Running In An 'o' Ring On A Bsa Mercury

[Hsing-ee]

I have reassembled my Mercury-S for the umpteenth time, with a new spring, new piston washer and new breech washer. It is STILL only doing 8.5 ft/lbs.
My BSA Meteor does 10 ft/lbs & is far easier to cock.

All that I can think of is that the piston 'O' ring takes a while to bed in. The piston is sealing well but it was difficult to push into the compression chamber, seems like a very tight fit.

Do I need to shoot a tin of pellets through it to bed it in? I have already changed the 'O' ring as I thought it might have a nick in it, but this had no effect. The gun should be doing about 11 ft/lbs I think.

Does anyone else have experience of the BSA piston 'O'-ring bedding-in experience?

Thanks chaps, its got me stumped.

[keith66]

I had a similar problem with a mercury some years ago, the o ring groove in the piston head was not deep enough causing excessive compression of the o ring and hence friction.
Favorite would be to put the piston head in a lathe and skim a few thou out of the groove, or at a pinch you could file it deeper with a round needle file if you were really careful, though this way it would be hard to avoid high spots. either way if you overdo it you will need a new piston head.

[gedfinn 2]

Sounds like its too tight mate ( Big )
Fit a smaller O ring, It Shouldn,t be Tight,!
If you go to any Hardware store, They will have a Box of O Rings ( 2 Boxes )
Have a sift through them, And look for the nearest & smaller size, ? Look in both boxes, " Imperial & Metric " Failing that have a look and see what I have, Cheers,

[Hsing-ee]

Thanks chaps, it may well be a shallow groove which is at fault .. I'm going to try shooting a few hundred pellets through it and then if it is still a problem, it's back to surgery for it ... maybe even a PTFE head from Dr Robb.