Brake Caliper Piston?
 

I ordered replacement pistons for a set of rear brakes that came off of a 98 Olds Bravada. The pistons I ordered from Rockauto are CARLSON 7825 for rear disc brakes. The pistons seem to be smaller in diameter than the ones I removed from the calipers. I looked at GMC Jimmy, Chevy Blazer and all the listings for 98+ S-Series on ROCKAUTO and they all call out the same CARLSON P/N of 7825. How close of a fit should pistons fit in the caliper? I thought maybe I ordered the wrong piston but I ordered correctly and the parts I received are the correct P/N. But I do not understand how there could be quite a bit of slop with the fit of the piston in the caliper bore.

Update: I gave up rebuilding junkyard calipers. I am returning the pistons to RockAuto and will buy rebuilt calipers at auto parts store and use the junkyard calipers as core returns.