It is matter of luck and on 78K miles on my car I joined the club.

Being technical person, I read a lot (probably too much), also below if high res scan I made of breakage. It is no brainier - material had defect, looks like some air was trapped when stud was cast and internal rust developed over 20 years. Also one can see that metal has irregularity.

Yes, dilvar is exotic material that matches aluminum head thermal expansion ratio. Yes, it will hold for 24hours at red line at Lemans. No it won't hold 20 years of street use, just not made for that.

I'm not going in to debate what kind of studs are better (get's touchy )

High res scan of broken stud:






I'll post whole rebuild tale once I put car together and drive it for awhile.
Oleg.

 

Those first-generation Dilavars didn't have proper corrosion protection, hence the failures.

The last generation ones as used on the 993TT, 993RSR, & 993 GT-2 are FAR better and after several hundred engines & 18 years, I've have not experienced any failures. From long-term experience, these really work MUCH better than aftermarket studs at keeping the heads from loosening.

 

A question about studs:

Do you have to replace them if you remove the headers/cylinders? Are they a "only use once" item even on a low milage engine?

Thomas

 

Not at all,....unless you have those 1st generation Dilavars, head studs are usually reused over & over again.

 

In my case coating had pretty much nothing to do with breakage - stud had internal rust build up.

First Generation is very relative term. Here's brief history of Porsche using Dilavar studs in production that goes back to 1975. Paul Frere book is good reference,
also I'm quoting http://porsche.wikidot.com/forum/t-2...der-head-studs and current PET.


1975. Dilavar studs, first used in 930 Turbo Carrera engines.
1977. Dilavar studs, a non-magnetic steel alloy, found their way into 911S.
1980. The first improvement to dilavar studs was made for 1980 SCs. The stud changed in appearance, to an almost jewelry gold finish.
1981-2.The newer version was coated with a gloss-black paint-like substance obviously designed to withstand corrosion.
1996. TSB 9604. Another revision. Old p/n 930.101.170.02 replaced to 993.101.170.51
Current PET:
M64.20 (993 3.8RS) and M64.60 (993 TT) list 993.101.170.53 which is Dilavar.
Regular 993 lists steel studs 993.101.172.02.
All 964's list 993.101.170.51 Dilavar.

My head hurts



Thomas, according to TSB 9604 993.101.170.51 and up can be replaced individually, all pre 1996 dilavars must be replaced in groups

 

Ok, 1st generation...can you clarify? The engine is a stock 964 MY92. Do you mean 1st as in build date or studs from engines before the 964 model?

Thomas

 

Ist generation Dilavars were used from '78 to '94. The first engines to receive the latest version was the 993 RSR/GT-2 in 1995, then the 993 Twin-Turbo in '96.

These latest ones are fully-threaded with a black coating.

 

Ok, so in this case mys friend should replace them then since it's a MY92

Thanks,
Thomas

 

I would see what he has since some of them had mild steel ones (magnetic) and people on a VERY tight budget will use them.

Personally, I don't like the steel ones on these big-bore engines due to head distortion that causes head leaks between the upper & lower studs. The steel stud's expansion rates are too small for these alloy cylinders which causes the heads to slightly bend when hot.

 

So is 993.101.170.53 the correct part number for best option? Thanks.

 

I saw them yesterday and they are black. The engine has never been opened. The car is a Euro RS.

Thomas

 

Interesting, how are these made. I don't know much about the material but the picture is reminiscent of oxides that are common in cast materials. What I am unsure of is the rust. How can it rust if it is encapsulated in the material and there is no oxygen or water that can penetrate it. I can only assume the part cracked and at the inclusion which was a stress riser and the elements worked once exposed.

In the case of magnesium castings oxides are common and usually a result of improper screening of the material during the casting process. If these inclusions are part of the process I don't see how the design would make a difference more than changing the manufacturing process or screening the parts using radio-graphic, ultrasonic or magnetic particle testing. Then there would also need to be a criteria established to determine what an acceptable amount would be and how close to the surface it would be allowed.

Most likely the point of initial failure started in the area in red (specifically the dark small area in the center of the circle) the area in blue looks to be some minor scratch that allowed oil to permeate the material once the material began to fail this exposed the oxide to the elements resulting in rust. I don't think they had anything to do with the actual failure until the stud was weakened by the dark spot failing. This may be something that was inevitable before it ever was installed in the engine. More than likely the stud would not have broken if it wasn't for the small spot at 12 o'clock

 

I concur with Cobalt's metallurgical failure analysis. Great advise!

 

I took engine apart. Will do precise measurements after I take heads apart. From what it looks broken stud did not do any damage and even caused leak:

After all I think 20 years of metal fatigue will get dilavar's apart. Matter of time/luck when you'll get it. I think they should be looked as $1000 gaskets and replaced once in 20 years


4-6 side:




Exhaust side leak - I have it on both banks, so it was not cause by stud.
Here's how 1-3 side looks:






PS. I think taking valve covers off and checking for broken up studs should be mandatory for PPI, especially for people who do not DIY. If you let "wrench" do this job - I may guess few $K out of pocket

 

Hi friends!

So what studs do you advise to use on Carrera 3,2 1984? From 993 or 993 turbo with part number 993.101.170.53?