Help w/Briggs 18 HP piston size and honing

[Kernopelli]

I bought an old Grasshopper (same one I had posted about previosly) for my teenager to make some money with this summer. I knew when I went and looked at it that it was going to need a rebuild or at least freshened up.

I tore it down and now I am getting ready to rebuild it. It has 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 to hone but without knowing what the standard piston measures how do I know FOR SURE what to hone the cylinder to and how big of piston I need to order and also how much piston/cyl. clearance am I supposed to have?

Any input from the Gurus would be greatly appreciated.