Polymer Man wrote:
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.


an o-ring makes a horrible dynamic seal , get a u-cup type seal ,

but for an 1/8 cross section o-ring [.139 ]

the width of the groove should be .179 - .186

the depth on the radius from the bore dimension should be .110 - .113

the clearance between the bore and the piston o.d should not exceed .003
total or .0015 per side

if you want to use a back up ring or an anti extrusion device the groove
would be widened accordingly

the surface finish on the bore and the groove should be 10-20 micro
inches for best seal life but can get as high as 32

the finished surface marks should all be perpendicular to the direction
of travel

not hatched like an engine that uses cast rings


I recommend everyone that plans on doing hydraulic work get a set of
catalogs from Hercules bulldog , full of good information


18003335617 option 4

or order on line