There are a number of tricks out
there to restore a worn pston and
sleeve and alot of people willing to
give them a try.Too many times Ive
seen guys shelve a perfect car or
plane or whatever because of a worn
engine cause they simply didnt have
the cash for a new one.Below Im
going to list four ways to get quite
abit more life out your engine
starting from the easy to the
damned hard.I have tried all four
and have had good results with
three of them.
1.Add castor...Older cast iron piston
and sleeve engines used castor oil to
achive compression instead of a
taper.The same holds true for a worn
ABC engine.Infact I have engines
with gouges on the piston and
sleeve that operate because of extra
castor oil I put in the fuel.
2.Sleeve pinch...I failed with this
one.The idea is to pinch or compress
the the sleeve to improve
compression.One way is to use a
hose clamp and very lightly tighten
it around the sleeve and heat it up
on a stove.Then you chuck it in
water to cool it and the sleeve will
keep its pinch from the heat
treatment.I dont care who says what
but this is the worst way to treat
your engine and you will destroy it
because you wont get it perfectly
round.The better way is to send it to
be done by guys who have
destroyed alotta engines perfecting
the art.They seem to have great
success and alot of peaple are happy
with them.My problem with it is that
while its true that the sleeve does
wear,the piston is what wears the
most and causes compression loss.
3.The ball bearing trick...Right,take
your piston and remove the con rod
and pin.Now get a perfectly smooth
flat surface.Now find a ball bearing
that will fit perfectly inside the
piston.The fit should be loose.Now
place the piston upside down on the
flat surface with the bearing
inside.Now the idea is to tap on the
bearing to expand the piston
abit.Tap,fit tap,fit tap,fit.You get the
idea.What you use to tap the bearing
with is up to you.I used a shaft with
a light hammer.
4.Cut a oil groove...Although this one
is the hardest,I like it best.If your
nitro rc piston to liner fit becomes
less that perfect, it is EASY to refit the
piston to the liner. Buy yourself a
VERY GOOD tubing cutter from Sears (
http://www.sears.com/shc/s/
p_10153_1...0P?vName=Tools) about
$32.00. It will be a once in a lifetime
purchase that you will use over and
over. Don't ever ever ever ever use
this cutter for anything else. The
wheel that comes with the cutter will
last almost indefinitely, but you
should pick up a pack of spares.
Keeping the cutter razor sharp is
critical.
OK, now I will tell you how to use
this tool.
Almost all nitro rc pistons have an oil
groove at the top of the piston. It is
usually VERY NEAR the top of the
piston. If the piston does not have an
oil groove, it is easy to make one
with the tube cutter. The cutter will
EXTRUDE the aluminum in the oil
groove making the piston tighter in
the liner. It is CRITICAL to be very
careful here not to get the piston too
tight. You want to push the piston
up into the liner and when it stops,
look into the exhaust port and see
the wrist pin in the MIDDLE of the
exhaust port window. This is exactly
how tight you want the piston to fit.
This re-fit of the nirto rc piston can
be done over and over. The fit will
go away quicker than if the piston
was perfect to start with, but this
method will recover a loose fitting
piston/liner.
I didnt type out the last one,just
some pirate copy/paste.I didnt come
up with any of the methods myself
but I can tell you that if done right
each method will work and give
your engine that extra life.Enjoy
Edit Quick Reply Reply
The Foll

All of these (besides pinching) sound like horrible ideas. Yes, you will destroy your engine if you try using a hose clamp, but they now make special rings designed to do it evenly. There is a thread around here where a guy used a tool holder, and it seems to work perfectly. Doing ANYTHING to the piston is not necessary, and will not last. The sleeve is tapered, no pinch means this taper is gone... squishing the top of the piston isn't going to bring that taper back.

I like the theorys, and would like to try a few of the above methods. The whole problem with the theory of the repinch is you cannot take a perfect circle and compress it and expect it to be round. Where did the extra material go too?

I like the piston expansion idea. If the sleeve is tappered and you ran it until no more pinch, a fatter piston would work assuming that the sleeve has not worn to the point where it now has become a strait bore.

I beleive the caster oil theory, but assume the premix ratio is beyond normal, 16:1 maybe. If you have a nitro motor with little to no pinch left and drop a few drops of afterrun oil in the cylinder, it will start.

The last idea of putting an oil grove in the piston will probably work but I would assume have the shortest life out of all the fixxes mentioned above.

If you dont learn something new everyday, you waisting your time on this rock.

Toolman thankyou for the positive reply.I could tell you about the amount of people I know including myself that have tried all but the pinching methods that have engines up and still running after quite a few liters of fuel,but this being a online forum it would be a moot point as it would be hard to prove.

Proanti...If it looks like a duck and it quakes then it must be a duck and clearly your snubbing my post.The piston is proven to be the part that wears the most and as such in my mind and many others it would be the part that needs attention.As toolman said ''Where does the extra material go?''Now if you read the first paragraph of my post you would relise that it leans more to the people who have the very difficult choice to between feeding their kids or buying a new engine.Guys who have a love for this hobby but find it hard to justify the cost.The recession left a huge dent and it seems like another is on the way.As for the tool you mention,it retails for more than two or three engines.As for getting the sleeve pinched,Id much rather pay $32 for a tool that I can use as many times I want on almost any engine I want than pay the $25 it would cost to have the sleeve pinched once.Now Im not dissing the guys how offer the service,but it remains a sevice alot of guys out there will think twice about.
My post was meant to be helpful,and you clearly sound very bias or not willing to try something different.

Well I have some experience with a few of these techniques.... The pipe cutter method does definitely work...as it widens up the oil grooves and pushes the material outwards, which increases the diameter of the piston at the area of the oil grooves...This definitely does work, but doesn't seem to last overly long before it wears down.... The ball bearing trick doesn't seem to work well on the higher end engines, the aluminum is far too hard and brittle, it would crank before it would stretch....Very similar material to Hypereutectic pistons...Hard with great wear properties, but quite brittle....... 100% the piston is the part that wears out, it is much softer material then the chrome lined sleeve...The sleeve goes thru very little wear from to the piston, and usually any wear is caused from foreign material going thru the engine........The piston just isn't hard enough to make any significant wear into the sleeve....... I have my best results pinching the sleeve, but I use equipment that would be too costly for the average hobbiest....And even with the proper equipment there is a substantial learning curve to perfect the technique... I now have about 100 repinched engines in circulation now with nothing but excellent results, most times the lifespan of the repinch lasts longer then the original ......The amount of pressure and accuracy needed to properly crimp the sleeve is beyond anything you could do with a hose clamp/screw clamp........I have dyno tested the repinched engines many times over and they perform as strong or stronger then the original factory fitment.... Repinching the sleeve works awesome, but it is a little bit of a black art to perfect

I completly agree that proper repinch is the best way to go.However those tools are simply not avialible in South Africa.Ofcoarse,the guys who developed this tools went through alot of trail and error perfecting them so obviously they wont just publish the dimentions for people to make there own.I can completely understand why and they should defintly be rewarded the way they see fit but the most of us simply dont have the monotery resources to develop and perfect our own tools so it seems these guys will have a monopoly on the whole repinch concept.So while this is the situation,the above methods are what the guys will keep on using.Ahh capitalisim...

The repinch process is not as much voodoo as people think. Ibelieve I was the guy Proanti was talking about who was using tool holders off of a high speed machining center to repinch my own and all my friends and family's cylinder sleeves. It works better than most people would think... you really have alot of control over how much you compress the top of the sleeve - and it only takes 5 or 10 minutes if you take your time.

Here is a pic of a similar tool holder that we use in the shop. The shows a complete tool holder with a collet installed. The nut (which takes a spanner wrench) tightens down over top of the tapered collet, which is setting inside of a tapered hole. As the collet is forced down into the hole, the diameter in the center gets smaller. All collets have a range in which they will work precisely to hold different diameters, and there are many different sized collets.



Here is a link that shows a picture of just a collet:


Now, there are different types of collets, and different types of holders. The ones we use at the shop are about the most accurate and most expensive type that you will find - most of the work I do requires extremely precise runout of the cutting tool as we commonly use endmills in the .005" to .010" in diameter... that cost $175 each. In order to make a tool that small and that expensive last, it has to be running almost perfectly true. That having been said, I suspect that someone could find a cheap tool holder on ebay for less than $50 that would do the job for pinching sleeves. Finding a lone collet the size you need might be a little tougher on ebay, but you can get them new direct from the manufacturer. One thing to remember... the inside diameter of a collet when not compressed is the largest diameter you can accurately hold inside that collet. You can generally go a mm or so smaller and still get good accuracy, but squeeze it more than that and accuracy starts to suffer quite a bit.

Now... when you pinch the sleeves with one of these, you can't just slap it in and pinch it straight... you would end up compressing the entire sleeve creating interference on the entire stroke of the piston. What you would need to do (this is what I do) is simply cut a short piece of precision shim stock, say .003" thick and .150" wide, to wrap around the top of the sleeve before you put it into the collet. When you tighten the nut on the tool holder, only the top of the collet will now engage the sleeve, prividing the taper you desire. You can actually put index marks on the tool holder nut so you can "sneak up" on just the right amount of pinch with tremendous accuracy by checking, pinching, checking, pinching, etc etc. . Actually, if you know the thread pitch and the taper of the collet and holder, you can trig out the exact amount you are compressing the sleeve for any amount of revolution of the nut. It is so accurate that you can literally go about .0001" on the diameter between checks. The shallower the taper, and the finer the thread pitch, the more accurate you can be on the pinch.

I am finding out that there is not really any need to put so much pinch back into the fit that you need to break the engine in again. The piston will already be shaped to the sleeve when you remove it, and you won't really change the shape of the sleeve with one of these precision holders... so all you need to do is pinch it enough so that you just get a little drag on the piston at the top portion of the stroke. If you measured it, you might only see .0002 to .0004" total shrink on the sleeve diameter. I have done this for a few people now, and it absolutely restores power. Since the pinch is kept to a minumum, you don't have to go through the break in process quite so drastically as on a new motor so you save all that stress and wear on the connecting rod and wrist pin.

Ihave not had to apply heat to get the repinch to hold.

At any rate, I really think that this is something that anyone could do if they get the right tools and take their time. As in any industry, sales and marketing people like to make the average citizen feel like their product or service offers something that can't be had anywhere else... that it's different beyond the wildest of our imaginations... Most of the time it's an exaggeration at best, and a flat out lie at worst. Stuff is usually simpler than we think.

Cheers.

That was very helpful oacker thank you so much.I was thinking something in the lines of a 3 jaw chuck from a lathe using the shim the same way you do.You think that might work?

I agree with the tool collet as that is the exact same unit I use to pinch my stuff. Been using it for a couple of years now. My problem is, WHERE does the material go? You cannot take a perfect circle and squeeze it and expect it to be round. You are displacing material to somewhere where does it go? That is my problem, it just does not add up in theory. I still use the pinch theory, but would like to explore other things that actually make sense.

You are only decreasing the ID by about .0001, so the material you are not displacing all that much material. You do change the shape a little, but that's why you have to break your engine in again.

I think it boils down to "how round does it have to be." The bottom line is that no circle is PERFECTLY round anyways... we can get pretty darn close with specialty jig grinders and such, but never absolutely perfect. Ihave actually measured the total runout of the pinch area of one of these sleeves before and after repinch... it doesn't change but a tenth or two (.0001 - .0002). Hardly enough to cause an issue. If the piston were spinning around in a circle as it goes up and down then I would say roundness of the bore would matter, but that's not how it works. The piston wears into the shape of the bore, regardless of whether it's a couple tenths out of round or not. It could almost be an oval shape and still work.

I suppose you could have the inside of the sleeve chrome plated and then set up and jig grind a precision taper inside of the bore... you would get a hole that's probably more round than the pistons come stock... but I really don't think there would be any benefit, and it would cost alot more money and take alot more time. Pinching sure seems to be an economical way to get performance back.

As a side note, I picked up a used os .18tm for the revo... I bought it soley for the carb cause it's a direct fit into the 3.3, but I think the existing engine parts are actually rebuildable. New bushings in the back plate and conrod, a re-pinch, and I think I will be good to go. Now, the repinch on this one is going to be a bit different than what Ihave done thus far.... someone beat the tar out of this thing, and the piston almost goes all the way through. Looks like they loaded the fuel up with tons of castor oil to get compression up and keep it running... it was clearly running HOT.. and it was a real bear to get the inside clean. Maybe there is too much wear on the piston, I don't know... I'll pinch it and see what happens.

Cheers

Iwould think that a 3 jaw chuck might turn your sleeve into a triangle shape... notice that the collets are basically 6 jaw, and only have about .04 between the jaws. The openings between the jaws are where the extra material goes that tool man is talking about. The more of those there are, and the smaller the spaces are, the more round you will get your sleeve. On a precision collet, your talking ten-thousandths of an inch... on a 3 jaw chuck out of a lathe, I can imagine a scenario where you could get several thousandths of an inch out of round - there is no where near the control on a typical 3 jaw lathe chuck that there is on a precision collet... and unless you put a set of soft jaws on and tightened them together to bore a hole just a few thousands smaller than the sleeve through them, I just don't think you would get a good enough fit to keep the sleeve round.

There are some precision collet adapters for lathe chucks that would probably work well as they are similar to the tool holders I showed above.

cheers.