Short stroking vs sleeving?

[Shed tuner]

I personally think many have took adding weight and applied this to all pistons...this is bad. A Diana piston weighs 300g, an 80 Piston weighs 330g...toooooo heavy for sub12. Now a HW55 piston weighs 165g, an old 50S piston weighs 175g or so....this is to light for uk 12

The magic range is 220 to 250g for sub12 depending on spring room...a new 97k piston weighs more than this and a good tune will lighten the piston down, 20 to 25g off and a stroke reduction gets the feel just like a 25mm piston gun

Hey Tony,

I'm not quite ready to disagree with you on this, but I am thinking there's more to it.

(Assume for the moment as we all are on this thread that we are talking 12FP, with a gun that is basically sorted in terms of stroke / bore for 12 FP use.)

I reduce my (non weighted) TX pistons from 220 to ~200, as I find the action snappier (not thumpy).

You are probably right in terms of absolute efficiency (i.e. me doing this will probably cost me something in power, all other things being equal), but I'm not convinced this is the only goal - for me the real priorities are
1) hold insensitivity
2) "feels nice"
3) sensible cocking effort

(I'm taking accuracy and consistency as a given! )

To that end I like the slightly lighter pistons. I also seem to end up having a stoke 2-3 mm longer than I think you'd have, which I guess compensates for the power.

I also like to have a bit of preload (I guess minimum 15KG, sometimes more like 30), which stops bounce, and I find helps consistency.

One experimental TX setup I'm playing with right now has a 165 g piston, and a long spring with probably around 40KG of preload. It is amazingly snappy, gas-ram like, and fine to cock. I need to shoot it more to decide if I prefer it over my "softer" (200g piston) std tunes. But I definitely prefer both of them to the full mass 220g piston.

Pity you are at the wrong end of the country

Rgds - JB

[bigtoe01]

Jon

Not really tinkered with a 200g TX piston build, but i will I have however tinkered with a Diana 440 which now belongs to NoWayNoHow (Andrew), I originally built this gun for Darrin Lynn and it features a sleeved comp tube to 25mm and a massively modified LGV piston with aluminium skirt to drop the weight. I Originally built it at 200g, it was way to snappy, I was able to build a bronze tophat and add a washer or 2 to bring the weight up and around 220 to 225 it became very sweet. The gun is seriously efficient, Darrin had it a few weeks and found cocking it awkward due the fulcrum placement, when he decided to let it go I reworked the power down and the cocking did ease a little, if anything it could stand a bit more off the power again. I seem to remember in the Diana I ended up with 81mm stroke using the standard LGV rod although re profiled to suit the T06 trigger. The shot cycle is fast, almost gas ram fast, however the recoil is soft and no thump at all...very sweet thing to shoot, Andrew's son Elliott is shooting it so if you see him at a HFT match he may have the Diana with him.
Now I did a second 440, this one i attacked very differently. I noticed the Diana52 piston had a longer rod than the 440 piston (5mm) so starting with a 52 piston I reworked the skirt to massively drop weight, added a rear bearing and sleeve and reduced the OD of the piston so it can only ride ont he front seal and the rear bearing...stroke is 77mm and bore is 28mm.
This one again is stunning, i tried a stock 52 piston and it was horrid, 300g house brick thumpy etc.

here is the prototype piston I did for the 440, the final one had stock stroke and no front bearing, this one is short stroked also.

https://dl.dropboxusercontent.com/u/...s/IMG_4654.JPG

here is the piston in Andrew's 440..

https://dl.dropboxusercontent.com/u/...e/IMG_4620.JPG

and the spring/weight/rear guide

https://dl.dropboxusercontent.com/u/...e/IMG_4607.JPG

Im running an LGV spring in the 440's, however im now running development springs in newer builds under test, these are either 32 33 or 34coil and have varying preload of 30 to 50mm depending on spring and gun... I just installed one to my .177 AGT80...running at 10.7fpe its nothing short of incredible to shoot. The springs are capable of more power, its just I have set the 80 to run at this power for consistency and accuracy and its looking like its superb at both.

[TonyL]

I think it was Mick (T20) who recently pointed out on another, similar thread, that one of the reasons why the narrower bore / longer stroke format worked so well, was that, with less surface area, there was less pressure on the piston seal face.

Also, pellet start pressure was achieved when the piston was further from the cylinder end wall, making this architecture less slammy.

Piston weight, obviously, as above, huge factor.

Love the way this thread's developing.

Gotta go for now, head and eyes knackered, will look in again tomoz!!

[bigtoe01]

I think it was Mick (T20) who recently pointed out on another, similar thread, that one of the reasons why the narrower bore / longer stroke format worked so well, was that, with less surface area, there was less pressure on the piston seal face.

Also, pellet start pressure was achieved when the piston was further from the cylinder end wall, making this architecture less slammy.

Piston weight, obviously, as above, huge factor.

Love the way this thread's developing.

Gotta go for now, head and eyes knackered, will look in again tomoz!!

Tony i do remember what Mick was saying, there is more however. The seal you use comes into play also, a properly sized o ring in a good accurate compression tube is about the most efficient seal you can get, you can actually run a shorter stroke with an O ring Vs a parachute style seal for the same power with the same spring.
I found the O rings start compressing air much quicker as the piston starts to move, the parachute seals have to build pressure up to hit efficiency which means for the first few mm of travel air is actually passing by the seal. Now what this means is the bounce point on the piston can be further back with an O ring than it is with a synthetic seal...if this is the case the gun needs less stroke, so you lower peak pressure and you move the bounce point closer to the breech face.
I actually though it would be awesome to have a release valve on the underside of a comp tube that you can set so you could in theory fine tune the peak pressure Vs stroke Vs bounce point and tune the shot cycle, that really would be something imo it would only need a 0.5mm or smaller hole and a small boss welded to the comp tube just back from the breech face on the under side..do it at 10deg from the underside to clear any cocking lever and just have the relief for it in the stock so it can slide back and forth. The valve only needs to be a ball and spring with a spring tensioner so a grub screw in a tube with a ball bearing.

I should Patent this LOL

Anyway, if someone makes one and markets it make sure to give the kudos to where you got the idea

[wonky donky]

I have been working on relief valves to try to fine tune firing cycles mainly in a HW77.

This is the first prototype valve I made wich fits inside the piston head The white stuff on the valve is Molykote.

The early system works in the same way as the Sharp Innova "blow off" valve & is adjustable except mine was semi spring/pneumatic. It does work but only in "O" ring conversions & dovetail fitments & to a limited degree. Later plans were to build a complete new replacement drop in piston with synthetic bearings & a redesigned rod, top hat, spring & guide caliber specific.

[greenwayjames]

"I should Patent this LOL"

"I have been working on relief valves to try to fine tune firing cycles"


Hill and Williams beat you to it by about a hundred years. I have often thought why no one else had followed it up in later years

[bigtoe01]

"I should Patent this LOL"

"I have been working on relief valves to try to fine tune firing cycles"


Hill and Williams beat you to it by about a hundred years. I have often thought why no one else had followed it up in later years

do show please

[greenwayjames]

do show please

Do show what? Hill and williams made air rifles in the early part of the twentieth century. They are real collectors items and scarce as hens teeth. I wish I had one to show

[bigtoe01]

Do show what? Hill and williams made air rifles in the early part of the twentieth century. They are real collectors items and scarce as hens teeth. I wish I had one to show

until i see it how do i know they had the same idea i have?

it would need to be exactly the same idea also remember

so, can you show me what they did?

[greenwayjames]

Calm down dear. I was just informing you that valves in the transfer port operated by the piston had been tried before by Hill and Wiliiams amongst others. I remember seeing an article on them in an old Airgunner. Collectors Corner I think. No one is accusing you of pinching others designs.
There is little that is new in this world

[bigtoe01]

Im completely calm, im interested to see what was done before that is all...its no good saying its been done already and not showing me how its been done

[greenwayjames]

I imagine their patents have expired by now even if the principle of operation is the same. Get yourself down to the patent office an search the old abridgements for patents covering airguns. I believe there were a whole series of articles done on airgun patents going back into the 19th century in Guns review. That may save some legwork

[bigtoe01]

I imagine their patents have expired by now even if the principle of operation is the same. Get yourself down to the patent office an search the old abridgements for patents covering airguns. I believe there were a whole series of articles done on airgun patents going back into the 19th century in Guns review. That may save some legwork

I really can't be arse'd m8, i just dream up stuff and try it, if it works i keep it, if it doesn't i ditch it.

[greenwayjames]

I really can't be arse'd m8, i just dream up stuff and try it, if it works i keep it, if it doesn't i ditch it.

Dixi

[bigtoe01]

Dixi

which means what?