BSA Mercury Seized Piston

[Hsing-ee]

What is the best way to loosen a seized piston in a BSA Mercury? I am assuming a plastic piston head in this case, or at least Birmingham Cheese buffer washer welded to the compression cylinder.

[thisisdonald]

I just got round to putting one of these back together today.
Mine is a mk1 and the plastic head had swollen and the steel ring had jammed it solid.
I tried days of different loosening concoctions before hammeri g it out with a long screwdriver!
This obviously damaged the cocking slot, but as i was making a new, shorter piston head, it didnt matter as i had to extend the cocking slot anyway.
I managed to get the piston into my quick change toolpost holder and get a milling but into the lathe chuck for that job.
Good luck with yours..they can be a bitch..if i had the tools and materials i would have tried to make a puller of some kind to attach to the latch rod

[Shed tuner]

hmm.. just thinking aloud.... get a length of say 10mm or 12mm ID 3mm wall tube. Drill and tap a 6mm grubscrew through one side, near the end. Insert said tube down cylinder, over piston rod. Tighen grub screw to engage in the notch where the sear goes. Drill a hole through the top end of the steel tub and put in a T-bar handle or somesuch. Hit / pull / twist on the handle.

Or use a flat steel say 6mm thick that will pass through the cocking slot, and engage in the sear notch, and hammer down.

[Hsing-ee]

hmm.. just thinking aloud.... get a length of say 10mm or 12mm ID 3mm wall tube. Drill and tap a 6mm grubscrew through one side, near the end. Insert said tube down cylinder, over piston rod. Tighen grub screw to engage in the notch where the sear goes. Drill a hole through the top end of the steel tub and put in a T-bar handle or somesuch. Hit / pull / twist on the handle.

Or use a flat steel say 6mm thick that will pass through the cocking slot, and engage in the sear notch, and hammer down.

That is a clever idea. I will try Guy's WD40 cure first and then try Jon's pipe-bomb method if it doesn't release.

[Vic Thompson]

I use a Large Headed 6" long coach bolt when I come across BSA's with seized pistons. One side of the head is ground down to fit through the cocking slot and engage with the piston, whilst the threaded end is packed with washers capped by a nut. I use a shim under the bolt, hold it against the body tube and do the nut up. That in my experience shifts any stuck piston without causing damage. If it's a short thread on the bolt you have to keep adding washers as you tighten, but I've cut a thread all the way down the bolt and this will pull the piston all down untill the head of the bolt reaches the end of the cocking slot.

HTH

Vic Thompson.

[thisisdonald]

Hi vic.. i canny get my head around that... any chance of a wee sketch?

[Hsing-ee]

I use a Large Headed 6" long coach bolt when I come across BSA's with seized pistons. One side of the head is ground down to fit through the cocking slot and engage with the piston, whilst the threaded end is packed with washers capped by a nut. I use a shim under the bolt, hold it against the body tube and do the nut up. That in my experience shifts any stuck piston without causing damage. If it's a short thread on the bolt you have to keep adding washers as you tighten, but I've cut a thread all the way down the bolt and this will pull the piston all down untill the head of the bolt reaches the end of the cocking slot.

HTH

Vic Thompson.

Genius! Like a bearing-puller...

[magnummike]

I like that one Vic as I've struggled with a few Merc's recently

[Vic Thompson]

I've sent a PowerPoint Sketch across to thisisdonald with a request that he host it if possible as I don't know how to do so myself. In the event that he can't do so, and if anyone else can oblige, please let me know and if necessary I can forward the Sketch on to you.

Thanks, Vic Thompson.

[A9TPK]

When mine was stuck I reversed the head on a sash cramp (which is normally used as a spring compressor) then attached strong nylon cord to the latch hook at the end of the piston and the screw head on the cramp (having first screwed it right down). By tightening the sash cramp it was effectively turned into a puller and I managed to pull out my piston which until then had resisted all other attempts to remove it.

My problem now is how to remove the ring of old buffer washer left behind which is stuck to the side of the cylinder. Any suggestions? I have tried wet & dry but am concerned the i seem to be removing metal either side of the ring but not the ring itself.

Terry

[copperjacket]

Good advice thank you.

Mercurys seem to be fashionable again - I have 2 under way for my children. One is all but complete, the other I thought I would try to to do away with the buffer washer and effectively stroke but made a mess of drilling a new securing hole. I suspect I may need to source a new piston.

I am going to put open sights on both - still have them after all these years but have managed to loose the screws.

[Benelli B76]

Its amazing that BSA did not know that nylon expands due to absorbing liquid (hydrolysis). I am forever turning down these old piston heads to get them back to cylinder size. You should soak nylon in warm water (conditioning) before arriving at the the finished dimensions at the factory. The alloy piston heads seem to jam due to continual smashing against the end of the chamber.

Baz

[Shed tuner]

Hi Vic, yeah, nice idea, I see that working too...

[les allam]

If you have no luck with the above methods, I have a banding wire tensioning tool that I used on my BSA scorpion pistol ,that had a seized piston .I am only in Barking.
Regards
Les..

[thisisdonald]

Received and hosted here.... thanks mate