Here's the explanation, in Mezgers' words, in 1972 (I don't know if he maintains this opinion today...)

The development of the Porsche type 917 car
Hans Mezger, Dipl-Ing
Proc Insts Mech Engrs 1972, Vol 186, 2/72
pp 17-18

"In general it could be stated that for the 917 racing car, steel is used only if and where titanium and other light alloys are not or less suitable.

A special problem had to be solved in connection with the vertically divided crankcase and the mounting of the cylinder heads. The two-bolt connections are shown in Fig. 10.

Magnesium and aluminium have a mean coefficient of thermal expansion of approximately 22E-6 to 24E-6 per degree centigrade.

The corresponding value for titanium is 8.2E-6 per degree centigrade. The combination of a titanium bolt and a magnesium or aluminium part causes the bolt to expand only about one third of the expansion of the corresponding part. The 'cold' initial stress is thus enhanced by an additional expansion differential which will either induce a bolt rupture or a distortion of the magnesium or aluminium parts which in turn entails an undesirable reduction of the initial stress.

http://forums.pelicanparts.com/uploa...1265230693.jpg
(C) IME 1972

If normal steel bolts of a thermal expansion coefficient of 11.5E-6 per degree centigrade which is about half of that of magnesium or aluminium-are used, the difficulties are essentially the same. The solution of the crankcase and cylinder head bolt problem was found to be Dilavar. This is a special steel alloy having a thermal expansion coefficient of approximately 20E-6 per degree centigrade. When the engine is heated, Dilavar bolts will thus expand almost at the same rate as magnesium and aluminium. In addition the cylinder head bolts are insulated by fibreglass, otherwise the cooling air flow would cause the temperature level of the bolt to fall far below that of the cylinder."

So now we need Dilivar case bolts. Please.......
The theory is well documented and beaten to death.
My claim, weather you choose to believe it or not , is that the difference in the expansion coefficient at the temperatures achieved by Porsche engines makes no practical difference.

The number for aluminum 23 is pure aluminum not silicon/aluminum alloys as used in all Porsche cylinders. As you know, there are many different alloys used in cylinders. I've seen numbers as high as 17 for stainless steel and numbers as low as 17 for high silicon/ aluminum alloys. 17=17 just a thought. So what is the expansion coefficient for Porsche cylinders?
And once again why would a slightly higher clamping pressure on a cylinder head when the engine is warm be detrimental?
If the EC of high silicon /aluminum cylinder was actually lower than Dilivar (possible?) wouldn't that be BAD ?
I would agree that it is difficult to ascertain the one right answer. I do know that no one who has ever used my product has ever complained to me about that product. Can the same be said for the Dilivar head stud?

I just got off the phone with a customer who rebuilt his engine with Dilivar and let it sit. Some time later he came back to his engine (it never ran mind you) only to find a broken stud. New Dilivar stud broke before the engine ran....... His project rebuild is on this forum right now and after removing the Dilivar his project now has Supertec studs. On the Forum right now.........

Henry - think you misinterpreted the wording - it says Dilivar for the head studs - steel for the case bolts (as does the picture in figure 10) Later reference in the paper is made to the use of spherical washers on the through bolts, but I didn't include that as it did not relate to the use of Dilivar - sorry if this has caused confusion.

Not trying to hose down your product, which by all accounts, has a great reputation for quality.

Just adding some published, peer reviewed information, to the discussion.

Regards
John
(PS I do not consider myself a peer to H Mezger, or even yourself in this regard)

I am confused but is this thread about presenting information or defending a product? This is an reoccurring subject and I am curious, has it ever been resolved? What do the folks at LN Engineering say? Don't they recommend head studs to go with their cylinders? Raceware, APR do they have reps here defending their products here or is this just going to keep coming up?

Just for the record Henry your studs are in my engine and they have been fine, I did not build it, I just paid for it.

John T.

I gueuss I got confused. I thought it said:
"The solution of the crankcase and cylinder head bolt problem was found to be Dilavar."

Doesn't it also say "fiberglass coating". Later found to be a desaster.
But without the fiberglass don't you run the risk of "the cooling air flow would cause the temperature level of the bolt to fall far below that of the cylinder?"
I'm not sure but do Dilavar studs come with a fiberglass cover. :)


You've quoted a remedy recommended by one guy, 30 years ago to repair a 1200hp 917, who had no idea what we know now.
Dilavar is unstable crap.
The question was "head studs, would you use these ?" The answer from me is "no ****** way, I would use these".
John it has not been resolved. As mentioned before "there is no right answer". It is great fun agitating the animals caged by the "Porsche can do no wrong" myth.
Charles @ LN makes billet cylinders (great product BTW) and billet aluminum has a different expansion rate than cast silicon aluminum. He ask me for some head studs years ago and I dropped the ball and never sent him any. I think I was low on stock at the time (as usual).
I will let him answer the question of what he recommends.


For all of you confused by this exchange, I'm sorry. I think the teasing has gotten out of hand. Buy what you want. For most applications anything will do and "will do" is good enough for most.

For the record, there are many excellent choices that have been tested with our cylinders by various builders and end users:

ARP
Supertec
993TT Dilivar

Others I don't see as often, but I know they have been used successfully as well:

Raceware
Performance Developments

All I care to see is an increased tensile strength over the standard steel stud with the Nickies. In our early testing, studs with tensile less than 190,000 psi (or thereabouts) would loose torque.

My only complaint I have is that I do not understand the very high torque value provided by ARP with their studs. We threw out their recommendations of torques with the ARP 2000 head studs for the 356, 914, and VW type 1 years ago and settled on 24 ft/lb with them happily.

I purchased a '78 SC with broken studs in 1992. At the time, the hot set-up was Dilivar on the bottom and steel on the top. I figured if Dilivar was better than steel I would use them on both top and bottom. I got the motor back together and in the car when I met my wife and moved from Florida to Texas. As you might guess, the 911 took a back seat to my new life. To this day I have never driven this car and only heard it run prior to pulling it down for the stud replacement.

It was stored in my garage and one day while sweeping up I heard a loud "pop" followed shortly by a louder "bang". About one minute later a second round of the same sounds, both coming from the direction of my old friend, the 911. I looked under the car as was very surprised to see two head nuts with washer and about two inches of stud lying on the floor. This was all happening with a motor that had never been run! The car was never outside nor was the engine ever allowed to get wet; ever.

I think I paid $24 each for those Dilivar studs back in "92 which is about the same as what I just paid for a set of studs from Supertec. What I can tell you without reservation is that if looks have anything to do with it, the studs I got from Henry are hands down my favorite. The fine pitch threads on the nut end of the stud make torquing the heads butter smooth. In my opinion, Dilivar is useless in this application. If you build race motors that are going to have the studs replaced every six months, maybe. But if you're building a street motor or anything that will be in the car for more than a year, forget Dilivar.

Like Henry, I have 22 "perfect" Dilivar studs for sale cheap. Any takers?

Lindy

Those are the "old" formula Dilivar studs. We've been discussing about the merits of the 993TT Dilivar studs. Those 911 SC dilivar studs should have been pulled from the market, like a Toyota floor mat/accelerator pedal/ecu..............

Dilivar is Dilivar: the only difference between what broke in my car and that that comes from a 993 TT is the length of the thread. I don't think the question here is whether or not a particular stud will hold the head and cylinder to the case but rather, how long will it do this job? My experience (that's all I have to offer) is that Dilivar does not lend itself to a very long life.

Lindy

Come on Dave.......
"Old Formula" of course, why didn't I think of that? (he says as he slaps his forehead)
All this time I thought we were talking about Dilavar but now I see we were really talking about don ta da..... Super Dilavar.
New and improved Dilavar, able to leap tall buildings in a single bound. And yes, it does have a smaller Carbon Footprint, because we all know Porsche cause Global Warming.

So if I get this right, all the specs posted above and in previous posts are suspect because they are the old formula?
So if we are to believe that expansions rates are the single most important element of a head stud, what is the new and improved expansion rate?

Take your time to answer, I'm ok, I'll wait :)

Wow, now you take pot shots at Toyota. Bar none the best production car on the planet.
Isn't it interesting how a recall for a rather minor issue becomes front page, almost "Global " news when the US government becomes a competitor of Toyota?
The Trans Sec tells the American people to "stop driving their Toyota". WTF
Of course there is concern, but a warning that "your pedal may stick, be careful and get it checked" should suffice for a thinking public.

I would like to go on record as saying " I would rather push my Toyota than drive a POS Chevy!!!!"

Rant over............

Henry,
You say you "researched" Dilivar and were able to find expansion coefficients and yet, someone on this forum found them. Everybody with any sense knew Porsche had the Dilivar studs redone when the 993TT came out. Yes, "dilivar is dilivar, it is an alloy created under a specific formula, but there is also a process of manufacturing an alloy that can make all the difference. The 911 3.0liter dilivar got their bad rap, deservedly so.
Tell me, ol' wise one, how many 993TT studs have failed? Do your "research". I can wait, to paraphrase. :)
In the mean time, take a anti-condescending pill, and wait a few hours to reply. I grow tired of your self-righteousness.

What some critters call "condescending, self-righteousness" others accurately label HUMOR. BTW: taking a pill is never a cure for confusion.

Like I posted earlier "there is no right answer". It is great fun agitating the animals caged by the "Porsche can do no wrong" myth.

Is rattling on the cage door working for you ? Because I'm having fun :)

Dave,

How many 911 SCs do you think were rebuilt with Dilavar studs? How do you think that number compares with all of the 993 TT that were ever built? I don't know but I'll bet it's at least ten to one. I don't know anyone who has a 911 that hasn't either replaced there studs as a precaution or as a necessity. One of the reasons you don't see many 993 TT failures is because there aren't that many out there, comparatively.

The 911 SC Dilavar stud got their "bad rap" because of what they were made out of, not because of an inferior manufacturing process.

I think a 911 SC gets used a lot more like a daily driver than a 993 TT and is subjected to the environment and the real world. I think that's what this thread is about; the real world answer to the question "Would you use these"? Since Dilavar is known to fail as a material in this application, why would "anyone with any sense" use it?

Lindy

Lindy,
There are theories that failures were attributed to the coatings on the studs, and a failure by the vendor's manufacturing process. Pick any that may apply......

As Steve and Charles alluded to earlier, they have had good success with the RSR/TT studs. Are you saying they don't have "any sense"? These studs certainly see more thermal stresses and higher forces than an 180HP SC.

I as stated in an earlier post, price versus performance, can't be used in an argument to use these studs. Over $800 for a set versus Supertec's studs, Raceware or ARP for less than $600. That argument makes more sense than RSR/TT studs are "bad".

Don't know about the "daily-driver" theory. The 993TT is certainly more civilized, sophisticated than the SC, but then it also has AWD, more creature comforts. More all-weather than the SC. Right about the production numbers of the two breeds, but its been over 14 years......we certainly would have heard something by now if the studs started mysteriously snapping.

Really, this debate has gone on a long time (too long). None of us on either side of the fence will waiver. It's your money and your engine.

My last word on the subject........I promise. :D

You are a walking contradiction. Trash Porsche, but whatever you do, don't trash Toyota? Give me a break.

That's me, just another critter in the zoo. I always get a kick out of the name calling.
When your agument fails, go with name calling.


walking contradiction
http://forums.pelicanparts.com/uploa...1265311523.jpg

So after all the name calling and postering, self advertising and so on MOTLEY911 ever get his engine built? This thread has been going on since June and it's actually getting silly and a little insulting.

I want to know if anyone has ever heard of a 993tt head stud failing? Just curious. I have a motor going back together soon and have to make some decisions.

I havn't directly but Danny Stahl, a porsche mechanic and owner of Stahl Motorsports here in West Palm Beach told me he's seen a 993TT head stud break in a motor.
I guess you could google Stahl Motorsports in West Palm Beach, Florida and call or email him for details if you want more info.