Servicing a 70s Mercury - Spring Choice

[ZAKDINGEL]

as far as i remember the serial number is stamped on the underside of the breachblock, next to the cocking link, starts with a z , hope you see this.

bsa serial number dates (z mk1 1971-1973)---(za mk2 73-74)--(zb mk2 74-78) (zc mk3 78-85) Hope this helps you

[leetrenchard]

Rebuilt mine with standard chambers spring and.seals, shoots v sweetly at 10.5 in .22

[TonyL]

Hi James,

And a very warm welcome to the forum.

A most splendid resource and with many knowledgeable, helpful folk.

And welcome to the fun and rewarding hobby of springer tinkering.

Personally, and as many above, I'd go for the standard spring. Sources for all the parts you'll need (some mentioned above) are Protek (yes, there's a spares section on there), John Knibbs, Chambers and TR Robb.


If you were ever to need to shorten a spring, there's an excellent article in the March issue of Airgun World on this very subject.

[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..

[Rellum]

Rebuilt mine with standard chambers spring and.seals, shoots v sweetly at 10.5 in .22

Ditto nothing wrong with Chambers stuff if a little expensive