Dave, you just might have hit on what your problem is if your engine is still standard bore size and you are just replacing pistons. If the cylinders haven’t been bored they are likely tapered from wear and the majority of the wear is always at the top of the ring travel where the pressure is the greatest. The same thing could happen if the engine was bored and honed a little too large. What has happened is that you likely have too much piston to bore clearance which is allowing the piston rock in the cylinder. This could very likely cause a piston to crack. I would suggest you have a machinist check the cylinders for taper and bore size then measure the piston for the clearance. Cast pistons are supposed to run very tight in the bore, some engines have an allowable clearance of slightly less than a thousandth of an inch, .001”. I would bore the engine to .020” oversize and get eight new pistons. Usually if you bore the engine to the exact twenty over standard size, the clearance will be correct as it is built into the piston. I’ll bet you won’t have any more problems if you do this. Also make sure your distributor is in good condition and the timing is set to spec. I would have the distributor run on a Sun machine and check the centrifugal and vacuum advance for proper operation. Too much advance can cause detonation which is really hard on cast pistons. I think someone above mentioned this also. Sometimes you have to spend more money to save it. Good luck.