You are correct....0.5mm = .020. When he gets the sleeved block back, just take it to a machine shop with the pistons (all 4, not just 1 like a local shop did recently and ruined a motor) and the shop will bore and hone to spec. The pistons come with P/W clearances. More than likely, if he spoke to the shop actually sleeving the block and let them know he was going 84mm, they will send the block back bored to 83mm. Most sleeving companies will do this to help you save money at the machine shop. In that case, 1mm (or .040) is what the machine shop will bore.