Educate me on Piston Weight

[sd5782]

I have been learning a bit about tuning, and have found the posts on SCR very interesting. I have sleeved a couple of transfer ports down with brass tubing in the past, as well as drilled some out, and now see what the science is behind it.

I have searched, and haven't found discussions specifically on piston weight. I know that my Mercury S will have to be lightened, and perhaps my 26mm HW77. I want to experiment on some less costly stuff first; a Baikal 512M, and an older HW30. The Baikal has a very heavy solid guide that I haven't weighed yet, and surprisingly the Macarri kitted HW30 wants to make the Beeman peep sight move around.

I would guess that there is perhaps a general weight range for power levels maybe? Perhaps the weight is more related to spring wire size?
I have read that the later HW77 piston is heavier than the first, so I would guess that a 6, or 8fpe gun would be lighter yet. Are there any general starting points, or directions to go from? It is all very fascinating.

[tinbum]

I agree this would be a good subject to get the more knowledgable talking about.

I lightened my 25mm 77 piston which worked well in conjunction with expanding the TP.
As I understand it a lighter piston reduces inertia and accelerates faster, but if the escape route for the air is too restricted the lack of weight results in piston bounce. That in turn increases felt recoil.

I look forward to some replies from people who aren't just guessing!

[Shed tuner]

I agree this would be a good subject to get the more knowledgable talking about.

I lightened my 25mm 77 piston which worked well in conjunction with expanding the TP.
As I understand it a lighter piston reduces inertia and accelerates faster, but if the escape route for the air is too restricted the lack of weight results in piston bounce. That in turn increases felt recoil.

I look forward to some replies from people who aren't just guessing!

That's about the size of it. Heavier pistons tend to be more efficient, but will take more energy to accelerate, however they need less peak velocity as they decelerate slower. Balancing piston weight, spring, port and stroke is all part of the fun

[Black Beard]

There are too many variables to make general rules except:

Heavier pellet needs heavier piston to work well.
Efective piston weight is piston weight plus a third of the spring weight. A compressed coil at the spring guide end adds no weight but one at the top hat end adds all its weight (it acts like iit is part of the piston/top hat).
Heavier pistons and pellets can get away with smaller transer ports.
Heavier pistons mean more power but also more recoil.

[Olas]

Here's an animation and some writing that fills in some details

http://www.arld1.com/images/swfs/pis...etdynamics.swf

[TonyL]

Pretty much as per posts 2, 3 and 4.

Also from my understanding, whilst a heavier piston may result in higher initial recoil, if a lighter piston was to induce more piston bounce, it's this surge recoil that is the most detrimental to accuracy.

It's far too easy to generalise, as said above, and each individual set up, in terms of bore, stroke, TP volume, spring etc will have a different "balance point". Some will have pistons which are too heavy for any configuration.

And then you remember the reports of how good those ally-piston TXs are!!!!

I suppose that, if you go too light, this can always be easily addressed by fitting internal weights.....

Gotta go out now.......