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Thread: A few questions about pistons and tuning?

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    A few questions about pistons and tuning?

    Hi all,

    I'm thinking of doing some piston tinkering, now a few questions to the tuners amongst us

    If you can get a decent mirror polished saddle/area on the rear of the piston, is it really worth buttoning the piston for smoothness?

    Is it actually worth buttoning the piston, does it really make that much difference?

    What is the best weight for a piston?

    What is the best way of lightning a heavy piston, machine off a bit of the girth in the middle or drill holes?

    As for seal sizing, what's the best way and what's the sliding fit supposed to be like, should it glide down under it's own weight or a gentle push from your finger?

    Thanks for any advice.

    Pete
    Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in

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    Quote Originally Posted by look no hands View Post
    Hi all,

    I'm thinking of doing some piston tinkering, now a few questions to the tuners amongst us

    If you can get a decent mirror polished saddle/area on the rear of the piston, is it really worth buttoning the piston for smoothness?

    Is it actually worth buttoning the piston, does it really make that much difference?

    What is the best weight for a piston?

    What is the best way of lightning a heavy piston, machine off a bit of the girth in the middle or drill holes?

    As for seal sizing, what's the best way and what's the sliding fit supposed to be like, should it glide down under it's own weight or a gentle push from your finger?

    Thanks for any advice.

    Pete
    Hello Beesa Pete.

    Buttoning (or Delrin ring):- Would always have to be seen as a good idea to reduce metal to metal contact. However, on reassembly, I always grease the rear of the skirt. Then, once the piston is pushed fully forwards I always smear some more moly grease on the inside circumference of the cylinder, behind the piston. Recommended by Steve Pope in his fitting kit instructions so it has to be good advice and it's always worked for me. Using synthetic bearings would reduce the amount of lubrication required in this vital area.

    Best weight for the piston? Blimey, that's a BIG question and the answer very much depends on the individual gun, bore and stroke dimensions, transfer port size, preload, spring force and even the pellet used.

    In terms of seal fit, I'd prefer to go for a "push fit" rather than the piston dropping under its own weight thinking, but it does seem to be down to personal preference.

    Sorry it's all sounding a little of a cop-out, but like so many aspects of springer tuning, much of it is down to the individual.
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    Quote Originally Posted by TonyL View Post
    Hello Beesa Pete.

    Buttoning (or Delrin ring):- Would always have to be seen as a good idea to reduce metal to metal contact. However, on reassembly, I always grease the rear of the skirt. Then, once the piston is pushed fully forwards I always smear some more moly grease on the inside circumference of the cylinder, behind the piston. Recommended by Steve Pope in his fitting kit instructions so it has to be good advice and it's always worked for me. Using synthetic bearings would reduce the amount of lubrication required in this vital area.

    Best weight for the piston? Blimey, that's a BIG question and the answer very much depends on the individual gun, bore and stroke dimensions, transfer port size, preload, spring force and even the pellet used.

    In terms of seal fit, I'd prefer to go for a "push fit" rather than the piston dropping under its own weight thinking, but it does seem to be down to personal preference.

    Sorry it's all sounding a little of a cop-out, but like so many aspects of springer tuning, much of it is down to the individual.
    Hi Tone,

    Hope you had a good weekend boingering yourself stupid

    I know most tunes are down to personal preference and as for all the difference figures, I wasn't sure if there was a 'starting point' so to say or an idealish sort of figure to head for and then do your fine adjustments from there, I understand it's a complicated science which only a few have mastered over the years.

    I often read springer tuning threads and people get replies like "your pistons too heavy" and other such answers, I know the piston in project 'cock is a bit on the light side (according to JB at the time of us chatting about it) but I'll have to weigh the Supersport piston when it's out and see what would be best.

    Thanks for your input yet again Tone

    Pete
    Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in

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    assuming the supersport is 29mm and longish stroked, you cant go too light on the piston, so 250-300 g range ? buttoning good if rear of piston loose in the bore. if tight, less advantage.
    hth JB
    Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.

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    Quote Originally Posted by Shed tuner View Post
    assuming the supersport is 29mm and longish stroked, you cant go too light on the piston, so 250-300 g range ? buttoning good if rear of piston loose in the bore. if tight, less advantage.
    hth JB
    Thanks for that Jon, just what I needed to know, the piston in my Airsporter RB2 and the Brocock Indy have nicely finished/machined and mirror polished rear skirt/saddle and fit pretty well in the cylinder, all I want to do is take the 'spit and polish' tune up a stage after the usual kit fitting and spring end polishing etc, I may have a go at transfer port sizing as I have access to a lathe now but that's something I'll have to pester you and others about at a later date, as soon as my seals arrive and my spare piston, I'll do a measure up and so what figures I have.

    Many thanks for your advice yet again Jon

    Pete
    Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in

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    never messed with piston weights as I don't have a lathe.

    Rear piston bearings I don't mirror polish. I just use finish emery that still shows sanding marks. The marks will retain lube and I use moly paste on the rear bearing.

    Seal, sized so it can be gently pushed down with one finger for me. Again, moly paste rubbed in the sides but wiped off. The sanding marks retain enough lube.
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    Quote Originally Posted by robs5230 View Post
    never messed with piston weights as I don't have a lathe.

    Rear piston bearings I don't mirror polish. I just use finish emery that still shows sanding marks. The marks will retain lube and I use moly paste on the rear bearing.

    Seal, sized so it can be gently pushed down with one finger for me. Again, moly paste rubbed in the sides but wiped off. The sanding marks retain enough lube.
    What's your method of seal sizing Rob?

    Pete
    Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in

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    Quote Originally Posted by look no hands View Post
    What's your method of seal sizing Rob?

    Pete
    Crude but simple. I have no lathe so spin the piston up at slow speed in a drill. Emery applied at the same angle as the seal side and gentle pressure. Any more than that or if spun faster you stand a chance of overheating / melting the side of the seal.
    Just a matter of trial and error and hopefully not going to far. Never ruined one yet though.
    I try to be very careful around the leading / front / sealing edge.
    I've found it best with a spare pair of hands to hold the drill as I'd rather not clamp tools in a vice.
    Once the seal is down to the right size (able to slide down with a gentle push) I apply a smear of moly paste to the sides of the seal and wipe off. A small amount of lube is retained in the sanding marks.
    I've found the custom air seals very difficult to size as I've found them to be a loose fit on all the pistons so stopped using them and instead stick to O.E. seals.

    I use the same method to polish rear bearings on pistons and also to the Diana piston area where the crimps "cloverleaf " it.
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    Quote Originally Posted by robs5230 View Post
    Crude but simple. I have no lathe so spin the piston up at slow speed in a drill. Emery applied at the same angle as the seal side and gentle pressure. Any more than that or if spun faster you stand a chance of overheating / melting the side of the seal.
    Just a matter of trial and error and hopefully not going to far. Never ruined one yet though.
    I try to be very careful around the leading / front / sealing edge.
    I've found it best with a spare pair of hands to hold the drill as I'd rather not clamp tools in a vice.
    Once the seal is down to the right size (able to slide down with a gentle push) I apply a smear of moly paste to the sides of the seal and wipe off. A small amount of lube is retained in the sanding marks.
    I've found the custom air seals very difficult to size as I've found them to be a loose fit on all the pistons so stopped using them and instead stick to O.E. seals.

    I use the same method to polish rear bearings on pistons and also to the Diana piston area where the crimps "cloverleaf " it.
    I've just put one the new Oz seals on my Brocock Indy (Basically a Gamo action), it's the seal that covers the whole dovetail and reduces the lost volume, seem to fit pretty well but the normal type seal they do does seem a bot loose, I didn't size that one as I think I'll just shoot that one in, it'll be interesting to see what the Maccari seal is like as I've never had one of those before.

    Pete
    Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in

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    Quote Originally Posted by look no hands View Post
    I've just put one the new Oz seals on my Brocock Indy (Basically a Gamo action), it's the seal that covers the whole dovetail and reduces the lost volume, seem to fit pretty well but the normal type seal they do does seem a bot loose, I didn't size that one as I think I'll just shoot that one in, it'll be interesting to see what the Maccari seal is like as I've never had one of those before.

    Pete
    I only size if they need sizing. My last oz seal honked like a goose it was that tight and was a pig to size so I binned it.
    I did buy one of their gamo closed faced seals for the same reason you stated above but never got round to fitting it on the TX as it shot so well with a sized O.E. seal
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