The seller/manufacturer should specify the piston to wall clearance due to the differences in material expansion caused by different amounts of silicone used. High silicone content pistons expand much less than low silicone pistons.

I am going to guess that the pistons you purchased were low silicone, so you will need to run a larger piston to wall clearance to compensate for the expansion. Again, this information should be provided by the seller/manufacturer.