I have a cracked piston and rings on cylinder 3 and can’t seem to find and oem piston for cheap so I’m considering getting nippon k20a pistons and boring out my cylinders from a 85mm to 86mm to make the clearance for the k20a pistons. My question is what exactly do I need to do to be able to get them to seat right with no issues besides boring and decking both head and block?

 

The machine shop you use will bore and hone the cylinders to give you the correct surface for proper ring seal. After the rings have been properly installed and gapped, and the engine reassembled, you can just drive it normally but try not to maintain the same exact RPM for too long during the first 500 miles or so.

Before you do any of that, you need to consult with an engine compression calculator and do the math to find out what your static compression ratio will be with your chosen K20 pistons. The pistons have to be installed backwards to account for the difference in pin offset. You will also need to clay the pistons to check for proper piston-to-valve clearance as the valve reliefs will be shaped differently and in different locations versus the stock F20B pistons.