Reply With QuoteWelcome to Airgun BBS. The piston is the same as the Mk V and VI airsporter and you should be able to pick one up on here or on a well known auction site. Just tap the pin out on the piston head. You could modify the barrel hinge to take a bolt like the Mercury S. Most of the pins and washers etc are available (at a cost) from either Chambers or Knibbs. Chambers used to do a kit, spring, seal, guide, top hat, and lube (£52.21). So you are probably looking at £70-£80 to refurb it. Dep[ends on condition and how far you take it. But if it has good memories , does it matter. Probably not worth too much more than what you would spend on it. Don't see too many Mercs on here but there must be a few about. I have been looking for some parts for an S model (or a donor gun, any other than MK1 and 2) for a while with no result. Don't think a Diana 23 would have much effect on a rat unless you had the barrel down it's throat.
atb
dogsbody
. You can close the breech jaws up fairly easily on a Mercury as they are quite soft. I have used a small screw and nut with 2 washers. Put one washer on the outside of each jaw, put your screw though and wind the nut on.If you hold the nut with a spanner and use a screwdriver on the screw head you will see the jaws start to close up. Repeat till the breech block feels a good fit in the jaws. If you are on about the piece of plastic that sits in the cocking arm, that the pivot pin goes through, then a small tap washer or bit of leather with a small hole in will do the job of locating the pin. 
