Short stroking vs sleeving?

[Arthur John Smithsplease]

I understand the principal of short stroking by adding to the piston head or increasing the length of the piston rod.

If someone could explain to me in simple language what is entailed in the alternative approach ie. re-sleeving the piston/compression chamber (not sure which), I would be grateful.

I know that it is a much more complicated procedure, but I'm not entirely sure what it involves and why.

[bigtoe01]

Consider a hw80, 80mm stroke 30mm piston, swept volume is 56.5CC

if you push the power plant to max efficiency it will produce around 22fpe in .22, maybe a tad more (seen one at 24fpe)

Note I say max efficiency

now, for sub12 that same 80 will be running miles away from being efficient,so we need to change things.

You could massively short stroke it down to 55mm stroke to give 39CC, this does work, but the piston is too heavy. So you can sleeve the compression chamber down from 30mm to 25mm and use a 25mm piston, if we use the same 80mm stroke we get around 39cc which just happens to be the start of where things start getting good. Now the key is to have the right piston weight also, when you reduce the piston size down you also reduce the weight, for sub12 the efficient weights are around 220g to 250g.

You can of course fine tune the stroke to peak the efficiency, around 70mm is all thats needed for .22, i usually tweak around 70 to 73 for .22, 73 to 75 for 20cal and 75 to 78 for .177 and .25. Now note I set the stroke for 25cal the same as .177, the reason for this is 25cal is greedy on air, you only see this when you start testing with low swept volumes. This is one reason why 25cal sub12 80's can be very nice, lighten the piston to 280g or so and increase the TP dia a little and the rifles are quite efficient even at 12fpe and have nice shot cycles.

So, sleeving is all about reducing the swept volume while allowing the correct stroke to bring the efficiency of the power plant for the power output you are looking for. For 16/17fpe you could sleeve to 26mm, for 19fpe you could sleeve to 28mm and again keep stroke between 70 and 80mm and play with piston weight to get the shot feel nice.

I have no doubt a 26mm sleeved 80 at 16fpe in .22 with a 250g piston would be superb, maybe if i ever go FAC i will try it

[Arthur John Smithsplease]

Many thanks Tony.

I think I understand a bit better now.

Big job though.

[Arthur John Smithsplease]

I can't help wondering why the HW80 has been such a good seller at 12ft lbs.

Lots of people swear by them.

[evert]

Now note I set the stroke for 25cal the same as .177, the reason for this is 25cal is greedy on air, you only see this when you start testing with low swept volumes. This is one reason why 25cal sub12 80's can be very nice, lighten the piston to 280g or so and increase the TP dia a little and the rifles are quite efficient even at 12fpe and have nice shot cycles.

You can see the .25's low efficiency in springers at FAC levels as well, thats at least my experience.

[walnutfarmmick]

Many thanks Tony.

I think I understand a bit better now.

Big job though.

+1 ^^^^^^^

ATVB, Mick

[greenwayjames]

I can't help wondering why the HW80 has been such a good seller at 12ft lbs.

Lots of people swear by them.

Probably because it is easy to make them naughty

[bigtoe01]

Probably because it is easy to make them naughty

I think this reason and some clever nostalgia/marketing. many just look at JD having an 80 for hunting with an optima super moonlighter, they forget the 80 was sleeved to 25mm and he mostly used a Tasco AG...its all in the marketing

Its like HW's advert for the 97, they remind us it wins the FT or HFT events, they don't remind us the gun that won was massively modified, most run old HW77 power plants (25mm)

Getting back to the 80 and sleeving it, they shoot so nice when done because of the weight also, the few i have done for mates are all custom now, serious stocks with work on balance and barrel weight/shrouds to further increase weight, the .22's almost feel dead to shoot.

Sleeved 80's really are something quite special, anyone with a standard 80 at 12fpe should try one at some time, they are very very different..

Now consider the Diana 52, another power dinosaur, 105mm stroke 28mm piston (300g) i sleeve these to 25mm also, 250g or less pistons with 70 to 77mm stroke, they weigh around the same as a 97 and imo are a better gun, stupidly accurate. I actually think the 52 could be a serious contender for the HFT worlds in the right hands with it sat in the right stock.

[greenwayjames]

"Now consider the Diana 52, another power dinosaur, 105mm stroke 28mm piston (300g) i sleeve these to 25mm also, 250g or less pistons with 70 to 77mm stroke, they weigh around the same as a 97 and imo are a better gun, stupidly accurate. I actually think the 52 could be a serious contender for the HFT worlds in the right hands with it sat in the right stock."


I remember an article by one of the writers in Airgun World and it was a test on a 52 that Bowket had short stroked, sleeved the bore down and fitted one his universal 24 coil mainsprings The writer was impressed enough to buy it. That was a couple of decades ago. He thought it worth the effort of sleeving them down as he also sleeved HW80s and 35s.

[Airsporter1st]

I can't help wondering why the HW80 has been such a good seller at 12ft lbs.

Lots of people swear by them.

Most people's perception is that at 12 ft/lbs the HW80 is particularly understressed, resulting in excellent reliability.

It is the 'desert island' gun of choice for many people.

[bigtoe01]

"Now consider the Diana 52, another power dinosaur, 105mm stroke 28mm piston (300g) i sleeve these to 25mm also, 250g or less pistons with 70 to 77mm stroke, they weigh around the same as a 97 and imo are a better gun, stupidly accurate. I actually think the 52 could be a serious contender for the HFT worlds in the right hands with it sat in the right stock."


I remember an article by one of the writers in Airgun World and it was a test on a 52 that Bowket had short stroked, sleeved the bore down and fitted one his universal 24 coil mainsprings The writer was impressed enough to buy it. That was a couple of decades ago. He thought it worth the effort of sleeving them down as he also sleeved HW80s and 35s.

I have not seen those articles but i fully imagine JB would sleeve the 52 down. To date I have sleeved down to 25mm:

Magnum springers:
HW80
Diana 52
Diana 54

Mid power springers:
Diana 440
Diana 430

Short stroked:

magnum springers
HW80
Diana 52

Mid power springers:
HW97/77
HW95
HW57
HW85 (old)
HW35
Webley Tomahawk
Cometta 400s
Diana 280k

The magnum springers work better sleeved however due to piston weight.

[greenwayjames]

Just come off the phone after a long chat to JB about reduced bore size air rifles. He said that was the way ahead and he convinced Gamo the Hunter 1250 was not the ideal way to go for US markets. The 52 that was tested had the stroke shortened by about an inch as well as the cylinder being sleeved. The gunwriter wanted to do a back to back test against a 52 that another well known designer had done his conversion on. That person favoured lighter pistons but Bowket heavy. The magazine apparently said no. He told me he made a scratchbuilt air rifle with a 7/8" bore and stroke about 3 3/4" which showed promise but that was superceded by a rather different approach to reducing the front area of the piston to gain the benefits. This design was sketched out in one of his AGW articles and the result was a call from Fredrik Axelson saying he was trying to sell the same layout to a well known, now defunct, gunmaking company. I found that amusing, a case of great minds thinking alike. Quite an interesting and informative chat.

[bigtoe01]

Just come off the phone after a long chat to JB about reduced bore size air rifles. He said that was the way ahead and he convinced Gamo the Hunter 1250 was not the ideal way to go for US markets. The 52 that was tested had the stroke shortened by about an inch as well as the cylinder being sleeved. The gun writer wanted to do a back to back test against a 52 that another well known designer had done his conversion on. That person favoured lighter pistons but Bowkett heavy. The magazine apparently said no. He told me he made a scratch built air rifle with a 7/8" bore and stroke about 3 3/4" which showed promise but that was superseded by a rather different approach to reducing the front area of the piston to gain the benefits. This design was sketched out in one of his AGW articles and the result was a call from Fredrik Axelson saying he was trying to sell the same layout to a well known, now defunct, gun making company. I found that amusing, a case of great minds thinking alike. Quite an interesting and informative chat.

so he went 80mm stroke and 25mm bore on the 52, i tried that, then pushed down to 75 for .177 as it just felt that tad better, however piston weight, i also tried 280g and 300g 25mm pistons and they feel thumpy, the 250g piston i now use is much much nicer.

Anyone any idea about this AWG article and got pics...would love to see what he was up to back then?

I have heard about his 7/8th bore rifle, i have a 24mm test set up here that eventually i will have a go at finishing, right now its to heavy though so i need to remake the piston. I was initially going to try 85 80 and 75mm stroke to see how they did.

[greenwayjames]

would love to see what he was up to back then?

Why dont you ring him and ask. Leave a message. He always gets back - to me at least

[bigtoe01]

would love to see what he was up to back then?

Why dont you ring him and ask. Leave a message. He always gets back - to me at least

I like the old articles, i used to collect AGW as a kid and loved the JD hunting articles but stupidly sold them on and did not shoot between 20 and 36 so I have 16 yrs that I have missed.

I may call him, really though the old articles are what i like being honest