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...

[original45]

I will add another 5 mm or so.
I had already added friction washers both ends so that was extra 8 mm initially on 're fit.


Grrr hate it when it goes this way

Thank you

[original45]

Added 20 mm pree load on guide .ft lbs went slightly higher 3,15 .
I had a few shots registering 3,75. After 20 more shots but dropped again with next string.

O ring sits quite firmly in the piston head with no room for it to move either forward or back in its slot.

I even expanded the pellet skirts a little for a tighter fit but got lower fps ?.
I'm guessing o ring isn't sealing

[Shed tuner]

yup, sounds like a sealing issue someplace...

[Benelli B76]

If it is a nylon piston head, these expand over time because they absorb moisture (hydrolysis). Therefore the O ring groove will lose depth and an original O ring will be tight. Either re-cut the depth of the O ring groove or fit a thinner O ring to compensate.

Baz

[NickG]

Added 20 mm pree load on guide .ft lbs went slightly higher 3,15 .
I had a few shots registering 3,75. After 20 more shots but dropped again with next string.

O ring sits quite firmly in the piston head with no room for it to move either forward or back in its slot.

I even expanded the pellet skirts a little for a tighter fit but got lower fps ?.
I'm guessing o ring isn't sealing

You need to be aware of how o rings work, they need some room in the groove width, when they see some pressure they deform into the groove sealing the gap between piston and bore , your ideal section needs to just rotate on the piston when not in the cylinder , the od then needs to be slightly larger than the cylinder .010" on diameter is about right for general use , now you are all going to tell me that bsa cylinders are crap out of round and barrel shaped , but as o rings seal as soon as they start to move the deformation of the o ring I spoke of earlier should take all this out ,I think your problem is friction due to incorrect sizing of the o ring ,robbing you of power .

[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

[Shed tuner]

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

BSA scorpion pistol cylinder is also 28mm diameter Mr G; they are monsters