Quote:
Originally Posted by Mongoose930
I believe the head material would have more to do with the torque spec. than anything else.
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This is not correct. The torque is based on the friction to turn the bolt and the desired tension on the bolt. That is why they list dry and lubricated torque specs.
Here is where the aluminum comes into play. For steel and cast iron the length of threads that are needed is about one bolt diameter. For aluminum you need two bolt diameters. So a 1/4" diameter bolt needs to have a minimum of 1/4" of threads into the hole in a cast iron head. The same bolt in an aluminum head would need 1/2" of threads. Now there are modifications done to reduce the number of threads needed. If they tap the hole bigger and put in a heli-coil or other type of insert, you do not need as many threads. Depending on the outer diameter of the insert, it is possible to get back to one bolt diameter.