In the workshop

Having stripped down the old BSA Mercury Mk2 break-barrel last month, I’m going to now detail how to rebuild it, with some tips on how to smooth it out and how best to correctly lubricate it. In terms of polishing the internals to improve the smoothness of the firing cycle, it’s really quite simple. You only really need to hone the cocking slot milled into the underside of cylinder to make the cocking stroke run smoothly. As it’s only the rear end of the piston that comes into contact with the cylinder wall, that’s the only area you need concentrate on; everything else can be left as-is. As with any polishing, make sure all the abrasive dust is removed from the action – perhaps with an air blower – and with the rifle then thoroughly degreased and all traces of the old oils removed, it’s safe to start to rebuild.


For this restoration project, I’m fitting an V-Mach self-tune kit, but with an additional steel liner that inserts inside the piston to stop all grease from the mainspring being thrown onto the cylinder walls. As I’ve mentioned in previous strip-down articles, care should be taken when fitting this – it’s got sharp edges and is best pushed in by using the wooden edge of the workbench… never the palm of your hand.

When fitting piston liners, I always ensure the join of the liner is positioned right opposite the cocking slot in the piston (so that grease can’t ooze through and defeat the object). However, on the Mercury’s piston, there’s a slit opposite the cocking slot. Therefore, in this case, I offset the liner so that its join is well away from both the slot and the slit.


Now it’s time to refit the piston head buffer washer, and the piston head itself which, as I explained last month, is a separate addition to the piston body. The head is held in place by a small pin and, in the past, I’ve found that some new buffer washers need to be compressed a little in order to line up with the hole in the piston. If this is the case, put some light pressure on the head – I use a sash cramp 202– and you’ll find the retaining pin will simply tap in. Once both the buffer and head are secure, carefully push on the new O-ring – this is the main piston seal.

Rub a light film of grease onto the O-ring, and on the liner in the cocking slot. Now enter the piston into the cylinder, lining up its cocking slot with the corresponding cutaway in the cylinder. 203You’ll need to move the piston from side to side a little while giving it a gentle push to get it past the end threads in the cylinder. This is a little tricky, and you certainly don’t want to ‘snag’ your new O-ring at this point, or you’ll ruin its sealing properties. I use some 35mm film to help protect it.

204205Once past the threaded section, before you push the piston all the way down the cylinder, rub a film of grease on the rear of the piston (where you polished it), along with the piston rod. With the piston now pushed all the way, make up a swab on a dowel rod and rub a light film of grease onto the internal wall of the cylinder, behind the piston. In this rebuild, I’m replacing the standard coils with a special V-Mach mainspring, along with a spring guide and top hat – these slide into place, with the top hat at the piston end of the spring and the guide at the other, ready for the action to be closed. By ‘closing the action’, I mean screwing on the end block – but this must be done with extreme care as there will be a lot of tension on the mainspring. It’s a good idea if someone to help you with this procedure.

Because the Mercury’s trigger end block is such an awkward shape, it’s not easily done using a conventional spring compressor, or sash cramp – and getting the threads started off needs a lot of brute force… and a bit of technique. Here’s how I do it using two mail-order catalogues (a telephone directory would do).

Put the catalogue on the floor with a cloth wad on it to stop the pages ripping up. Then position the cylinder’s breech jaws onto the wad and, with a second catalogue on your chest, press down with your entire body weight to compress the spring. At the same time, you need to turn the cylinder to start the thread on the trigger block – this is where the assistant comes in handy.


The threads are very fine, so pay particular attention to not crossing them – it’s all too easy, under the tension of the spring, to start the block off at an angle and cross the threads. An assistant can help by slowly turning the cylinder as you concentrate on pushing down the block against the spring and keeping everything in line. In any event, keep the pressure on until you are happy that you’ve screwed the block in at least four full turns before you release the tension of the block. Like I said, the threads are fine, and I wouldn’t want to rely on the block holding up against the strong mainspring with just a few threads.


Fully tighten the trigger unit using a Tommy bar through the hole – but, again, you need to keep a watchful eye on what you’re doing. When screwing the end block/cylinder, the piston can sometimes turn slightly inside, thus putting its cocking slot out of alignment with the cocking slot in the cylinder.208209 If this happens, you can realign it using a flat-bladed screwdriver. I have a large screwdriver that fits perfectly into the hole at the end of the cocking slot, and simply turning it aligns everything.

Of course, you might think that fitting the barrel and cocking linkage before compressing the spring could stop this problem – but this would make closing the action trickier, in my opinion. If the breech were to ‘break’ open as you’re bearing down on the end block, you could end up having a nasty accident… or at least an embarrassing one, head first on the floor.



210If the old breech seal has degraded, fit a new one. Then rub a film of grease onto the insides of the breech jaws, and also the barrel latch and spring. Refit the latter into the barrel unit. Now, put a dab of grease onto the end of the cocking linkage and enter into the circular cut-out in the front of the cocking slot, following up by carefully sliding the barrel breech block into the jaws on the cylinder so that the hole in the jaws aligns with that through the breech block.


Before inserting the breech axis pin into this hole, put a liberal helping of grease on it. As the barrel latch is also held in place by this breech axis pin, you’ll have to put some pressure on it (the latch) so that the pin can slide all the way through. The best way is to use some dowel rod; I use a plastic pusher and then simply push the axis pin all the way through with my thumb.212

Finally, put a drop of oil or very light grease on the cocking linkage barrel axis pin and trigger sears – but don’t overdo the trigger. Too much moly grease can make the trigger very light and unpredictable, particularly given the age of this gun and the amount of wear the sears will have undergone. Wipe the action down, refit the stock and check the rifle’s power over a chrono.


In the case of this rifle, the power readout was extremely healthy and the firing cycle was surprisingly smooth for such an ‘old timer’, bearing in mind this particular rifle dates to 1978. With the V-Mach innards, its recoil was extremely light and there was no spring ‘twang’ whatsoever.

It was also very consistent over my chronoscope – which just goes to prove that BSA’s Birmingham airgun-making plant knew how to make a good gun even all those years ago. Like I said last month, the Mercury is a good punt if you see one going cheaply in your local gun shop.


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One comment on “In the workshop
  1. Max says:

    Thanks for this most useful article. Probably didnt do as good a job as you but all in all I’m very satisfied with my first attempt and managed to make a reasonable jig to compress the spring. Thanks again, Max 24/08/13

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