Machining a block, how it´s going???

Redfish, BJR and moper are right on. Pistons vary in their diameter right from the manufacturer, cast, hyperutectic and forged pistons all have different clearance specs even from differing manufacturers and different rings need different finishes. You're machinist is correct. Let him tell you what size pistons you need if you don't have the tools and/or experience to judge what the situation is. :)

It's also a good idea for your machinist to use a torque plate as the bore does change shape when the head is torqued down. It's not life or death but makes a big difference in longevity and sealing when very high cylinder pressures (high hp levels, super and turbocharging, nitrous) are involved.