Although the O ring seal sounds good, I'm not sure if it would work (I'd have to take it back out and have a look), if you look at the photo's on
THIS thread I posted earlier, you'll see the piston at the top of the second and third photo's, I'd be a little worried about cutting a groove in the front of the piston as the whole thing is under extreme pressure (I don't want to teach you to suck eggs as you already know this) and if I or the next owner (if I ever decide to sell it) want to turn it back to a rammer then I wouldn't want the piston to be weakened by having the groove cut into it, not sure how much 'meat' there is on that perticular part either and if it could be cut deep enough to hold an O ring in place, as you can see the rear part of the piston could easily take a bearing but I don't really feel it needs it.
Pete