Hi,
I do not know what it should be but I had a similar dilema with my sons little Honda motorbike.
I cannot remember the exact values now but I decided that the piston clearance is measured when cold and the piston should always have the slightest clearance when as hot as it ever will be.
I think my guesstimate were about like this...
I think I chose about 120 Deg C as my hot temperature (2 stroke air cooled) for the cast iron cylinder.
I also thought the piston can maybe get to 170 deg C as the oil would be cooking on the underside of the piston pretty well at that temperature.
I worked out the thermal expansion of the cast iron bore with a 100 deg C temp rise and the cast ally piston with a 150 deg C temp rise.
I worked out the amount the clearance would reduce due to thermal expansion and then added a thou or half a thou.
I think it ended up at about 3 thou clearance.

We have not seized it up yet and it runs smoothly with no piston clattering.

I have no idea what it should be on an 8/1 but that was the logic I used on my sons little bike engine.

Rob