Be aware orings are fatter than stated. For example a 1/8" oring is
..139".

With that said, the groves in the piston should be deep enough to
compress the oring around 25% against the bore. The groves should be
wide enough for the squashed o-ring to have somewhere to go.

So, an 1/8" oring (really .139) should have clearance between the OD of
the groove and the ID of the bore of .104". The groove would be around
3/16" wide.

The oring should be stretched some in its grove, but only minimally,
like 5%.

A rough bore will ruin the oring. Not a bad idea to hone it in the
direction of use. A bright finish is not necessary, just no sharp
roughness.

A lead in, chamfer or radius to the bore will prevent pinch cutting the
ring during install.