Head Studs

[VFR750]

I thought a stud could be analyzed as if it were a spring.

The whole system is a spring. Case-cyl-head in parallel with the stud-washer-nut. A softer stud is good, provided the stack does not seperate under the extreme combination: high cylinder pressure at min preload (cold engine)

Failed case threads are the other extreme: too much load for the available material capability at the materials temperature. Mg is weaker than Al.

[safe]

Dilavar has failed in my 3.2 engine and in most of my friends SCs.
All steel has leaked in my 964 3.6 and in my friends 993 3.6.

If Henrys empirical data shows as good results as he says, thats good enough for me, even if its not the best theoretical design on papper.

I have the Supertec studs in my 3.2 and it does not leak yet 3rd season now. Its not a race car but I track it as much as I can, 5-6 days a year.

[chris_seven]

These last two posts clearly show the reason why I can't come to terms with these contradicting requirements and a single solution.

I agree with the basic comment that good design practice is to use the correct clamp force to overcome lifting and the softest design compatible with this objective to minimise damage caused by expansion.

The 3.2 manual suggests a 2-Stage Procdure of 11lbsft followed by 90 degress for the head tightening and this is the same for the 964 motor.

The 993 uses a first stage of 15lbsft so has a little more preload but if leaks occur then it is insufficient and Porsche must have screwed up. The leakage that occurs is most likely to be when the engine is cold as clamping will increase as it warms up.

I would agree that using a stiffer stud will help with the cold clamping but it will also increase the forces produced when expansion occurs.

I don't think it is too much of a leap to assume that the Supertec Stud is stiffer than the original Porsche stud.

It is quite clear that the Aluminium cases survive this increased load and as it seems to solves leakage issues it must be a good thing.

However, the stiffer stud will not help the mag cased engines as the expansion forces still need to be controlled.

My main concern throughout these debates has been based on these simple issues.

Making the stud stiffer doesn't solve both problems and the original 7.7mm shank steel stud would be less aggressive in this situation than a stud with a larger diameter shank.

Thus the descisions that need to be made will differ depending on the detail of the engine.

As a last effort I would just repeat that I don't believe 'one size' fits all and that the stud that is good in a 993 may not be so great in a 2.7/2.8 300BHP RSR.

I am sure that Ti Studs will be a good solution for this type of motor.

[Lapkritis]

A gasket-less joint is going to be prone to leaking by nature. Demanding performance of that nature by clamping it under higher than intended pressures is questionable. Most written materials/opinions on these engines suggest to leave the cylinder to head as dry assembly with the CE ring. I'm curious if many builders use a conductive film gasket such as the permatex copper spray to fill surface imperfections. I plan to use this on my build regardless; I've used it on many engines in the past with great success and much higher cylinder pressures. Machining and tooling does not leave a perfect finish every time and this stuff fills those imperfections and does not add any significant measurable thickness.

[VFR750]

Chris,

Your comment about a single solution being elusive is, very true. There is no, one, perfect solution. In fact for a Mg case there might only be a few pretty-good solutions, and more leeway with the Al cases. There are many requirements for a good head stud, and they are at odds with each other. Porsche themselves have struggled with this.

Observation: Angle-of-turn, after a small seating torque is the best method for consistent preload. Pure torque requires consistent lubrication to minimize the effect of friction between the nut, washer, and stud threads. If given the choice, go with angle-of-turn.

I think Dilivar is a good material, with one serious shortcoming. For many, this is a deal-breaker. For others, with careful assembly, and paying attention to the protective coating, Dilivar works. Most people want to avoid the risk, and that is reasonable.

Titanium did not really exist as a financially viable alternative in the 1970s for a street car, so Porsche likely did not try it; they went with Dilivar. It probably seemed like THE solution, until studs started to break in larger numbers.

Even today, Ti carries a hefty premium. Theoretically it looks like a very good choice, but with little empirical data. For a Mg case, I think Chris is right, the Ti might be a better option than steel. But it needs to be developed, and it sounds like Chris is up to the challenge.

[Henry Schmidt]

Titanium? who knows
Before we first started making studs, we tried all the studs on the market. We were disappointing with every product available.
Supertec is an engine building company not a parts company so we would prefer to just buy the parts we need.
If someone comes up with a better product (proven by real world testing) we would be pleased to buy it with one caveat: better with the realm of cost effective.

[Green993]

Henry, what problems did you encounter with steel head studs like the 993 has?

Originally Posted by Henry Schmidt

Titanium? who knows
Before we first started making studs, we tried all the studs on the market. We were disappointing with every product available.
Supertec is an engine building company not a parts company so we would prefer to just buy the parts we need.
If someone comes up with a better product (proven by real world testing) we would be pleased to buy it with one caveat: better with the realm of cost effective.

[VFR750]

Henry,

A very good way to put it. You are in the business of building engines. A lot of responsibility. For you to entertain changing, is a huge gamble. I respect your position of wanting solid proof of a Ti answer. You should insist on that, you really can't afford to be wrong.

Ti might be "better", but it will take time to prove it. Hopefully Chris is on the right track. It would be nice to have an additional option.

[Henry Schmidt]

Originally Posted by Green993

Henry, what problems did you encounter with steel head studs like the 993 has?

Inadequate clamping force when hot creating an unstable joint between the head and cylinder.