Lapping nylon

A principal of lapping is the softer material (the lap) will pick up the abrasive and lap the other part. Nylon will pick up the lapping abrasive, and in turn wear the bore in your air gun.

Things are lapped to get a very precise fit and a fine finish. Due to the extreme difference in coefficient of thermal expansion between nylon and your metal bore and the fact that nylon changes dimension with exposure to water (kinda like wood), you would not want an extremely close fit between your nylon piston and metal sleeve or you risk binding. You will need some clearance, probably around .005" plus or minus .001". It does not take lapping to get that kind of fit. A fine lapped finish would probably not be highly beneficial to the function. And I don't think nylon will lap well. My opinion is your best bet is to just turn your piston on a lathe.

If you want a high performance plastic that does not absorb water, has good lubricity and high wear resistance and a low coefficient of thermal expansion and friction, look at polyimide (Nylon is polyamide and is very different) such as Dupont Vespel.

Though some plastics polish well (Delrin for example) if you want an actualy lapped part, you should probably use metal.

Auto engine pistons lapped? No! And getting the 0.5" off with lapping is also not the right way. :-))

I'd take some PA with MoS2 in it (the black one) and simply turn it on a lathe. Just make it a nice gliding fit. I have done that with pistons (diam 12mm) for a compressed air engine to my satisfaction.

Nick

Poor choice of material, a high durometer rated Urethane would be far better. It'll last far longer. It will also not change dimension with the changes in relative humidity.

Poor choice of methods too. You need a lathe and a very sharp cutting tool to get a decent surface finish on your nylon. About US$500 could get you well equipped.

You might try over at yelowforum.com and check through the posts there. From there go to the bulldurum forum, available from the pulldown at the top of the first one given.

The only pistons I know of that are lap fitted are on model engines.

Cheers Trevor Jones

Polymer Man--Thanks!

Calculations indicate that I'm looking at 2000-2500psi and thought lapping might result in less gas leakage. I'll give Vespel a look see.

Tut

Nick

Thanks for your comments--I'm not sure what PA is but assume its polishing compound. I have four different grades--from rouge to black.

Tut

Trevor Jones--

Thanks for your comments--my last auto engine rebuild was over 30 years ago--guess thing change.;-))

Tut

PA is Polyamid. I would use it for the piston.

Nick

Things have not changed that much in that time.

Cylinders get bored using a boring tool, The bores are sized to fit the pistons, not the other way around. Odds are good that your local engine rebuilder is using the same machine he was using 30 years back. The bores are then honed with abrasive stones to provive a finish.

About the only part of the engine rebuild process that may involve lapping, is the final seating of the valves to the valve seats. Even that is more of a shade tree mechanic thing, as the pro's tend to get good seal using their proper tools in good shape.

I think it's fair to say that abrasive finishing of nylon is a non-starter. Any abrasive that can cut into the nylon will produse a vast multitude of stringy threads and a rough surface, not conducive to sealing the piston. Any loose abrasive, such as lapping compound, will embed itself into the nylon and provide you with a quite effective way to destroy the cylinder of the air rifle.

Suit yourself, though.

What make of air rifle? Chinese? European? American? Parts are readilly available for many makes, and cheaply, too. Not as much fun as building your own, but the chance of success is greatly increased.

Cheers Trevor Jones

Thanks again for your comments. The air rifle is a $135 Chinese one imported by Compasseco--it shoots extremely well but needs some refinement. This is the only model the importer doesn't sell extra piston sets and springs for. The original machining on the piston assembly wasn't quite right and gouged the piston sleeve when fired. The sleeve then roughed up the plastic piston creating a sealing problem--(a plastic piston sits on a metal assembly.) It still shoots fine (dime sized groups at 35-40 yds) but the cronograph says the exit pellet velocity has fallen to around 815 fps--down from 1100 fps. My plan was to hone out the sleeve and then replace the plastic piston with a tight fitting nylon one.

Tut

I don't think that will work very well. You need a piston made of something softer and elastic with a lip on it to seal sort of like an automotive brake cylinder uses.

In fact, that's what I used a few years ago to fix the $35 cheepo Chinese single shot spring air pistol I got to teach the kids gun use and safety. I searched around and found a car wheel cylinder piston cup which fit the cylinder bore.

I punched a hole in its center and used a self threading screw to fasten it to a hole I drilled in the center of the front face of the piston.

HTH,

Jeff

Jeff

Your car wheel piston cup may work. Thanks

Tut

That plastic piston is likely urethane. To make this work you really need a lathe, or you can hope to get lucky on the stock sizes of seals as suggested by another poster. I put your chances of seeing ANY improvement at about zero, using nylon and abrasives to size the seal, and the chances of it working at all only slightly higher.

One other thing you could do while you are in there is to "button" the piston. Buttoning is adding sliders to the piston to take up the space in the cylinder and hold the piston centered. By placing three buttons around the front of the piston, and trhee more around the back, you can decrease drag an make the firing cycle much smoother. Normal practice is to ensure one set of buttons is opposite the cocking lever, with the others evenly spaced around.

Check out Jim Maccari's website and see if he has a seal kit for your model. Google "JM's spring tar". He may be able to upply an oversize seal, or , depending on how large you have to go to hone out the damage, you may be able to use one of his stock seals as is or with a small amount of adjustment.

Cheers Trevor Jones

Nick--Thanks for the PA info....

Trevor--some good ideas in your post. Jim Maccuri actually made the main spring for the rifle. What would your recommend for button material?

Thanks again everyone

Tut

Pretty sure Macarri is selling either delrin or teflon buttons on his site.

Cheers Trevor Jones

Trevor--

Thanks I'll check him out.

Best Wishes

Tut