Educate me on Piston Weight

[Shed tuner]

Thanks Jon Well chuffed! loving the short port shot cycle! Many thanks.

this weight loss changed the consistency of these jsb's as well. Was 6fps spread for 8.4, now 10. Was 3fps for the 7.9, now 5. Was 7fps for the 7.3, now 3. Again, will check after a few tins.

Cool, I figured you would lol

Yeah, losing piston weight will reduce consistency... adding preload will get it back, but go easy on the power. Longer, weaker spring can help, but more likely to slam. Your numbers are excellent, so just shoot and enjoy

[bigtoe01]

Diana 440TH, 28mm piston, 76mm stroke, 4mm TP 5mm long, piston weight 215g all in inc seal, shot cycle is like a schung!!! when you fire it, barely moves, follow thru on 35yard targets .177 is easy, you can watch pellets drop off targets.

Diana 440TH, 25mm piston, 80mm stroke, 255g piston, TP as before, near as good BUT different, more dull thud like a 97, i like it but not as much as the one above...will make an alloy piston for this one.

Diana 52, .22, 75mm stroke, 25mm piston, 265g, 4mm dia 5mm long TP, shot cycle fast, smooth, very slight recoil, best .22's i have...

All these guns are better than my 97's, and way better than the TX I had (which was nice but you guys have pushed that forward now) and the reason?


The ultra short transfer port

and the fact Diana comp tubes are machined from a billet so never leak (apart from the port seal)

[brucegill]

Cool, I figured you would lol

Yeah, losing piston weight will reduce consistency... adding preload will get it back, but go easy on the power. Longer, weaker spring can help, but more likely to slam. Your numbers are excellent, so just shoot and enjoy

Cheers Jon. Can't put my finger on the differences in shot cycle between short and long ports. Seems you can run a shorter stroke yet it feels nicer. Smoother and less recoil?

[bigtoe01]

Cheers Jon. Can't put my finger on the differences in shot cycle between short and long ports. Seems you can run a shorter stroke yet it feels nicer. Smoother and less recoil?

makes power easier as its more efficient.

now we need someone brave enough to machine a comp tube for no transfer port (.177 only) with the comp tube sealing using an O ring machined onto the end of the barrel. The cocking arm would need to be shorter also for the shoe to work ok.

[brucegill]

makes power easier as its more efficient.

now we need someone brave enough to machine a comp tube for no transfer port (.177 only) with the comp tube sealing using an O ring machined onto the end of the barrel. The cocking arm would need to be shorter also for the shoe to work ok.

Ahhh, cheers Tony, that makes sense.

Like that idea. When I work out how to cut threads on the lathe I'd give it a go

[bigtoe01]

Ahhh, cheers Tony, that makes sense.

Like that idea. When I work out how to cut threads on the lathe I'd give it a go

dont ruin a good comp tube Bruce, better to make a new one me thinks

[brucegill]

Lol, you know it! Just can't find any 30mm OD 2.5mm wall tubing anywhere

[Shed tuner]

I think you described it pretty well Bruce, more power, but softer / calmer that the long port, short stroke guns. As Tony said it's simply more efficient. My 5.3 port (shortened both ends) port is even better, and is really hard to keep under 12 at just 75mm stroke. Good stuff

Ref the "no-port" variant, I've already o-ringed the barrel in preparation, but as you say machining the threads off the end of a comp tub is a bit of a pain as it means shortening the cocking arm to cope for the "further forwards" locked up position

Alternative is to machine a barrel to sit even further back in the action (and hence comp tub) - this is they way I'd do it, no cocking arm / comp tub thread machining - just bore thru the end plug and as you say Tony o-ring the barrel to seal. I've a scrap .177 (failed poly experiment) barrel, so might do this on my "playing" gun. Also got a scrap end plug, so no real financial loss !

I think though I'm more tempted to go for a 1mm port (no conventional breach seal, same barrel o-ring), such that you can radius the inside edge for flow, as opposed to being at the mercy of the position of the pellet skirt in the barrel. Would also stop crap getting into the comp tube.

Another sealing option with the 1mm port variant would be an annular o-ring groove in the face of the (full OD) barrel, but a bit too tricky for me to machine.

Gonna sleep on it for a few more days before starting work

[brucegill]

I just can't find the bloody tube for a new comp tube anywhere

on your 5.5 TP rifle, do you just run one oring in the breach (or a HW seal)? I might have a go on a tatty mk1 I have here as I could just shorten the end catch rather than the UL (the balls in the UL Not the catch on this one )

[Snooper601]

Hi Bruce, item number 121170069408 on egay.

Cheers

John

I just can't find the bloody tube for a new comp tube anywhere

on your 5.5 TP rifle, do you just run one oring in the breach (or a HW seal)? I might have a go on a tatty mk1 I have here as I could just shorten the end catch rather than the UL (the balls in the UL Not the catch on this one )

[T 20]

Hi Bruce, item number 121170069408 on egay.

Cheers

John

Hi John

I was just going to suggest that Bruce contacted you, but you beat me to it.



All the best Mick

[brucegill]

Hi Bruce, item number 121170069408 on egay.

Cheers

John

Thanks John! Missed that one lol. Much appreciated.

Anyone want to go half's or even thirds on it lol. £77 posted for 3m of the stuff....

[Shed tuner]

on your 5.5 TP rifle, do you just run one oring in the breach (or a HW seal)? I might have a go on a tatty mk1 I have here as I could just shorten the end catch rather than the UL (the balls in the UL Not the catch on this one )

Yeah, just one TX o-ring. Hence I moved the (shrouded) barrel back to take up the extra 2.4 mm.

For the no-port option, it's not about shortening the underlever, it's the cocking arm, as the comp tube would be further forwards - unless you do as I suggested and bring the barrel back - which is easy on the shrouded ones, but a pain on the Mk1/2s - unless you shorten the underlever too lol

[Snooper601]

Thanks John! Missed that one lol. Much appreciated.

Anyone want to go half's or even thirds on it lol. £77 posted for 3m of the stuff....

Go on then, I'm going to need a 5hit load when I get my lathe home from Mick's anyway to make TX tubes and 25mm HW77/97 tubes

Cheers

John

[Shed tuner]

Thanks John! Missed that one lol. Much appreciated.

Anyone want to go half's or even thirds on it lol. £77 posted for 3m of the stuff....

I'll pass thanks, I'm thinking of outsourcing the work...

John, you busy ?