Hello Men, I have been lurking in the background for awhile. Great site. I am getting ready to rebuild an old B&S 18 HP I/C opposed twin. It is a Model # 422437 0719. I bought a B&S manual and have scoured the internet but can not find what the standard pistons are supposed to Mic. out at or how much piston to cylinder clearance I am supposed to leave. I know the standard bore is supposed to be 3.4365-3.4375 on all the 422400-422499 motors (as well as most of the other old opposed 18 horses). I have measured my cylinder bore and am a little oval shape with the higher numbers coming out right at +/- 3.4375. I don't want to hone my cylinder out any more than I have to and it "looks" like .10 over is all I'll need but without knowing what the standard piston measures how do I know FOR SURE what to hone it to and how big of piston I need to order OR how much piston/cyl. clearance am I supposed to have? Any input from the Gurus would be greatly appreciated.