Quote:
Originally Posted by PeteKz
The material properties of the steel head studs in a Porsche air-cooled engine do not change due to time, mileage, stress cycles, or heat cycles. The temperature that a steel stud would have to reach to change its material properties is far higher than it could experience in use.
IIRC, SAE Grade 8 steel bolts are heat treated around 800F. I expect the steel head studs are made to similar specs. To change the material properties of the heat treatment would require the stud to get close to that temperature, so let's be conservative and say 500F. Remember that the studs are in the airflow under the shroud. So they don't get as hot as the exhaust side of the head. Either the heads, valves, cylinders or pistons would fail and the oil would be cooked long before the studs could get to that temperature range.
If there is mechanical damage to any of the studs, including corrosion pitting, then absolutely change them. Otherwise, there is no mechanical reason not to reuse them. Follow Henry's recommendation to use the same studs all around (even though Porsche originally used Dilivar on the bottom row and steel on the top row from 78-on).
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You're missing the point. Not all steel is the same. Not all manufacturer have the same thread processing (some cut some rolled) or the same material spec or heating treating process. Those processes matter when equal clamping force is the goal.
Unless you are willing to have your old/used studs analyzed, why risk it?
The head studs on many 911 engine are enclosed by the cylinder so they are not exposed to the cooling air. Heat cycles on stressed hardware matters.
As for Porsche's choice to stagger the head studs on 78-on, that is an inaccurate statement. All 78 and later 930 Turbos, 964 and 993 engines used the same stud top and bottom.
The use of staggered studs was an attempt to mitigate warranty issues and was never designed as anything else.