How light can you go with the piston?

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tim
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Joined: Mon Apr 27, 2015 1:09 pm

How light can you go with the piston?

Sun Apr 10, 2016 2:41 pm

I know you will lose power if you reduce the weight of the piston considerably but by how much?
Making a piston from Titanium retaining a steel latch rod you should see a weight saving of about 95 grams which is a saving of about 36% on the standard set up and further savings can be made by replacing the front guide with a guide made from Delrin, again I this should save about 10 grams. So a total weight saving of 105 grams.
I am sure this weight reduction will result in a major loss in power but also a reduction in recoil.

Has anyone done any work in this area if so what do you recommend as a limit to the weight saving and which way did you go with the spring?

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KeithS
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Re: How light can you go with the piston?

Sun Apr 10, 2016 5:55 pm

An interesting post matey ..

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gary martin
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Joined: Thu Feb 19, 2015 8:21 pm

Re: How light can you go with the piston?

Sun Apr 10, 2016 6:17 pm

tim wrote:I know you will lose power if you reduce the weight of the piston considerably but by how much?
Making a piston from Titanium retaining a steel latch rod you should see a weight saving of about 95 grams which is a saving of about 36% on the standard set up and further savings can be made by replacing the front guide with a guide made from Delrin, again I this should save about 10 grams. So a total weight saving of 105 grams.
I am sure this weight reduction will result in a major loss in power but also a reduction in recoil.

Has anyone done any work in this area if so what do you recommend as a limit to the weight saving and which way did you go with the spring?


my piston weighs 176g inclusive of latchrod. this is 24.8% lighter than the std piston i had. not too sure how light you could go, but too much will result in piston bounce.
there are also other factors involved, transfer port dimensions, weight of pellet, pellet composition, the list is in all likelyhood more extensive. i know a couple of very well respected tuners, who, i am guessing would struggle to give an accurate answer. they prob have a gut feeling via expeirence.

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tim
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Re: How light can you go with the piston?

Mon Apr 11, 2016 4:00 pm

my piston weighs 176g inclusive of latchrod. this is 24.8% lighter than the std piston i had. not too sure how light you could go, but too much will result in piston bounce.
there are also other factors involved, transfer port dimensions, weight of pellet, pellet composition, the list is in all likelyhood more extensive. i know a couple of very well respected tuners, who, i am guessing would struggle to give an accurate answer. they prob have a gut feeling via expeirence.[/quote]

Gary Thank you for the information. With your piston reduced what did you do with the spring
At the moment I am just thinking out loud, reducing the piston weight as you say will Introduce piston bounce but at what weight will piston bounce occur?
Again is this consistent or will the variables of pellet tolerances change the point at which the piston bounce occur?
Transfer port size although constant on my gun can differ between guns with manufacturing tolerances.
I have seen that transfer ports in the centre of the breech work most efficient, there is an optimal transfer bore size for gun calibre and that a stepped port has the greatest efficiency of moving compressed air from compression chamber to barrel.
I believe that the spring set up can alter the harmonics of the spring when in action and can this be tuned in or out to the advantage of consistency, some work needed to verify this.
I am also thinking about the piston weight will the position of the weight on the piston have any advantage for example take the weight of a standard piston and replicate the same weight but by producing an ultra light piston then adding weight only at the nose of the piston (say with a heavy metal like Tungsten) or at the tail of the piston what will this change?
I feel sure work has been done in these areas but I cannot find any papers published, I would like to do some testing myself so if anyone has findings in these areas it would be good to start me off.

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wastrel
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Re: How light can you go with the piston?

Mon Apr 11, 2016 4:39 pm

looks to me like this post needs a-
metallurgist, physicist & a ballistics-sist!
it`s all sistics to me!
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Sake-san
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Re: How light can you go with the piston?

Mon Apr 11, 2016 5:58 pm

I can only suggest that you really, really try to define what precisely it is that you wish to achieve at the end of the exercise. What is the precise, goal or purpose of your tuning ambitions?.
There is simply so much potential subjectivity otherwise (e.g. smoothness, noise volume and type... twang et al, consistency, sight picture movement, etc etc). Plus for many having had a tune done or completed by themselves there is an inevitable "must be good" feel good factor than can cloud reality.
Both Nick Gibney (Nick G) and Dave Price (Wonky Donky) have been happy to share their rather significant experience and learning. They both produce very, very good (if a little different) tune options.
I would start by conversing with them, if I were in your position.
Have fun.
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gary martin
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Joined: Thu Feb 19, 2015 8:21 pm

Re: How light can you go with the piston?

Mon Apr 11, 2016 7:50 pm

tim wrote:my piston weighs 176g inclusive of latchrod. this is 24.8% lighter than the std piston i had. not too sure how light you could go, but too much will result in piston bounce.
there are also other factors involved, transfer port dimensions, weight of pellet, pellet composition, the list is in all likelyhood more extensive. i know a couple of very well respected tuners, who, i am guessing would struggle to give an accurate answer. they prob have a gut feeling via expeirence.


Gary Thank you for the information. With your piston reduced what did you do with the spring
At the moment I am just thinking out loud, reducing the piston weight as you say will Introduce piston bounce but at what weight will piston bounce occur?
Again is this consistent or will the variables of pellet tolerances change the point at which the piston bounce occur?
Transfer port size although constant on my gun can differ between guns with manufacturing tolerances.
I have seen that transfer ports in the centre of the breech work most efficient, there is an optimal transfer bore size for gun calibre and that a stepped port has the greatest efficiency of moving compressed air from compression chamber to barrel.
I believe that the spring set up can alter the harmonics of the spring when in action and can this be tuned in or out to the advantage of consistency, some work needed to verify this.
I am also thinking about the piston weight will the position of the weight on the piston have any advantage for example take the weight of a standard piston and replicate the same weight but by producing an ultra light piston then adding weight only at the nose of the piston (say with a heavy metal like Tungsten) or at the tail of the piston what will this change?
I feel sure work has been done in these areas but I cannot find any papers published, I would like to do some testing myself so if anyone has findings in these areas it would be good to start me off.[/quote]

i am using the std transfer port dimensions. as for spring i use air arms mk2 spring. stroke is 82mm. calibre .177.
Sake San is correct, ask what you wish to achieve, then you can start experimentation.
Gary.

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Nick G
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Joined: Tue Apr 14, 2015 8:12 pm

Re: How light can you go with the piston?

Mon Apr 11, 2016 8:24 pm

would just like to add , that I did the piston for gary, You can indeed go lighter , I have made skirtless pistons for the hw77 at 150 gram, with the correct trans port and spring combination this gives an extremely fast firing cycle . with a very small recoil and surge , though the action sounds aggressive , and the gun is more hold sensitive than I like .Not to everyones Taste.

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gary martin
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Re: How light can you go with the piston?

Mon Apr 11, 2016 8:56 pm

Nick, top bombing at the PJBRC yesterday mate, that was a very good score. i was pleased with my own result with the lack of set up time. i shall be shooting piston class all year, so hoping to get some major improvement.

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tim
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Joined: Mon Apr 27, 2015 1:09 pm

Re: How light can you go with the piston?

Wed Apr 13, 2016 4:16 pm

Ok it's all about what you want to achieve with the modifications.
Can I just say from where I am coming to ask the question.
Firstly I purchased the Pro-sport in .22 cal on a wim it looked to good to be left in the gun store it had to be in my hands.
Form the get-go I was not happy with the trigger action and my shooting was not consistent so I firstly went back to the store and asked for the trigger to be looked at.
They had the gun a few days and said that I should be happy with it now, To be honest I could not tell any difference still not happy.
So I dismantled it and completed some work on the trigger myself including making a new trigger blade as the hole for the hinge pin was oval allowing the trigger to float about in the action.
The components in the trigger action all required some work to make it work smoothly, because of this I decided to look at the compression tube, piston and the cylinder.
I have access to some very precise measuring equipment and I found that the compression tube was barrelled meaning the bore has a larger diameter in the centre portion of the stroke and was not truly round.
The latch rod was not in the centre of the piston and at the point at which the sears interact with the latch rod it was bruised. The bruising coincided with the high point of the latch rod indicating to me that in manufacture the latch rod had been struck to try and centralise it Maybe it is better than it was but still out, this would not have helped the trigger action.
The PS cylinder bore is again not a true cylinder and has a smaller internal diameter part way down the bore, to fit the compression cylinder along with the piston into the PS cylinder the Delrin bearings had to be smaller than the this tight spot so that at the point that they work at they are loose.

I am now looking to manufacture new parts myself and was just thinking out loud shall I make them in steel or a lightweight material hence my question.
I know I could do some number crunching to come up with some facts and figures but a page full of calculations bases on Hooks law and the like is not any where the same as having done it shot it and evaluated it in the real world. Starting points and the finished product are usually worlds apart, I hope you can make sense of this.

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