993 stud?
 

Is 993 fully-threaded head stud made of dilavar?
If so, is it more reliable than original dilavar of '73?

thanks.

I found black coated fully threaded headstuds holding my 3.0L together. I'm pretty sure they're 993 studs, as I haven't heard of any others being fully threaded. They are not magnetic, so I believe them to be dilavar. I'm going to reuse them.

There were no dilavar studs in 1973. They were all steel back then, and are fine to reuse. The turbos got dilavar studs when they came out, and the n/a 911's got dilavar studs on the exhaust side sometime in 1977.

Rob
1980sc

I thought the new 993 stud was steal, but I also heard the are not fully threaded anymore. What part number are we talking about, 993.101.172.02?

Beat me too it emcon. THose are steel 993 head studs. Next step up would be like raceware or ARP..

If they are steel, why are they non-magnetic?

Exactly!! They're dilavar I believe.

Since the dilivar studs are very notch sensitive, I doubt a fully threaded stud would be dilivar. As for them being non-magnetic, maybe they are a new alloy with some nickel. Are they black or shiny?

The 993 fully threaded studs I have are a shiny gold color, but completely coated with black epoxy or paint. I have a full set of 24 of these studs in both of my 3.0L's. They were installed into both engines by the previous owners.

Rob
1980sc

I was told by someone (I think Early S, but I am most likely very mistaken) that the fully threaded steel studs were prone to rust in the threads. Is this so? If not why do we not get hardened steel (M8.8) all thread rods and have them cut to size?

The current 993 steel studs (993.101.172.02) are no longer fully threaded. I don't think they ever were coated (but the dilvars were, IIRC)

Tom

I have never read that Dilavar was notch-sensitive. If they are, it wouldn't matter where the thread was, in the end or the middle. Otherwise, they would fail at one of the threaded ends. Please let me know.

Sherwood Lee
http://members.rennlist.org/911pcars

I'm thinking now that these black coated studs I have probably are Dilavar. Otherwise, why would they be coated? The old style steel studs have no issues with corrosion and snapping. Only the old Dilavars.

Since the new version of the 993 stud seems to no longer be coated, or even fully threaded, then perhaps they are steel now. I wonder if the non-coated style is magnetic?

Rob
1980sc

PS: You can see this stud I'm refering to on page 84 of Wayne's engine rebuild book. It's listed as the "993 full-threaded headstud."

I wish I kept my old fully threaded 993 studs, but here is a pic of the replacement studs from Porsche, they are coated, partially threaded and steel.
http://forums.pelicanparts.com/uploa...1063397549.jpg

How long ago was that Bill? I got new 993 steel studs from Porsche 2 months ago, and they were not coated.

From Chris Streit's discription his weren't coated either.

Tom

The block was a '97 euro 993, studs were replaced this past winter, that pic is the new studs and was taken in Mar.'03.

I guess notch sensitive was not the right wording. I just removed six broken dilivar studs plus the 6 unbroken ones. One of them had severe cracks and was probably days away from failing. Since this thread was started I've tried to find out what the problem is with them. I analyzed the material of one of the broken studs at work. I didn't write down the material composition. I used a portable nuclear analyzer so even if I had written them down they still would be a little off. I remember them to be about 14% nickel, 4% chrome, 2% moly, 1% vanadium, and the balance iron (this particular analyzer doesn't read carbon). I could find no other common (or uncommon) material that matches this composition. I'm not a metalurgist, but my guess is that the nickel was added to increase the thermal expansion (I don't have the table in front of me so I could be off) and there is not enough chrome to fiend off stress corrosion cracked (SCC). SCC is caused when you have a material that is highly stressed, like a stretched bolt, with significant heat and some type of corrisive which may just be humid air. There is obviously a lot more to this issue and I'm not finished researching it. I'll post back to this thread as I learn more.

That's *very* interesting stuff. Thank you for posting your findings.
I'm wondering though, with such a large percentage of Fe, why Dilavar studs are essentially non-magnetic?

According to Forbes Aird's, Racer's Encyclopedia of Metals, Fibers and Materials, Dilavar Ni13 is made up of:

13% nickel
5-6% manganese
3-4% chromium
+ dashes of other alloying metals.

Furthermore, "... exhibits a thermal coefficient of about 11.3 ppm - closely matching aluminum and magnesium. It has about the same density and elastic modulaus as other steels and, in the temper chosen by Porsche, it delivered a UTS up to 180ksi and a yield strength of at least 116ksi, with more than 12% elongation."

On paper, Dilavar looks to be a very tough material. Perhaps the problems with it in our engines has more to do with a lack of stringent manufacturing QC by Porsche's vendor(s) and/or by overlooked environmental issues (salt air, water corrosion, etc.), the reason why PAG tried various coatings to prevent/reduce breakage.

Let us know what you discover.

Regards,
Sherwood Lee
http://members.rennlist.org/911pcars

As for Dilavar being non-magnetic. Carbon steel is in the ferrite phase and has a body center cubic (BCC) crystal structure at room temperature. At a temperature of about 750C carbon steel converts to the austenite phase with a face centered cubic (FCC) structure which is non-magnetic. Nickel is an austenitic stabilizer in steel. At about 8% Nickel and above, it suppresses the austenitic transformation temperture in steel to room temperature. So at 13% nickel, Dilavar is non-magnetic.

I noticed when I was drilling out studs, that the shavings are magnetic. This leads me to believe that there is some ferrite phase left.