Thank you for the reply.

TDC makes sense to me. One of the recommendations I read in favor of doing the test at BDC said to remove the rocker arms, which would ensure that the valves stay shut. That seems like a lot a trouble, though.

Someone on this board, I think it was leathermang, recommended doing the test at BDC because 100 psi in the cylinder puts an enormous amount of force on the piston. For a 617 motor, 100 psi works out to about 1000 pounds on the piston. On the other hand, if you get the piston reasonably close to TDC, that force will result in a small amount of torque. If you get it within, say, 10 degress of TDC, the torque from 100 psi will be in the neighborhood of 65 or 70 ft-lb, which seems managable to me.