I'm attempting to fit a 1.5 inch nylon cylinder into a 1 inch diameter
metal sleeve. The nylon will act as a piston for my spring piston
pellet gun. I've heard for years that auto engine pistons are lap
fitted for each cylinder and would like to give it a try. Could someone
give me the basic idea of how this might be done for 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.
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.
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.
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.
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.
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.
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