BSA Airpsorter S Rebuild - Advice Please

[Kevev]

Hi all, I am seeking advice and/or, recommendations on the rebuild of my BSA Airsporter S Mk1.

Having already stripped the gun I have cleaned everything up and prepared it all for rebuild with new parts as required.

I have given the inside of the gun’s cylinder a light honing with some 600-grit wet and dry, on a drill jig and cleaned it all off thoroughly using a solvent cleaner and compressed air gun.

I fitted a new steel piston with a Meteor O-ring seal (I had previously been advised to use a Meteor O-ring, as it has a smaller section) and new buffer washer sourced from TW Chambers.

I also bought a new TBT spring kit. This was supplied to me with the longest spring available, so that I could make it the correct length. This I was told should be the same number of coils as on the original spring. So, after counting the coils on the original spring as 32, I modified the new TBT spring to the same number of 32 coils. After this modification the new TBT spring was considerably longer than the original, even with the same number of coils. The helix angle is obviously different and contributing to this, but is it a problem?

I started putting it all together by first inserting the piston body, with its new piston and seal into the cylinder. I had smeared a small amount of the Tinbum grease on the O-ring seal, as I had been told not to use oil on it, which seems alien to me for an O-ring. The piston assembly was very difficult to get into the cylinder and I had to push it very hard to get it all the way home inside the cylinder. It seemed to me to be far too tight. I would have expected it to slide in the cylinder much more easily.

Anyway, once it was all the way home, I placed the original spring and its steel rear guide into the cylinder to take a reference measurement. The spring and rear guide was protruding 42mm out from the threaded end of the cylinder. I then removed it and replaced it with the new TBT spring with the plastic slip washer, top hat front guide and rear guide. This time the spring and rear guide were protruding 80mm from the threaded rear end of the cylinder.
It looked to me like it was too much, but I just don’t know as I have no experience with this kind of rebuild. With the original spring I can hold the barrel in a vice and push the trigger block, cushioned in a towel against my chest, onto the barrel and screw it into place. There is no chance I can do this with the new TBT spring.

If I must make up a spring compressor and jig to hold the trigger block that is not a problem, but I am concerned that there will be too much preload on the spring making it difficult to cock the gun. Even impossible if the spring becomes spring bound. Then there is the issue of exceeding legal power limits.

I would greatly appreciate the thoughts and advice of more experienced gun owners and enthusiasts.

Is my piston too tight? Is my spring too long?

Cheers,
Kev

[JLB]

I think you've answered your own question. Piston head is too tight and too much preloaded on the spring. As far as I'm aware, the mk1 had a leather Piston seal and the cone shaped piston, was this the case with yours. I'm currently looking at the o ring conversion for my mk3 which also has a coned piston, the part is available from Chambers gun spares.

John.

[junglie]

The mercury seals are the same as airsporter ones, it's the meteor o ring that some advise to use however it was too loose in a mercury when i tried it so used the original one. The S model i worked on a few years ago had a piston so tight in the cylinder even without the piston head fitted it was a terrible fit. So i would try it without the o ring if i were you. The standard bsa spring is hard enough to fit in them without trying to cram longer ones in btw & should make good power imho, I hate refitting them or mercury's tbh as I can't use my spring compressor on them & can't be bothered to build another one just for the few bsa's i work on. I know folk love the S models but I was very unimpressed with the insides of the exellent condition (externally) one I worked on, imho the mercury's especially the S model & challengers are far better guns to shoot & make full power easily. Edit the mercury challenger I fitted the same standard bsa spring to new had about an inch & a half to two inches of preload on it & was a bugger to fit, the old mainsprings will be shorter in that case by an inch at least. I clamp the actions in a padded big record vice & screw the trigger block on & with welders gloves on After a few tries & usually a few swear words !

[junglie]

I think you've answered your own question. Piston head is too tight and too much preloaded on the spring. As far as I'm aware, the mk1 had a leather Piston seal and the cone shaped piston, was this the case with yours. I'm currently looking at the o ring conversion for my mk3 which also has a coned piston, the part is available from Chambers gun spares.

John.

None of the s models has the cone shaped old piston head design it was discontinued well before they were made.

[Kevev]

I think you've answered your own question. Piston head is too tight and too much preloaded on the spring. As far as I'm aware, the mk1 had a leather Piston seal and the cone shaped piston, was this the case with yours. I'm currently looking at the o ring conversion for my mk3 which also has a coned piston, the part is available from Chambers gun spares.

John.

Hi John,

Yes I probably did answer my own, question going by my own personal experience in engineering, but I still though it prudent to ask the question as I have no experience with air rifles.

What I do know is that the Airsporter S Mk1 with serial number prefix EP did not have a leather seal with a cone shaped piston. The original piston from my gun was aluminium with a pronounced radius on the front edge. The hole for the locating pin was behind the groove for the O-ring seal. The new piston I bought from TW Chambers is steel and does not have the pronounced radius on the front edge and the locating pin hole is further forward inside the O-ring groove.

Cheers,

Kev

[Kevev]

The mercury seals are the same as airsporter ones, it's the meteor o ring that some advise to use however it was too loose in a mercury when i tried it so used the original one. The S model i worked on a few years ago had a piston so tight in the cylinder even without the piston head fitted it was a terrible fit. So i would try it without the o ring if i were you. The standard bsa spring is hard enough to fit in them without trying to cram longer ones in btw & should make good power imho, I hate refitting them or mercury's tbh as I can't use my spring compressor on them & can't be bothered to build another one just for the few bsa's i work on. I know folk love the S models but I was very unimpressed with the insides of the exellent condition (externally) one I worked on, imho the mercury's especially the S model & challengers are far better guns to shoot & make full power easily. Edit the mercury challenger I fitted the same standard bsa spring to new had about an inch & a half to two inches of preload on it & was a bugger to fit, the old mainsprings will be shorter in that case by an inch at least. I clamp the actions in a padded big record vice & screw the trigger block on & with welders gloves on After a few tries & usually a few swear words !

You're absolutely right about the Meteor O-ring and that is what I have fitted. I have no idea why I said a Mercury O-ring other than my age and the possible confusion it induces sometimes ;-) . I will edit the thread and correct my error.

I will do as you have suggested and see how the piston fits without the O-ring fitted. I will get my micrometer's out and measure the OD of the original piston and its O-ring groove and see how they compare with the new one. Not sure I will get my telescopic bore gauges far enough into the cylinder chamber (highly unlikely) and I'm not sure how to go about finding its original and standard dimension?

Regarding the spring compressor, I agree it's a bloody difficult trigger block to get a hold of. Having said that I had no problem getting the trigger block back on with the standard spring, which was sticking out 42mm. I don't know if that is because its a very old spring and has been sat in the gun barely used for the last 45 years.

Looking forward, I have bored out and milled out a piece of 3" diameter delryn round stock and the trigger block fits snuggly and square inside it now. I can put this in the lathe chuck and use a live centre in the tailstock to compress the spring. Once I have the spring at the correct length. I'm going to chat to Nick at TBT about that, as he is away until Monday. He has been very helpful so far.

Cheers, Kev

[lightning22]

Welsh Willy does a parachute seal conversion, that's what is happening to the Mercury S I'll be rebuilding.

He's currently sold out, but no doubt will be stocked up again soon.

https://www.woodfield-gcp.co.uk/

[Kevev]

Welsh Willy does a parachute seal conversion, that's what is happening to the Mercury S I'll be rebuilding.

He's currently sold out, but no doubt will be stocked up again soon.

https://www.woodfield-gcp.co.uk/

Unfortunately, for me, he appears impossible to contact. I’ve sent him several emails and had no response. Trying to find a telephone number to call him has so far been futile.

I will only chase people for so long, if they aren’t interested in responding to me then I’m not interested in doing business with them.

[TonyL]

80mm offspring protruding sounds far too much. Even if you did manage to "cram" it in, you'll probably end up with a harsh shooting, over-sprung beast. Threaded section will be circa 12-15mm? I think I'd want to end up with about 45mm max preload, so if the threaded section was 15mm long, about 30mm of spring protruding. Shorten the spring (and re-finish etc) and re-try. And with more modest preload levels, you should be able to re-fit by hand (I find a thick, padded, grippy glove helps).

[TonyL]

Re piston head fit, as above, do try without the 'O' ring fitted. Make sure the cylinder is nice and clean and no residue left over from before. It might be that the groove on the piston head needs altering slightly so that the 'O' ring isn't such a tight fit (referred to as "crush" and one of the areas tuners who play with 'O' rings like to experiment with).

[Dornfelderliebe]

80mm offspring protruding sounds far too much. Even if you did manage to "cram" it in, you'll probably end up with a harsh shooting, over-sprung beast. Threaded section will be circa 12-15mm? I think I'd want to end up with about 45mm max preload, so if the threaded section was 15mm long, about 30mm of spring protruding. Shorten the spring (and re-finish etc) and re-try. And with more modest preload levels, you should be able to re-fit by hand (I find a thick, padded, grippy glove helps).

I had my new Mercury apart last night. Only similar but still, spring length 215mm and 35mm protruding from the end of the cylinder.

Regarding testing without an O ring. I restored a seized up Scorpion pistol a while ago.With it's original nylon piston head, I used a cordless drill and small circular file to turn the o ring groove down far enough for the seal to fit nicely.

[Kevev]

80mm offspring protruding sounds far too much. Even if you did manage to "cram" it in, you'll probably end up with a harsh shooting, over-sprung beast. Threaded section will be circa 12-15mm? I think I'd want to end up with about 45mm max preload, so if the threaded section was 15mm long, about 30mm of spring protruding. Shorten the spring (and re-finish etc) and re-try. And with more modest preload levels, you should be able to re-fit by hand (I find a thick, padded, grippy glove helps).

Thank you, that’s really helpful information and advice.

Much appreciated.

Kev

[Kevev]

Re piston head fit, as above, do try without the 'O' ring fitted. Make sure the cylinder is nice and clean and no residue left over from before. It might be that the groove on the piston head needs altering slightly so that the 'O' ring isn't such a tight fit (referred to as "crush" and one of the areas tuners who play with 'O' rings like to experiment with).

I’m going to measure the original alloy piston OD and its groove and then do the same with the new one to see how they compare. When I stripped the gun down the piston assembly needed tapping out with a drift, but the buffer washer was in a right poor state, as I had been told to expect, and the gun hadn’t been used for years so I wasn’t surprised it was tight to get out. Very surprised at how tight it was when it went back in with new parts though, and I knew something wasn’t right.

I’ve mentioned it in previous post, that I have many years of experience in both mechanical and electrical engineering, so I fully understand the fundamental principles, but have no experience with guns. Hence the desire for advice. I’ve been involved in the design and production of automated machinery for the food industry for most of my life, and a lot of that machinery uses pistons and cylinders to disperse accurate portions of a variety of food product, at varying viscosities. They use a variety of seals on the pistons, from simple O-rings to dispersal seals, which are what people in the gun world refer to as parachute seals. The cylinders vary in diameter from 3/4” to 4” and are driven by both pneumatic pressure and electrical servo motors. If any of these pistons were tight in their cylinders, something would be seriously wrong.

Thanks for taking the time to offer me your thoughts.

Kev

[Kevev]

Just to update and inform everyone who is following this thread, I have found nothing wrong with the new steel piston supplied to me by TW Chambers.

The O.D. of the original alloy piston is 27.40mm and the O.D. of the new steel piston is 27.45mm. An insignificant difference of 0.05mm and one that makes no difference to the fit inside the guns cylinder chamber. I have tried the steel piston in the cylinder chamber without the O-ring and it is as free as a bird, which you would expect with 0.275mm (just under 0.001") clearance. Assuming the cylinder is 28mm inside diameter.

The O.D. of the O-ring groove on the original alloy piston is 22.26mm and the O.D. of the new steel piston O-ring groove is 22.25mm. Again an insignificant difference of 0.01mm, and on the minus side which should make the O-ring 'crush' less by a minuscule amount. Nothing to worry about and certainly nothing to cause it to be tight in the cylinder chamber.

I tried fitting the O-ring back on and gave it a good dosing of oil and this time it went in a lot easier and could be slid up and down by hand. So my conclusion is that the problem was definitely a lack of lubrication.

I plan to give the cylinder chamber a bit more of a substantial honing and maybe relieve the O-ring groove a little too, once I have measured things with the O-ring fitted. Then I will mix up some gun oil and molybdenum grease to lubricate the piston O-ring with and then put it all back together again, well lubricated. I'm confident that will do the trick and if I do get any 'dieseling' it surely can't last long?

[Dornfelderliebe]

Try to get the absolute minimum of lubricant in front of the seal to minimise dieseling. You may then be able to let the new seal 'wear in'. Your kit will need a period to bed in any way.
I would put unadulterated moly on the rear end of the piston and follow your kit's instruction's for the rest. After fitting the piston I like to put grease in the rear of the cylinder where the back of the piston bears against it.