BSA mainseal o ring to cylinder ,how tight fit

[original45]

When fitting a replacement o ring to a piston head how tight in the 28 mm cylinder should it be.
Been sent a couple to try one seems very tight and the other pushed in with little effort at all.

Now I appreciate the cylinders vary I hear , and it's difficult to advise on how much effort to fit right and seal correctly .
But I'm guessing as these arnt parachute seals that will expand lip under forward pressure so I'm thinking it needs to be tighter requiring a fair bit of pressure by hand to push in and down cylinder ?.

Appreciate some advice please .

Thanks

[Phil Russell]

just my opinion / experience ... you do not want the O ring to be a firm, hard to push in type of fit but neither should it be too loose. I find it is best when you can feel some slight resistance. The O ring can then move back against the groove on firing to form a very good seal.
I have recollections that I once used a Meteor ring in an Airsporter that seemed to have a tight cylinder when fitted with the standard O ring and it worked well ... although my memory may be wrong and I might have found a ring that was just a tad smaller than the original Airsporter. The Airsporter is usually 1 1/8" od, the Meteor 1".
Cheers, Phil

[original45]

Thank you Phil.
I will go with that and pop in the old chaps n raid his o rings and try a few see which feels best .
The o.m old seal was a little thicker than the ones sent me .
Thanks again

[oliver13]

I automatically fit a smaller cross section O ring to BSA piston seals, the original is way too thick. After about a million shots & thirty years it will be worn down to a D section & be working alright, then the buffer washer collapses...
I forget the exact numbers, but basically if BSA supply an 1/8" section O ring, fit one about 3/32". You want it to be a snug sliding fit, not a tight one. Very little resistance to sliding down the piston but not actualy dropping under it's own weight.

[Shed tuner]

as both the above chaps have said.. the std BSA item is way too tight. At most, a firm push with one finger should allow the piston to move. Ideally, just a gentle push...

[original45]

The size given above and finger pressure advice are excellent thank you.
I've borrowed a few big boxes of o rings to go through tomorrow.
Will certainly try to find the size mentioned and much appreciate your help.
Cheers gents

[original45]

I automatically fit a smaller cross section O ring to BSA piston seals, the original is way too thick. After about a million shots & thirty years it will be worn down to a D section & be working alright, then the buffer washer collapses...
I forget the exact numbers, but basically if BSA supply an 1/8" section O ring, fit one about 3/32". You want it to be a snug sliding fit, not a tight one. Very little resistance to sliding down the piston but not actualy dropping under it's own weight.

I've had a few dozen shots with it ,and a few dozen more over the crono.
With a rws superdome 14.5 g I haven't got 3 foot pounds .it's making 2,65 to 2,89 ft lbs using a standard bsa scorpion main spring and the o ring size I could find close was a 3/53" I noted you suggested 3/32" whilst the piston didn't feel tight being 're fitted would the differance in size from suggested make such a differance.
Transfer seal is 100% sealing

It shoots very nicely no vibration or twang but I'm concerned as its so low knowing the volume .

Further advice appreciated please gents

[Shed tuner]

add a little preload to the spring and see if it gets better or worse. that will tell you a lot...

[Papa g]

I've had a few dozen shots with it ,and a few dozen more over the crono.
With a rws superdome 14.5 g I haven't got 3 foot pounds .it's making 2,65 to 2,89 ft lbs using a standard bsa scorpion main spring and the o ring size I could find close was a 3/53" I noted you suggested 3/32" whilst the piston didn't feel tight being 're fitted would the differance in size from suggested make such a differance.
Transfer seal is 100% sealing

It shoots very nicely no vibration or twang but I'm concerned as its so low knowing the volume .

Further advice appreciated please gents

I've noticed from this post you say a "bsa scorpion" spring which is a pistol spring, in a 28mm cylinder which is the same diameter as the Airsporter, mercury & lightning/supersport which are rifles so that may be part of the reason for the low power.
Regards Graham