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

[greenwayjames]

[QUOTE=bigtoe01;6084874] if you look inside a 77 comp tube you will always see scrore/polish marks where the piston has rubbed during flight.
[/QUOTE




A rifle has to be cocked before it is fired so how is it possible to state the score marks are caused as you say. Surely those marks are made by the side thrust on the piston when the rifle is cocked. When the piston is in flight there is no side thrust if it is released by a central rod.

[greenwayjames]

[QUOTE=Gwylan;6084886]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[/QUOT

Thanks. The spring has such force that it must determine the direction the piston goes.

"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."
That was my guess too

[Rob M]

When the piston is in flight there is no side thrust if it is released by a central rod.[/QUOTE]

Once the central rod is away from the trigger block the piston could be affected by sideways thrust caused by bends in the spring. I'm a strong believer in buttoning, anything that cuts down metal to metal contact has to be a good thing.

[Snooper601]

[QUOTE=greenwayjames;6084919]

if you look inside a 77 comp tube you will always see scrore/polish marks where the piston has rubbed during flight.
[/QUOTE




A rifle has to be cocked before it is fired so how is it possible to state the score marks are caused as you say. Surely those marks are made by the side thrust on the piston when the rifle is cocked. When the piston is in flight there is no side thrust if it is released by a central rod.

If springs were perfect with exactly 90 degree flat ends, then yes it might push the piston straight, but in the real world a spring NEVER pushes straight down the axis of the cylinder, ever. It only takes a thou of variation in the flatnes of either end or the winding of the spring to allow vibration and other forces to make it do everything EXCEPT fly straight. Even where the position of each cut on both ends of the spring can affect the way it pushes, if both cut ends are on the same side of the spring it will be slightly stronger on the other side as there are less coils and therefore it will not compress dead straight or release dead straight.

The idea is to reduce all friction to the minimum, and control everything where possible, to reduce any chance of unwanted vibration sapping power from the intended ultimate goal, pushing that pellet out of the barrel with the minimum of recoil and barrel movement.

Cheers

John

[bigtoe01]

[QUOTE=Snooper601;6084961]

If springs were perfect with exactly 90 degree flat ends, then yes it might push the piston straight, but in the real world a spring NEVER pushes straight down the axis of the cylinder, ever. It only takes a thou of variation in the flatnes of either end or the winding of the spring to allow vibration and other forces to make it do everything EXCEPT fly straight.

The idea is to reduce all friction to the minimum and reduce any chance of unwanted vibration sapping power from the intended ultimate goal, pushing that pellet out of the barrel with the minimum of recoil and barrel movement.

Cheers

John

John, if you set the spring guide into the trigger block, and you reduce the OD of the piston a few thous so it glides on the piston seal you can get them to fly without touching the walls of the cylinder, Hector over on the Diana forum designed his pistons to fly just like this and they work very well. However these pistons usually suit sliding breech guns better as the breech pushes the piston backwards and is then drawn forwards.

Setting spring guides into trigger blocks works really well, however you have to get the guide dead central and it has to be around 9.9mm bore for a HW piston. Just loose enough to allow the piston to fly with little friction and tight enough to stop the chatter. My plan is to add a bronze rear guide to my 77k over the winter with a rotating piston, no buttons on the piston as they will not be needed, the piston will never touch the sides of the comp tube in flight. I will however have to make sure the comp tube is an exact true fit...it will take time to get this one dead right but i feel worth it as it should be very close to vibration free when done.

[steve m]

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.

buttons on the comp tube serve a good purpose if your having the tube blackened,thus enabling no metal to metal ,which means no marking on the blackened tube
ste

[iceni1]

When the piston is in flight there is no side thrust if it is released by a central rod.

Once the central rod is away from the trigger block the piston could be affected by sideways thrust caused by bends in the spring. I'm a strong believer in buttoning, anything that cuts down metal to metal contact has to be a good thing.[/QUOTE]

My old man went thru this years ago and found no advantage what so ever. If you take the seal of and tip the comp tube verticle the piston falls straight out
Zero Friction.
He spent his lifetime tuning bike engines that were turning over at 20000 revs so there was not much he did not know about pistons.
If the lubrication is doing its job there should be no metal to metal contact.

A Air rifle piston is a very basic item compared to a car or bike engine, Which can be made of the most exotic materials and are both oval and taper ground.

[bigtoe01]

buttons on the comp tube serve a good purpose if your having the tube blackened,thus enabling no metal to metal ,which means no marking on the blackened tube
ste

aye they do, done it a few times, also blacked tubes without buttons and they looked just as good a few months after....it all depends on the comp tube fit in the first place. For example, the new comp tube im my most recent 77 is near an exact fit, no rock at all, infact I had to hone out the main cylinder just forward of the threads for the trigger block to allow it to slide perfectly...no buttons needed. I will check this over the winter when i do the piston and guide conversion.

Also remember the buttons on comp tubes wear, have a look at an older prosport and you will see.