My Mercury Ratting Rig

[T 20]

On a Mk4 Airsporter it's 20mm long plus the loading tap so less efficient but it makes the power with a little love .

That's a very good description, Crowbar --- "with a little love".
Sadly, it seems to me that we have to put quite a bit more love into Airsporters and Mercuries than we have to put into other manufacturers guns with a similar swept volume in order to get them running nicely at 12 ftlbs.

As the Airsporters and Mercuries are what I'd call short stroked guns, I've just nipped out to my workshop and measured the Mercury cylinder to determine what stroke it could support, if I went down that route.
The actual compression part of the Mercury cylinder is 95mm long which is a result of the overly long piston head, this equates to the cylinder being 58.5cc in volume, yet the gun was designed to use just 40cc of that space.
By way of giving a little comparison, the HW80 has a swept volume of 57cc, the Mercury could have been built to the same swept volume with the existing cylinder.

Another thing I find a bit strange is that for export market above 12ftlbs BSA seem to have left the stroke alone but increased the piston weight by fitting a steel piston head.

The Airsporter's being the successor to the pre war Standard has the same stroke, when you get to the introduction of the Mercury in 1971, BSA simply used the piston, cylinder and back block from the Airsporter coupled with the barrel from a Meteor for their new Break Barrel --- a logical way to go but they could have gone a little further.


Anyway I'm off to look at my Mercury a bit more before deciding which way to go with it.





All the best Mick

[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

[Big Andy]

You know what Mick..if I had to get rid of all my rifles the one I would keep would be my Mercury, it’s been fettled by Ray Neil with a buttoned piston and double O rings and shoots lovely. One of the best rifles BSA ever made in my humble opinion, also the one I hankered after as a lad but could never quite afford.

[crowbar]

It will never be a Beat all so will you be Crying with the big "O" ring, Free falling with a Petty parachute seal or
a little piece of leather that's well put together?

Sorry I will take the Valium in future.

[T 20]

You know what Mick..if I had to get rid of all my rifles the one I would keep would be my Mercury, it’s been fettled by Ray Neil with a buttoned piston and double O rings and shoots lovely. One of the best rifles BSA ever made in my humble opinion, also the one I hankered after as a lad but could never quite afford.

I wouldn't argue with you over that Andy, it sounds nice. --- besides which you're bigger than I am and only live up the road.

Unfortunately, as my Mercury is at present it wouldn't get into my top 20 of guns to keep --- it's just plain nastiness itself.

I've tried double O rings in the past after reading a John Bowkett article on tuning the HW77 in AirgunWorld --- I always stick to just the one O ring now as I didn't get on with two.



I was looking at my Merc again this afternoon thinking 70 grams may be a bit over optimistic to remove from the piston weight in order to get the Mercury piston down to 270 grams.
Then I realised I'd already made a piston using an Airsporter latch rod for my prewar BSA, so after a quick strip I had the piston out and on the scales. :-

image.jpeg




By no real coincidence the prewar BSA piston is very close in length to the Mercury. :-

image.jpeg



So I'll definitely be building a new piston for my Mercury when I get time.






All the best Mick

[Big Andy]

I wouldn't argue with you over that Andy, it sounds nice. --- besides which you're bigger than I am and only live up the road.

Unfortunately, as my Mercury is at present it wouldn't get into my top 20 of guns to keep --- it's just plain nastiness itself.

I've tried double O rings in the past after reading a John Bowkett article on tuning the HW77 in AirgunWorld --- I always stick to just the one O ring now as I didn't get on with two.



I was looking at my Merc again this afternoon thinking 70 grams may be a bit over optimistic to remove from the piston weight in order to get the Mercury piston down to 270 grams.
Then I realised I'd already made a piston using an Airsporter latch rod for my prewar BSA, so after a quick strip I had the piston out and on the scales. :-

image.jpeg




By no real coincidence the prewar BSA piston is very close in length to the Mercury. :-

image.jpeg



So I'll definitely be building a new piston for my Mercury when I get time.






All the best Mick

I may be bigger but you are cleverer

[T 20]

I may be bigger but you are cleverer

I doubt that Andy --- anyone trying to tune a gun with a crap trigger isn't that clever.





All the best Mick

[T 20]

I doubt that Andy --- anyone trying to tune a gun with a crap trigger isn't that clever.

And a crap cylinder and a crap 5.6mm barrel as well.

I really must put a wanted ad up for a 5.5mm Lightning or Supersport barrel before I finish machining.

[TonyL]

Any progress on this, El Maestro?

[T 20]

Any progress on this, El Maestro?

I'm still on the case, Tone.

image.jpg



The reason I haven't updated you is because of the simple fact that the Mercury Latchrod isn't long enough to reach the piston head so I'm having to machine the piston in three parts.
The top hat will have to be steel and form part of the piston head with the latchrod threaded into it in order to get the piston length I need.
The top hat section will have to be let into the piston from the front and welded in to keep everything nice and strong.

As I've just finished the latchrod my next job is to machine the piston head to length and bore through it at the spring ID then open up half the depth to a larger ID so that I can leave a step on it for the top hat to sit in.
The idea is that the step stops the top hat pulling through the front of the piston --- once this is all set to length I'll weld the top hat in.

This lot would all be easier to explain with pictures which is why I haven't posted for a day or two.




All the best Mick

[T 20]

The top hat will have to be steel and form part of the piston head with the latchrod threaded into it in order to get the piston length I need.

This lot would all be easier to explain with pictures which is why I haven't posted for a day or two.

Like this :-

image.jpg



image.jpg



Next up the piston body will need finishing to final size while I still have a hole in the front of it for the revolving centre to run in --- then I can weld the top hat in.

[TonyL]

Nice work, sir.

[T 20]

Next up the piston body will need finishing to final size while I still have a hole in the front of it for the revolving centre to run in --- then I can weld the top hat in.

Well that wasn't quite the right order as I missed a step out.

I finished machining the piston body for now and machined the front to let the top hat in. :-

image.jpg

image.jpg

image.jpg

Now this isn't quite ready for welding as I have to put a V groove on the weld line for maximum weld penetration.
But before I get the welder fired up I need to cut the cocking lever slot in the piston as it'll be much easier to do now without the latch rod in.

I was aiming for an all up piston weight of around 275 grams --- As it stands now the piston components weigh in at 290 grams.
Cutting the cocking lever slot will remove between 13.3 to 14.8 grams depending on how long I cut it so I hope to end up between 275 and 280 grams, this will be tried in the gun at this weight for testing.
I could make further weight reductions by fitting Delrin bearings but I will be trying as is for now to see if any further work would improve things --- I am considering increasing the stroke a tad depending on what this piston does.



All the best Mick

[evert]

Impressed by all the piston work you've been able to turn out the last weeks!