I had a hi miler 216 with the same situation. I was able to pull thte rod and piston out from the bottom, I did have to cut the ridge to get the piston out. When you do put the piston in upside down a .002 feeler between sidewall of piston and cylinder should just slip a .003 will bind. When you pull the rod cap it should slide off easily if it is hard to get off, the hammering probably made it out of round on the big end. Need to check the wrist pin and wrist pin bushings for wear as the noise could be piston slap, rod bearing, most likely, and wrist pin noise.
P gossett
