Fully threaded head studs - what are they?
 

In the process of tearing my engine down. I have the heads off, but not the cylinders yet. Best I can tell, the whole stud is threaded? I wasn't expecting that. Any idea what iteration of head studs I might have? I'm pretty sure they were replaced at some point in the engine's life.

Todd

I believe that would be 993 Dilivar. Which is a good thing.

yup, off the 993 models.

So, definitely reusable? I was hoping to have the case tumbled to clean it up. Maybe just hot tank it then?

Todd

Actually 993 twin turbo. Regular 993 are all 24 steel studs.

Steve Weiner is a big proponent of these 993TT studs for performance usage. They're the latest and greatest iteration of Diilvar studs and that's supposed to be a good thing.

These are the good ones, used on 993RS and 993tt
http://forums.pelicanparts.com/uploa...1362088663.jpg

Looking through some paperwork that came with the car. Looks like these used to be part number 93010117002. Superseded with part number in the pic above.

Todd

Nope, the .02 studs are also fully threaded but were used on base 993

Ok I see where that comes from, originally the base 993 used the .02. They eventually issued a TSB to only use the .51 but that was for all air cooled 911 rebuilds
http://i1087.photobucket.com/albums/...Bcasestuds.jpg

Still good, or recommendation would be to replace them?

Todd

My $0.02, replace 'em. Use a good aftermarket stud (Raceware, Supertec, etc.). The ".52"'s are outrageously expensive.
I put the .52's in my rebuild (2002) before the prices went through the roof. No problems with them ever.

Actually the Dilivar stud that is 930.101.170.02 was the smooth style only threaded at the ends. The 911 turbo engines have always had all 24 studs in dilivar flavor. That 930 part number is currently superseded by the 993 all-thread ending in .53

The all-thread appears to have arrived with the 965 3.6 turbo engine around 1993-1994. They used 12 of the regular 930 dilivars and 12 of the all thread (ending in .51).

A noteworthy aside here is that the 964 engines (even the 965 3.3 turbo) used 930 dilivar studs for all 24 studs.

Then the 993 comes along and the regular 993 gets steel studs p/n 993.101.171.02 (now superseded to .03 which is also steel). I have these 993 steel studs on my "race" engine, which I bought from Pelican last year.

Then the 993RS and 993TT come along and they get all-thread dilivars for all 24 studs.

Weird how the dilivar usage hops around on these engines, isn't it?

We might be interested to know what the 996TT uses, since that engine is based on an air cooled case. It uses 24 studs that are p/n 996.101.172.70, which coincidentally are the same studs the GT3 uses. They're fairly cheap @ only $17 each. So they must be steel? I suspect the water cooling negated the need to install the dilivar studs.

Damn - cheapest I can find them is $38/ea. $900 for 24...doh! Pelican Stock

Makes the Supertec studs a bonafide bargain, doesn't it? :D

I bought the Supetec head studs. They are art.

Definitely confusing. The paperwork I have says 93010117002 - Dilivar Head Studs X 24 ($599). But, they are definitely fully threaded. I don't have a date, but based on other paperwork with the car I would guess early 90's perhaps, but that could precede the 993. A mystery. I'll leave them in and let the engine builder decide what to do with them. I had thought about having the case media tumbled, but I guess I'll just have it hot tanked.

Todd

The subject of head studs has been debated in this forum for many years and prices just seem to keep on increasing.

Standard steel studs are around $400 a set with nuts and washers.

Good Quality aftermarket studs seem to average at around $650 a set

The latest 993 Dilavar parts are $1500 without nuts and washers.

All of this for a stud with a torque of 24-25 lbsft.

Grade 8.8 bolts can be safely torqued to 34 lbsft so I wonder why we need to use ultra-high strength super alloys that were developed for the hot sections of Gas Turbines when simple bolts will probably do the job.

4 feet of a Grade 8.8 All thread has a cost of about $4 and would make 6 studs so 24 studs would be less that $20.

Seems quite good value.

Grade 10.9 all thread is about twice the price an even so cost would be less than $50 per engine.

Seems like quite a good saving :)

Hmmm, that is an interesting thought..

They will expand too much, and the torque will be lost. If you torque them higher the barrels will be distorted when the engine is cold. So now you see the problem. Also the material for the 993TT studs is not Dilavar.

There is no reason why a Grade 8.8 Steel would expand any more than the standard steel stud. The Coefficient of Expansion of most engineering steels are within 1-2% of each other and this variation is negligible.

Dilavar expands around 50% more than ferritic/martensitic steels and and they don't lose preload - at least until they break.

The description used by our Host for 993 studs is as follows:

993 - fully threaded, DILAVAR, 911/911 Turbo (1965-98)

I was trying to have a discussion on why we all believe the more exotic the material the 'better' the stud when in fact the application is not particularly demanding in terms of metallurgy.

My old 500cc (Single) Velocette Venom Thruxton used to produce around 45 BHP which is equivalent to 270BHP from a 3.0 litre and it had 4 really cheap studs that were nothing special.

Manx Nortoms produced around 55BHP in 1963 (330 BHP equivalent) and again used 4 x simple studs.

Today with all of the modern advances we seem to need $1000 of high end Precipitation Hardening Superalloys to do the same job that was done in the Sixties with materials that were not much better than mild steel.

I guess this is what we call progress. :D

Do you mean #993.101.170.53 and if so, what material are they?

Head Stud Mfg.
 

Chris
When will your first batch be ready?

Mike Curnow

hummmm wonder if the threaded part allows expansion to match the cylinder expansion when heated or the threads allow stud expansion without breaking?

Mike, I am not sure that I am brave enough to take this step just yet :D

I don't think that the overall forces will change once the engine has reached a stable operating temperature as the threads won't affect the expansion of the stud once it has reached this equilibrium temperature.

I can imagine that there could be a peak level of clamping force due the temperature difference between studs and cylinders whilst the engine is warming.

It is interesting to consider that the increase in surface area created by the threads could influence the time for this equilibrium to be reached and reduce the force created by this difference.

This would be a very subtle change and Dilavar is likley to have a much lower level of thermal conductivity than a conventional steel stud so warm up time to reach equilibrium would be longer.

The other Stainless Steel and Nickel Alloy Studs - Casper Labs, Supertec and ARP, would also have lower conductivity and are not fully threaed.

There is a theory that very closely pitched and fine concentric grooves in the parallel portion of a test bar can increase the yield strain measured during a tensile test.

Post yield these grooves have no impact on either elongation at failure or Ultimate Tensile Strength.

Although threads do resemble concentric grooves a 1.5mm pitch is much too coarse to create this effect.

If, as I believe, Dilavar studs normally fail due to SCC resulting from interganular corrosion then increasing surface area could have a negative influence as more grain boundary will be exposed to the environment but I think the surface treatment is designed to eliminate this type of failure.

The basic idea is certainly worth more thought. :)