The piston thinness was a problem I have found on the Xray XB4 shock absorber, they are 2mm thin against 2.5mm for TRF (+20%) or 2.7mm for Kyosho. (+26%)

A way to test the thick influence is to use Xray piston inside Kyosho shock absorber with shim between the E-clip and the piston to remove backlash.

The other way (used to have better xb4 shock) is to counter bore the axle hole of a kyosho piston in order to put them in the xb4 shock