When I modeled the BMW M12 engine, my engine analyzer program shows a peak of 2419psi at 9250rpm and making 636.3hp. I used gasoline with an octane of 120 since the witches brew they used is not available in my engine analyzer program. I may have gotten lucky with that, but my guess is that it is pretty close.

If I can chime in here with some different points that have not been considered here.

Building engines is about applying good sound engineering and sound practices. You have to match what you calculate with what you measure. There are many factors that come into play here.

These engines do move around and a lot too. Look at the witness markings on the case halves. The cylinders do distort as well. As its has been posted before, this can be calculated to some degree. Yanking down on the studs will certainly do more damage than you know about.

Here is my opinion. If you consider your self to be an engine builder, totally different to an engine assembler, you would check the calc's and check the assembly. We do. We have special tooling that is fitted to the cylinder that we can torque the head stud nuts and measure the inside diameters and taper in the cylinder bores. Why would you not do this? Leave it to chance??

As a note of interest, the cylinders depending upon inside diameter and manufacturer, deflect differently and in different places up the cylinder bore. We know what cylinder pressure the engine will have and the cylinders are honed to match. We know cold what they measure and have tested then at running temps to get the clearances inside diameter and taper we want.

Also, you have to remember that when tightening down the head nut, you will also deform the head nut washer platform in the head. Much of the tensile that you want going into the stud (stretch) goes into deforming the washer platform. This is why we have special insert washers to stop the head from collapsing inwards around the stud. It gets worse the older the heads.

Pretty simple stuff here. Nothing hard, just have to be aware of what is going on and not leave it to chance. Do leave anything unchecked. If you want to use more clamping force, then measure the consequences of this. Make sure what you see on the engine backs up what you calculate.

All this hype over studs kinda goes no where when the basics are never considered. Remember, if the engine fails, why did it. Find out why and fix it. If it ran good with issues, does it matter what parts were used? No it doesn't.

I have saying, there are only two types of people in this business. Those that see it wrong and can fix it , or those that do not see it at all. Pick a side!!

Interesting thread...and I drive a water pumper.

Naive question: If, as this thread suggests...the various studs present in an air-cooled engine run in different environments, then why use the same studs throughout? Why not match these studs to their particular environments?

911 engines (like most other engines) have design compromises.
Porsche tried running two different stud materials in the same engine back early 3.0 with disastrous results.
It is both impractical and cost prohibitive to create a different stud for every temperature environment.
What makes the most sense is to choose a stud material with the least number of compromises.
A material that is impervious to corrosion failures, offers a predictable expansion rate that is greater than plain steel and a clamping force more in line with the 911 engine dynamics.

Yep....like 993 Twin Turbo head studs!

Why do you make this claim?
What is the chemical formula of Dilavar?
What is the actual expansion rate?
How can you claim that Dilavar is impervious to corrosion fractures?
What is the clamping force of the internal lower studs (say #5 cylinder) vs the clamping force of the external #6 cylinder stud?

Mezger claimed that the "Dilavar" studs in the 917 engine had to attain and retain a certain temperature in order to function properly. To that end, they coated the studs with resin and fiberglass.
Do you believe that the "Dilavar" in the 993 stud is a different material?

http://forums.pelicanparts.com/uploa...1617037714.jpg


Oh...Oh...Oh wait: I know the answer....."all the cool kids use them"

This is a seriously underrated post. Professional and to the point without resorting to childish "aLL tHa KewL KiDS" ****.

I am curious though, do you have a go to stud, or does it depend on the application?

These guys are the go to studs.

Sorry. I'll see myself out.

I did some digging on the composition of Dilavar in the past. I am just going to copy and paste what Chris Seven responded to my questions .back in 2015.
http://forums.pelicanparts.com/911-engine-rebuilding-forum/849320-wanted-pristine-but-broken-dilvar-head-stud-analysis.html
As I said before chemistry and characteristics of material is science.

Can't really help with a stud but the basic composition of Dilavar is relatively well known.

%Carbon: 0.63 to 0.73
%Silicon: 0.50 Max
%Manganese: 4.50 to 6.00
%Phosphorous: 0.30Max
% Sulphur: 0.30Max
%Chromium: 2.50 to 4.00
%Molybdenum: 0.20Max
%Nickel: 11.00-13.00
%Vanadium: 0.75 to 1.20
%Niobium: 0.10 to 0.40
%Nitrogen: 0.01 to 0.1

Its Werkstoff Number is 1.3937 and it is produced in Germany by Deutsche Edelstahlwerke amongst others.

The combination of Nickel and Manganese at the above levels will stabilise the Austenite Phase at Room Temperature which is why this alloy is non-magnetic.

It is generally used in a Solution Treated and Age Hardened condition with a UTS of 1200MPa and a Coefficient of Expansion (CoE) of approximately 19.4 x 10^-6 m/m/degK.

Elongation is generally good at around 10% and Young's Modulus (E) is typically 198 GPa.

It is safe to describe this material as a Precipitation Hardening Austenitic Stainless Steel.

Its Nickel Content may seem relatively low as most Stainless Steels with 11-13% Nickel tend to be either Ferritic or Martensitic (which also makes them magnetic) but the relatively high content of Manganese has a significant influence in terms of Austenite formation and retention. Unfortunately it has been shown that using high levels of Manganese in relatively low Nickel content steels has an adverse effect on resistance to pitting and crevice formation in corrosive environments.

Austenite, of course, has a Face Centered Cubic Atomic Structure (FCC) and this is the reason for the higher CoE than the more common Ferritic/Martensitic Steels which are Body Centred Cubic (BCC).

It is interesting that Invar is also an Austenitic FCC structure but exhibits a CoE if around 1.3 x 10^-6 m.m/degK which is quite contradictory. Its discovery in the 1920's was of great significance and initially the theory behind its low expansion was based on its confusing magnetic behaviour. Invar is also magnetic in the temperature range where its CoE is low and this is clearly an anomaly. There were attempts made to explain this behaviour in terms of magnetic moments but these proved incorrect and thus far no real explanation has been found but there will be an underlying magnetic reason.

The typical microstructure of a well produced Dilavar is very straightforward and comprises of fine grained Austenite (Niobium is added as a grain refiner) with Chromium Carbide precipitates which strengthen the alloy.

These carbides which should be coherent with the Austenite matrix for maximum strengthening form within the grains of the material but are also found to be present at grain boundaries and this is what gives rise to potential problems.

The basic mechanical properties of Dilavar are encouraging and the alloy should be not only strong but with a 10% elongation should be quite tough and used correctly I am sure that this is the case.

Typical failures of Dilavar studs show a brittle fracture in the direction of the maximum shear stress in the stud (approx 55 degrees to the tensile axis).

One of the issues with Austenitic precipitation hardening stainless steels is that they do have a strong tendency to suffer from Stress Corrosion Cracking (SCC) particularly in the presence of Chlorides. The relatively high level of carbon found in Dilavar does it no favours in this respect.

Stress Corrosion Cracking is a mechanism which generally occurs due to defect initiation at the grain boundaries of a material. The presence of chrome carbides in the microstructure will almost certainly help to create this type of defect. Add to this an increased tendency for the formation of crevices due to manganese then problems can occur.

As the length of the intergranular defect increases it effectively creates a short but growing crack. Once this crack reaches a critical length the preload stress present causes a brittle fracture.

There have been many, many significant catastrophes caused by this mechanism particularly in the oil/gas industries which operate in Sweet/Sour Gas environments.

The resin coating applied to the latest generation of Dilavar Studs appears to have all but eliminated the failures experienced due to this mechanism but the cost of these parts is still relatively high.

There is also apocryphal evidence that Dilavar studs suffer from Hydrogen Embrittlement but I am not sure that this is service related.

It is certainly possible that poor 'pickling' practices used during manufacture could result in failures of this type but there is nothing that would produce this type of failure in the service environment.

Dilavar alloys are now also commonly manufactured in China and some care is needed when sourcing this type of stud.

It would be interesting to test a modern Dilavar Stud for susceptibility to SCC but as a metallurgist with a background in Fracture Mechanics and Fractography I am reasonably confident that the early generation Dilavar studs fail for this reason.

It is fair to say that much of the knowledge we now have with regard to low Nickel Austenitic steels wasn't available in the late Seventies when Dilavar was first introduced.

You also have to question the use of Dilavar on the exhaust side of SC engines and conventional steel on the inlet side. Not really sensible.