[Henry Schmidt]

Originally Posted by winders

This thread says a lot:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/644752-993-head-studs-two-types-3.html

I don't get how you get "Inconsistent clamping force based on environmental changes" with the 993 Turbo dilavar head studs. Dilavar is a metal alloy that expands a certain rate. That rate is more in line with the rest of the engine than other alloys.

Simple enough. Not all of the head studs operate in the same environment.
Some are always hotter than the others.
If they are different temperatures, they will expand differently creating inconsistent clamping forces.
The greater the expansion rate the greater the inconsistency.
Now explain why Porsche used steel studs on top and Dilavar on the bottom of 911 SC. Only one reason: control seepage during cool down.
One more important question: The static head nut torque (cold) on a steel stud is 32NM and with cylinder expansion that preload becomes considerably higher when running. The engineers knew this and I assume deemed the warm clamping force desirable.
Now, if the Dilavar stud has a static torque of 32NM and the expansion rate is commensurate with the cylinders, the clamping force must be lower. How can both be correct?

BTW: In theory, the Dilavar expands at the same rate as the aluminum cylinders.
If that is true (that is the claim) then clamping forces would not increase as temperatures increase. Who in their right mind believes that 24psi is sufficient clamping force for a 911 head in full song?