Servicing a 70s Mercury - Spring Choice

[james_stan]

Thanks again. I found the serial number too - it's a ZB..

[Airsporterman]

Stick to the standard spring (nos) if you can obtain one, there used to be a member on here who had a supply of them (sorry I haven't read through the thread if this has already been covered) in my experience, any other spring fitted to these old BSAs tends to spoil them, makes them harsh to fire, makes them inconsistent and less accurate, in all, are generally not an improvement on the standard spring without a lot of messing around shortening/removing coils etc!
Just my view! (I have removed a few dreaded 'aftermarket' springs and the like from old rifles and replaced them with standard springs in the past)

ASM

[james_stan]

Hi all

Thanks again - started stripping last night and I got the trigger block off now and the spring out - the piston/rod seems a bit stuck though at the moment.
Is there a spec on the spring for its 'relaxed' length in case it's usable in the short term. This will definitely be the original from 40 years ago.
but it's done no work in the past 25 years - what's known on motorbike circles as a 'barn find'..

[Phil Russell]

The spring is the same as used on the Airsporter. The spec varies a little from 'authority' to 'authority' but for a standard spec round wire spring, which you almost certainly have:
21mm od x 14.5 id x 3.25 wire diameter approx 265mm long with 29 coils. If you go shopping for a new spring you may find Titan brand ones advertised for the Mercury. Note that these will no doubt fit the piston but will certainly be too long and you will not be able to fit it unless you are exceedingly strong. It will need cutting down and the end refinishing. Protek Supplies are currently advertising Airsporter / Mercury springs I believe. Or Chambers or John Knibbs.
The stuck piston you mention is a well known issue. The cause is almost certainly the buffer washer disintegrating and jamming the piston in the cylinder. You may be lucky and be able to pull the piston out or carefully use a drift to tap it free.
Good luck, Phil

[james_stan]

Ah yes 'a suitable drift' as Haynes manuals are fond of saying..

[james_stan]

Hmm all is not going well and I fear I may have damaged the internals..

I have managed to drift out teh rod with approx 2" protruding from the cylinder. The bottom of the piston skirt is just visible
at the top of the cocking 'keyhole' but it won't budge any further. I tried to rotate it but I fear this may have been a bad move. Suggestions
on how to proceed are welcome..
Cheers

James

[Phil Russell]

Don't panic. I doubt very much if you have damaged anything. My guess is that the piston is sticking on the disintegrated buffer washer and a grotty o ring on the piston head.
Can you push the piston back in? This sometimes loosens things up a bit so that when you drift (good word that) it again it comes a bit further out.
Also, a squirt of a 'lubricant'(WD40 maybe)) will do no harm both from the rear, applied around the piston body and from the front via the transfer port. Let the lube penetrate before 'drifting' again.
Cheers, Phil

[james_stan]

Well it's out - but not with out damage to teh nylon piston which gouged itself on some burrs I made on the cocking slot hammering the b'stard thing out.
I ran out of patience I think and got a bit brutal which is never ideal. The gouges are on the underside of the seal so in a pinch I guess it could be re-used.

Alternatively I may need to add an alloy piston to my list. Curiously the piston was held in place by two short pins and not one long one - did it break maybe

Thanks for the help and encouragement so far. Why did I think this would be simpler ??
Cheers James

[Airsporterman]

Getting it all back together again with a new spring in can be fun too!

ASM

[Phil Russell]

I think you may be Ok unless the damage to the nylon head has seriously damaged the O ring groove as the head is slightly smaller diameter than the cylinder. The pin(s) are certainly just a single pin. For a new one look at an old HSS drill bit rather than spend a lot on a new one from a spares supplier.
To put rifle back together can be awkward (as above). I either clamp the trigger block in a padded vice and offer up the cylinder or the reverse .. clamp the cylinder and offer up the trigger block. I do not like the 'rest it on the floor and use bodyweight to push down' method.
Cheers, Phil

[junglie]

And don't forget to clean up any damage or burrs to the cocking slot. The inside edge of this must be smooth & burr free or it will damage the new o-ring on re-assembly.A lollypop stick with wet n' dry glued to it is ideal to rub back & forth along the slot for this purpose.

[james_stan]

Thanks I ended up buying a new piston, pin and spring from Chambers (Protek are on holiday this week) before I read about the hss drill trick.. on close examination the pin had broken - quite possibly a while back as it didn't look clean. I have deburred the slot with a rat-tail file but I'll run some emery over it too I think. One question about the 'con-rod' the piston pin hole is significantly lager thin the pin diameter- I hope this is deliberate and not just wear in my case.. Old spring was 33 coils but only 235mm.. I hope to get it back together for the weekend. Can I expect the new piston seal to be hard to get in or
only when it comes to 'spring-time'..

Cheers

[junglie]

From memory the hole is larger to allow for compressing the new buffer washer behind the head. You have to push the piston head down on a flat surface hard to get the pin in usually.

[CapitalBee]

When I recently rebuilt a mk6 airsporter, I replaced the standard BSA O ring with a smaller section ring. This helped assembly and initial performance as I found the standard O ring was a blinking tight fit. I have some spare I think. If you want one drop me a PM.

[james_stan]

When I recently rebuilt a mk6 airsporter, I replaced the standard BSA O ring with a smaller section ring. This helped assembly and initial performance as I found the standard O ring was a blinking tight fit. I have some spare I think. If you want one drop me a PM.

Thanks - I have some BS212 orings as suggested earlier in this thread but if it turns out too tight I'll be in touch! BS210 was mentioned as a smaller option too.