnesium – with the added strength, you should be able to put together a high-compression high-performance engine.

At the same time that you are checking all of the threads on your case, make sure that the cam oil line threads are checked as well. These sometimes have a habit of becoming galled or cross-threaded, and may need to have an insert or case-saver installed in their bore.

-Wayne

[Doug Zielke]

Originally posted by BMB
.......can these headstuds be reused if they look to be in good shape or is there fatigue and stress on studs after removal...

Why would you?
There is no way you can easily tell if they are fatigued. Furthermore, the OEM (non-Dilivar) steel head stud is one of the cheaper parts in a rebuild.
(You can probably sell your old ones to Superman. )

[Superman]

Indeed. But you see as a statistician I recognize that everything has a probability that should auger into our decisions. JW has been building these cars for about the past thirty years. I'm guess that blindfolded, he can build two engines in the time it takes us to bolt one together. He says he has never seen a factory stud break. So according to my calculator, spending $600 or $400 or even $250 to insure against an event that has nearly no chance of happening...well....I'd rather have new shocks and take my chances with the head studs.

[Wayne 962]

Never seen a factory *steel* stud break. There is an important distinction there. They are indeed rock-solid, and more than fine to reuse. However, if any of the threads are messed up, then I would buy new ones.

Of course, I'd love to sell everyone new studs, but in this case, it's unnecessary...

-Wayne

[Doug Zielke]

It's very reassuring to hear how tough the steel head studs are.
Makes one wonder why Porsche engineers ever bothered with Dilivar at all.