Reducing weight of pistons

[HW777]

Having just picked up a 25mm Hw77 piston from Mach 1.5 Thanks mate, I was thinking about reducing the weight of this piston, but how far could I go before it becomes unviable to produce decent power, starts to slam or bounce upon firing

I would also like to do this with an Hw80/85 pistons, I know there are a lot of rifles with over heavy pistons, so no matter which rifles piston you may have done this too, I'd like to hear your thoughts and what weights you ended up with on the pistons

ATB

John

[evert]

You can easily go too light.
After taking the TX-piston down to about 200grams, I added some tophat weight to get it up to around 220-230grams if I remember correctly. It feels better with the added weight.
This gun has the stroke set around 84mm's.

[cooper_dan]

There is quite a strong relationship between the ideal piston weight and the piston diameter (and spring pre-load).
I don't know actual numbers but for example (maybe other people could chip in with weights):

22mm = 160g
25mm = 220g
28mm = 280g
30mm = 350g

There's a lot of messing with springs you can do as well so it's not just weight dependant.
Think my 25mm is about 220g

[Shed tuner]

There is quite a strong relationship between the ideal piston weight and the piston diameter (and spring pre-load).
I don't know actual numbers but for example (maybe other people could chip in with weights):

22mm = 160g
25mm = 220g
28mm = 280g
30mm = 350g

There's a lot of messing with springs you can do as well so it's not just weight dependant.
Think my 25mm is about 220g

that's a good rule of thumb, but if user longer softer springs (as I prefer to do), you can go 10% lighter and get excellent results with a bit less recoil.
Although I do think you 30mm is a little high - probably more like 320g (less 10% for me )

[HW777]

There is quite a strong relationship between the ideal piston weight and the piston diameter (and spring pre-load).
I don't know actual numbers but for example (maybe other people could chip in with weights):

22mm = 160g
25mm = 220g
28mm = 280g
30mm = 350g

There's a lot of messing with springs you can do as well so it's not just weight dependant.
Think my 25mm is about 220g

Even though it would be only milliseconds, I am looking to speed up the lock time, as I think it aids accuracy

I think the 25mm 77 piston is 220g so in theory I could lighten a little and play with piston weights but I think I'd rather braze on a bronze rear bearing and add Delrin inserts

[Shed tuner]

just did a little table showing the various piston surface areas relative to a std 25mm TX, and assuming std TX optimum piston weight of 220g

Pretty close to Dans data, except for the 30mm, as I guessed:

diameter area percent of TX weight std weight long spring
22....... 380...... 77%..... 170...... 153
25....... 491...... 100%.... 220...... 198
28....... 616...... 125%.... 276...... 248
30....... 707...... 144%.... 317...... 285

[cooper_dan]

just did a little table showing the various piston surface areas relative to a std 25mm TX, and assuming std TX optimum piston weight of 220g

Pretty close to Dans data, except for the 30mm, as I guessed:

diameter area percent of TX weight std weight long spring
22....... 380...... 77%..... 170...... 153
25....... 491...... 100%.... 220...... 198
28....... 616...... 125%.... 276...... 248
30....... 707...... 144%.... 317...... 285

Data is a very strong word for the random numbers that fell out of my head. I would take your numbers over mine

[HW777]

that's a good rule of thumb, but if user longer softer springs (as I prefer to do), you can go 10% lighter and get excellent results with a bit less recoil.
Although I do think you 30mm is a little high - probably more like 320g (less 10% for me )

When it comes to soft and long springs = coil bound or as close as and depending on pellet choice, I have had piston bounce

But what is the next step down from Titan or Vmach ?

[evert]

...I have had piston bounce....

But is that really a problem? And how do you measure it? Energy variations between pellets?

If you have a reasonably fast shotcycle that feels nice and soft to shoot, AND gives good stable accuracy with at least one available pellet, does it really matter that you have some theoretical piston bounce?

[Mach 1.5]

I have a 97 running a 25mm internals. VMach Spring, guide and top hat. 6 holes have been drilled into the piston, not sure on the weight and who by. Yes it's shoots really lovely but the piston resonates in the comp tube. Mach 1.5

[TonyL]

I have had piston bounce

?

You simply can't argue with physics. So, as recoil is natural and unavoidable, on most designs so will piston bounce, as I'm sure you're aware. Some designs / set-ups will deliver a smaller degree of piston bounce, in terms of its magnitude and also in the transition from recoil to surge, which I guess is quite crucial? Wonder if a very low power design with certain set-ups might not give piston bounce? Wonder if it's ever been tested? But it probably won't be of the design that many of us may shoot. As I'm sure you're already well aware, getting that cycle just "right" is one of perceived feel and comes from a harmonious balancing of so many different factors......spring preload, rate & force, piston weight (and its sectional density), bore, stroke, seal design, friction, transfer port diameter & length, pellet weight, friction and start pressure........

I'm happy if they feel "refined" - so smooth feeling and twang free. And, for most of us, the main thing we can measure (and also the most important) is accuracy. And real world, attainable accuracy, not just off the bench. I know you know this anyway, so not preaching to the converted and knowledgeable.

[greenwayjames]

During the 1980/90s Bowkett carried out many hundreds of low recoil short stroke HW77 conversions using his twin ringed piston head to retain lubricant between the rings, opened up transfer port, 24 coil spring made in batches for him by Mike Wade and heavy steel top hat within the piston to add weight and balance things out. They were quiet and soft shooting. He told me that the secret was in the balance of the internals. A lot of what he did goes against current thought but it worked. As he last worked on a spring air rifle three decades ago we will never be able to shoot a freshly JB converted 77. .

[Shed tuner]

During the 1980/90s Bowkett carried out many hundreds of low recoil short stroke HW77 conversions using his twin ringed piston head to retain lubricant between the rings, opened up transfer port, 24 coil spring made in batches for him by Mike Wade and heavy steel top hat within the piston to add weight and balance things out. They were quiet and soft shooting. He told me that the secret was in the balance of the internals. A lot of what he did goes against current thought but it worked. As he last worked on a spring air rifle three decades ago we will never be able to shoot a freshly JB converted 77. .

yes.. heavier piston work best with stronger springs (obviously somewhat shorter to keep it legal/smooth) and lighter with longer, softer springs.

The two firing cycles are quite different... I prefer the latter, but both work - as stated, it's all about balance.

What doesn't work is heavy pistons and soft springs, or light pistons and short strong springs.

[HW777]

You simply can't argue with physics. So, as recoil is natural and unavoidable, on most designs so will piston bounce, as I'm sure you're aware. Some designs / set-ups will deliver a smaller degree of piston bounce, in terms of its magnitude and also in the transition from recoil to surge, which I guess is quite crucial? Wonder if a very low power design with certain set-ups might not give piston bounce? Wonder if it's ever been tested? But it probably won't be of the design that many of us may shoot. As I'm sure you're already well aware, getting that cycle just "right" is one of perceived feel and comes from a harmonious balancing of so many different factors......spring preload, rate & force, piston weight (and its sectional density), bore, stroke, seal design, friction, transfer port diameter & length, pellet weight, friction and start pressure........

I'm happy if they feel "refined" - so smooth feeling and twang free. And, for most of us, the main thing we can measure (and also the most important) is accuracy. And real world, attainable accuracy, not just off the bench. I know you know this anyway, so not preaching to the converted and knowledgeable.

Perhaps I am becoming preoccupied with a shot cycle that is as quick as possible, while still maintaining all the characteristics of a well tuned rifle

A lightened piston will speed up the shot cycle but I will have to try and find the balance between a number of factors

Spring quality being one of them, I'm beginning to think I have trust issues with any other springs than the two main stay springs I use

[TonyL]

Like your good self, I believe that the characteristics and quality of a spring are such hugely important factors in the feel and power production. I suppose a cynic would simply comment, "Of course it will, numb nuts.....it is, after all, a SPRING powered air rifle". But it's also oh so easy to go tearing down other avenues like the bore and stroke and piston weight etc etc.....But I firmly believe that simply playing about with, and substituting, different springs can make such a huge difference.

Like your good self, I do really, really like the V-Mach springs. But I have also sampled SFS HW95 springs and rate those. As I do a good few of the WonkyDonky ones I have tried. Especially the one in my '99.

For some guns, the standard springs can also be superb, again, the 95 and 99.

And following Jim's advice, many have now tried the TX Mark III spring in various rifles / tunes with very pleasing results.