What is the reasoning behind "buttoning" pistons

[greenwayjames]

I have never seen the point in doing this to the rear piston skirt on a spring air rifle with central piston rod. OK it might make the rifle slightly smoother to cock but it cant make any difference to the firing cycle. Opinions from fellow shooters would be appreciated.

[thevoid]

Any item that can separate metal on metal contact in high speed environments can only be a good thing, remember that wear breeds more wear. In the case of air rifle pistons it will prevent piston and cylinder scoring. another point is that many piston skirts are hardened so the majority of the wear happens to the bore of the cylinder which is never good

[brucegill]

Buttoning the piston will always help like the void has just said. It can only be a good thing. But buttoning the comp tube would, I guess, only aid cocking. All that said, get a good fitting comp tube and piston and the difference is neglegable. I've had vglided 77's and a 97 but also owned a 25mm standard 77 that was just as smooth to shoot. But, it was a good example and coming back home to me soon

[DaveC_24]

The idea of buttoning is to reduce the friction between the rear of the piston and the walls of the compresion tube, my minimising contact area and also eliminating metal-to-metal contact, and this applies to both the cocking stroke and the firing stroke. Also, if there was previously a bit of play between the piston and compression tube, buttoning can help take up that slack and result in less wobble as the piston moves forwards during the firing cycle.

The result is less wasted energy (meaning more efficiency which could ultimately require less preload for the same muzzle energy) and less vibration from wobbling etc. during the firing cycle, making it smoother to shoot. Whether the effect of a buttoning job is massively noticeable will depend on the tolerances the individual rifle had in the first place compared to when "buttoned", though the reduced surface area of contact can only help in terms of energy efficiency.

[greenwayjames]

I hear what is said but surely the forces on the piston when the central rod is released are pushing it down the cylinder not against the walls of the tube. The piston skirt would automatically center itself so there is no friction at that point.

[bigtoe01]

I hear what is said but surely the forces on the piston when the central rod is released are pushing it down the cylinder not against the walls of the tube. The piston skirt would automatically center itself so there is no friction at that point.

in theory yes, in practice no, if you look inside a 77 comp tube you will always see scrore/polish marks where the piston has rubbed during flight.

I made up a a 25mm piston, 75mm stroke with a 26mm head to go in a 26mm compression tube, the piston has 3 large buttons that control the full stroke flight, the piston head is phosphor bronze...its supremely slick to shoot.

comp tube bearings..not really needed as long as the comp tube is a nice fit, slack ones need bearings however to centralise them.

[Gwylan]

I hear what is said but surely the forces on the piston when the central rod is released are pushing it down the cylinder not against the walls of the tube. The piston skirt would automatically center itself so there is no friction at that point.

There are so many things happening when a spring uncoils, that the piston will move from side to side as it moves down the cylinder. (I expect BTDT and the Prof will film something on Youtube to show this..!)
The spring guide doesn't have much effect on the piston. In the old days, tuners used to braze a brass, or phosphor bronze ring on the back of the piston to act as a bearing. The beauty of the TX, when it came out, was the front bearing ran in the comp tube, the rear one in the cylinder, and the whole piston and spring could rotate. If you strip an old, well used springer, you can usually see wear on the top rear of the piston, due to the cocking forces.

Gus