Dilavar Cautionary Tale

[jgparker]

I decided to do my valve adjust and head re-torque a little early on this rebuild (400 mile). Good thing I did.



You all can spare me the "I told you so"s, since I've already punished myself on this one. It just seemed like a good set of "burned-in" dilavar studs would have been safe, but I guess not. At least I got a nice new digital SLR today to take good pictures of carnage.

So, is there any reason not to go with the new 993 steel studs? (993.101.172.02). I'm convinced I can replace the studs without disturbing the cylinders (assuming no sealing surface damage). Is this reasonable?

Thanks,

JP
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[Doug E]

When I was researching studs for my rebuild I heard from one source that the 993 fully threaded studs shouldn't be used because the threads could/would come into contact with our older cylinders and that was a no-no. Then again, another shop swore by them so go figure

How are you gonna get the studs out w/o removing the C's, especially that broken one? From memory I don't think there is enough thread/stud exposed on top of the C's to get the stud removal tool properly seated but I could be wrong.

[Bobboloo]

Just so I understand correctly. By "burned-in" do you mean that the dilivar studs were used or were they new and burned in by the 400 miles on your rebuild? I've heard that once torqued that if you relieve the torque you can't re-torque them without risking breakage but maybe this is what you meant by the "I told you so" part of your post.

I'm just curious from a data point.

[jgparker]

Doug,

I've heard the 993 steal studs are no longer coming as all-thread, but are now smooth and coated like earlier Dilavar studs. The broken stud appears to have a wobble to it, so I'm convinced I can get it out easy. Guess I did not use enough red locktite. The remaining studs will protrude enough with the heads removed to get the Snap-on stud tool on them.

Bobby,

My Dilavar studs were replaced on the previous rebuild. I just figured that they had lasted 70K miles, so they were past any infant mortality failures. I've never heard that you can not re-torque Dilavar head studs, just that they should be avoided in general. I now fully agree with this idea....

Thanks,

JP

[Doug Zielke]

I'm not sure the idea of removing studs without pulling the cylinders is a good one. Even if you don't disturb the cyls, I think you're risking a leak from the base gasket area upon re-assembly. I'd take it right down and use fresh base gaskets.

[john walker's workshop]

plus, you have to clean up the threads, and blow out the holes with air. there will be lots of crusty loctite bits in there.

[Bobboloo]

Bobby,

My Dilavar studs were replaced on the previous rebuild. I just figured that they had lasted 70K miles, so they were past any infant mortality failures. I've never heard that you can not re-torque Dilavar head studs, just that they should be avoided in general. I now fully agree with this idea....

JP,

I understand your logic here but it's probably not a total coincidence that your stud chose to break upon re-assembly. First let me say that I'm not a metallurgist or even sure if that's how you spell it but I know that metals like aluminum and magnesium can only withstand so many stress cycles before they give up. (That's why racing on 30 year old mag wheels is considered somewhat risky.)

So I guess the question is. What are the studs made of? I don't know. I've heard steel before but I have a hard time accepting that because they are none magnetic and have a thermal expansion rate similar to the aluminum alloy that the cylinders are made of. My guess is that they're made of an alloy of some sort and if that alloy contains a metal like aluminum then maybe 70k miles of heat cycles that stress cycle the studs followed by a larger stress cycle like untorqueing and re-torqueing my do it in.

So what's my point? I don't really have one. I'm just trying to understand the ins and outs of old Porsche motors. It's a sickness I have.

I guess we should just accept the word of the "pros" when they say "Dilivar bad, Steel good." even though the "pros" 20 years ago said "Steel bad, Dilivar good."

[Fishcop]

JP (and others)

I take it you'd recommend I re-use my original steel studs on my rebuild?

Thanks

[cstreit]

The new 993 studs that you get from PCNA are not fully threaded. They look like the older style with threads only on the ends.

[Bobboloo]

JP (and others)

I take it you'd recommend I re-use my original steel studs on my rebuild?

Yes. Re-using steel is fine. Steel doesn't fatigue the way alloys do from stress cycles, that's why it's used in bridges and skyscrapers.

[911pcars]

Funny how steel can seemingly break, but Dilavar is a steel alloy and at approx. 170 Ksi, it's specs indicate it should be as strong as other high quality steel fasteners. It's not aluminum or some other non-ferrous metal. I suspect the early Dilavars died due to some combination of poor quality control during manufacturing, inadequate heat treatment and corrosion during service.

I'm going to have to guess that since Porsche still uses Dilavar for their head studs, it's okay - the current version anyway.

Are all SAE and metric grade fasteners the same quality? Should be, but there are low-quality examples being manufactured, imported, sold and used. Unfortunately, we can't tell the difference unless we know the manufacturer, we have access to a materials lab or an Apache helicopter blade falls off.

Sherwood

[jgparker]

I got the stud stub out OK. It took a very small pair of vice-grips and a lot of time. Here are the pics:





I'll post some pics when I pull the heads.

JP

[911pcars]

JP,
FWIW, the black, powder-coated Dilavars don't seem to break as often as the earlier silver or gold-plated ones. Sorry to hear it happened to your engine. BTW, I installed new black Dilavars on my rebuild (running since '98) so, knock on wood, so far so good.

Let us know how it's going.

Sherwood

[Steve@Rennsport]

FWIW, we have been using the 3.6 Dilavar studs in everything for the past 9 years without any failures.

These have been a mix of track, high-HP 930, and race engines. I'm very pleased about their ability to maintain even and consistent head torque over a very wide range of conditions.

[Bobboloo]

Thanks for the data point Steve. That's good to hear.

P.S. Your input is invaluable.

[jgparker]

I think I may have dodged a bullet on this one. There is no perceivable damage to the mating surfaces in the area of the broken head stud. Here are a couple more pics of the head and cylinder mating surfaces:





I plan to install 993 steel studs, replace the head gaskets, and put it all back together. It looks like I'm out the cost of the studs and a gasket set. Since I should have replaced the studs in the first place, I'm really only out the gasket set and my time.

Thanks,

JP

[911pcars]

What did Steve suggest a couple of posts ago?

Sherwood

[jgparker]

Thanks for the reality-check Steve. I know your right, that the new 993 Dilavar studs are much improved from the old studs, and will almost certainly be problem free. My choice to go with steel is this. 1) Wayne's recommendation is to use steel, and I got burnt last time I ignored his suggestion. 2) The steel studs are less expensive ($200 vs. $500 for a set of Dilavar) 3) Porsche uses steel rather than the Dilavar on 993 race/club models, and Porsche Motorsports recommends them for track use over Dilavar.

I'm not trying to be a smart aleck, this is just the decision I've made based on my situation and the information available to me.

Thanks for the help,

JP

[ChrisBennet]

Are the 964 studs Dilavar and the 993 ones steel?
-Chris

[jgparker]

The 964 started out using the 930 style dilavar stud (930.101.170.02), but then switched to the 993 style Diavar late (993.101.170.51). The 993 used Dilavar on most models (993.101.170.51), and steel on a few special models (993.101.172.02). This is what appears in the parts catalogs anyway.

JP