Do the seals go on the shaft or are they internal seals in the valve block? Are the seals o-rings or some special seal, say one that looks like a T in cross section or do they look like a funky Vee packing with a small o-ring in the big end of the main seal ring? Generally o-rings are not used as seals that are subjected to constant reciprocation.
There should be a good bevel on the bore in the block where the shaft goes in IF the shaft holds the seals. Something like a 30 degree angle and not a sharp corner which tend to cut the seals. Can the shaft go in either end of the bore? That asked maybe one end has the bevel angle and one doesn't indicating that that is the end in which to insert the shaft.
I know, a lot of questions but just trying to figure out what you have. Any good industrial seal supply house should be able to fix you up with the proper size seal you need. The width you give VS the shaft diameter does not sound like you are looking for an o-ring. Too fat for the seal diameter OR the o-ring uses high perfomance plastic split back up rings on either side of the o-ring.