85 mm - 2 mm over stock (3.346 inches) is the maximum bore using a Wiseco piston with TD rings (Alloy steel, ferrox coated, chrome faced)
By your measurement 3.331" (84.61 mm) - the cylinder is already worn very close to the wear limit of .60" (.15 mm)
Two stroke pistons are usually sized in .5mm (.020") oversizes
Your choice is to bore to 85 mm and install a new piston set or have the cylinder sleeved back to stock and install a new 83 mm piston or have the cylinder sleeved, bored to 84.5 mm and reuse the piston with a new set of rings. The problem with the last suggestion is finding rings for the piston you have. If it's a Wiseco piston, just get a set of Wiseco rings that match the bore. If it's a Polaris (Robin) piston, good luck finding rings - Polaris has discontinued all sizes. The rings usually have to match the piston. I know you cant install Polaris rings on a Wiseco piston while Wiseco rings may go on a Polaris piston, there will be excessive clearance on the lands and sealing will be poor which can lead to poor running at certain or all engine speeds.
Shop Owner and Mechanic with over 50 years experience