Larry and Larry,
I recently bought a set of ARP studs for the head and #2 and 3 main caps for my 261, and the ARP technical consultant felt that the 1.5 ratio was good. Thus, for a 1/2" cap screw, there should be 3/4" of tapped threads in the cast iron block. For a 9/16" thread (as for the center mains on a 261) this would about 27/32". All of the tapped holes in the 261 are about one inch, +/- 1/16", so the 1.5 time diameter rule is met. I have never measured the thread depth on the later Chevy sixes. The real requirement is that the threads on either the cap screw or in the block do not fail (that is, strip) before the tensile load in the cap screw or stud reaches its maximum. Note that the nuts for a high strength bolt are less than one diameter in length. For example, the nut height for a 1/2" bolt is 7/16". Since cast iron cannot carry as much stress as high strength steel, more threads are required in the cast iron to carry the load and to prevent stripping the cast iron thread if a lot of torque is applied to the head of the bolt (or nut, if one is using studs).
If somebody had a junk block sitting around, an experiment could be conducted to see just how much thread engagement is required to prevent stripping the cast iron threads for a given amount of torque. Some of the technical references say that only six good threads are required. This would be about 15/32" for a 1/2"-13 cap screw.
Hoyt, II #922
