My Mercury Ratting Rig

[TonyL]

Cheers, crowbar.

Good luck with the piston build, Mick. Obviously this is going to be more involved than first envisaged but will prove an interesting journey.

[Shed tuner]

yes it's a shame.. with a minimal element of design, the airsporters and mercs could have been so much better.

- longer stroke
- synth seal, not a massively over-crushed o-ring, or even just plain flat faced leather
- TP dimensions
- reasonable trigger
- breach bolt (why did they drop it !?!? )

None of this would have any material affect on cost

[T 20]

yes it's a shame.. with a minimal element of design, the airsporters and mercs could have been so much better.

- longer stroke
- synth seal, not a massively over-crushed o-ring, or even just plain flat faced leather
- TP dimensions
- reasonable trigger
- breach bolt (why did they drop it !?!? )

None of this would have any material affect on cost

I quite admire BSAs use of an O ring as a piston seal, Jon, as it was quite advanced for it's time as they started using them on the Meteor MK3 in 1969 --- Weihrauch only changed to synthetic piston seals on their 30mm guns in 1981 and their 25mm guns in 1983.
BSAs thinking was good on using an O ring as a piston seal but its use didn't quite work out and I suspect it wasn't all the factory's fault.
The Mercury cylinder is 1.10" ID so naturally BSA used a 1-1/8" (1.125") OD O ring to give 0.025" (25 thou") crush, which sounds a little bit tight but would still work well enough.
Now, I found out to my cost last year that supplied steel is allowed a + or - 1% tolerance which means the seamless tube supplied to BSA for the Mercury and Airsporter cylinders could have varied from 1.089" to 1.111".
If there was that variance in the cylinders it would mean the O ring crush could have been anywhere from 0.014" (14 thou") upto 0.036" (36 thou").
This variance in tube ID still shows up now and again on the forums where someone says their piston seal is a bit tight or a bit too loose in the cylinder.

This always makes me wonder if this could be the reason behind BSA recalling a batch of .22" Scorpion pistols as they were found to be putting out nearly 10ftlbs.
BSA sorted this by making the stroke shorter on the .22" Scorpion than the .177" Scorpion.




All the best Mick