Gasboat

Does anyone have knowledge of tools to pinch the top end of a glow fuel motor to get the compression back? I read sometime ago about the process but can’t find it.
Thanks Ron

mgnostic

I suppose you could modify the cylinder head to reduce the size of the combustion chamber. If the piston /bore is worn you are still going to get blow by and resulting loss of power. Pinch usually refers to a slight reduction in the cylinder bore at the top of the stroke. A new piston will help a little but you can't really change the cylinder liner without shifting all of its dimensions and thus its fit in the engine case and possibly even the timing of the ports. If it is a ringed piston you can get replacement riings custon made even if the engine is out of production. If it is lapped piston then you may be better off looking for a new piston and sleeve. Sometimes they just wear out.

1967brutus

No... "Pinching" is actually a technique in which the top of a tapered cylinder (as used in ABC engines) is squeezed a bit in order to compensate for wear and bring piston seal back. Used by the car folks to squeeze a bit more runtime out of a P/L set. I have no direct experience with it but from what I picked up results vary and a lot depends on the experience of the pincher.

mgnostic

I understand the idea in principle but I'd like to see how it is accomplished in actual practice. The closest I've ever come to that was being a little heavy handed with a Cox rod restting tool. The piston spread just enough to get tight at the top of the bore. I suppose it wouldn't take much to bring back compression but i would like to know how to keep the liner from going out of round or popping the plating.

the Wasp

My Feedback: (1)

what engine is it?
you may be able to buy a new cylinder and piston.

Gasboat

Wasp,
It’s an old K&B 3.5 inboard.
Thanks for all the comments.
Ron

the Wasp

My Feedback: (1)

here are some piston/liners kits for the K&B 3.5. click on my link, it brings you to the het. click the first link "MECOA shop on line", scroll down..
if you don't find a kit to fit your engine look around or call Randy.

EDIT, this link works fast for you, scroll down.

MECOA SHOP ON LINE

1967brutus

All I know, is that it requires the pincher to fabricate a tool for each specific type of liner to be pinched. The tool should ensure that roundness is maintained. It more or less resembles a collet as used in lathes (when doing precision work and chucks are not accurate enough), .
Chrome plating won't pop whatever you do because microscopically, it's not an uninterrupted layer but more all tiny separate specks of Chrome with a very firm bonding to the base metal, but Nickel plated liners are very sensitive to pinching, since Nickel more forms an uniform layer with less firm bonding, so a pinched Nickel plated liner is more likely to start peeling.
Opinions on the effectiveness of pinching are divided.

1QwkSport2.5r

Ah yes…. The infamous cylinder pinching subject surfaces again….

So let’s address what this is about, and why it’s a bit of snake oil. When a ringless plated cylinder liner is manufactured, it is machined with a taper within the bore of the liner. This is a machining process, not a mechanical force process. The brass liner is machined with the taper to a nominal size, masked and plated, and finally ground to proper size. There are specific tools used to grind the plated bore to size, specific to each individual bore size. This is a precision process. When an engine loses piston seal, it is NOT the liner wearing!!! The piston being aluminum, wholly softer than the hard chrome plating, is what wears down/out. Improper break-in and/or care/operating is what causes this wear to occur prematurely. With proper care, a ringless plated bore engine can easily last many many gallons of fuel worth of use.

Pinching of the liner physically squeezes the top of the liner smaller to recreate the machined taper/interference fit as it came from the factory. The caveat to this is it is not a consistent method to use; because the alloys of the liner itself can vary, the person performing the work, and the tools they use. The other caveat of pinching the liner is it physically shrinks the top of the liner, and this will cause excess clearance/disrupt the fit between the liner and crankcase causing leaks within the engine. Pinched liners never perform as well as a factory original engine, and they do not last nearly as long either. In other words, it’s really a waste of time. The actual pinching process is simple - often a lathe or milling machine collet and tool holder are used to squeeze the liner.

At the end of the day, all that is really needed is a new piston fitted provided the liner isn’t bell-mouthed at the bottom (which often goes hand in hand with the engine randomly quitting, running hot, and/or being very difficult to get a good needle setting. Looking at the piston will tell you if the piston fit in the sealing area is the fault of the engine running inconsistently. Most of the time, bell mouthing of the base of the liner causes leakage which will absolutely kill the ability of the engine to idle or transition off idle.

This was pretty long winded, so I apologize for that. It’s come up enough times, and often the replies aren’t accurate or correct. Just setting the records straight. The secondary market is the best place to search for parts for obsolete/out or production engines; though K&B/Mecoa still makes/stocks parts for many old engines. It just might be months or years before you can actually get them if the stock status is ANYTHING other than “stock status good”.

IMVVHO - the lifespan of a ringless tapered bore engine is directly influenced by the break-in process used, the and the fuel used throughout its life. I have had friends only get 2-3 gallons from an engine model I’ve had that I was able to get 10+ gallons from. I prefer to pre-heat my ringless tapered bore engines prior to starting them - I’ve found this preserves the mechanical “pinch” much longer. Just think - if you start with a straight cylinder, you don’t deform the piston by hammering it into the taper until it builds enough heat to straighten out.

Gasboat

That is very detailed. Thanks for the info. I’ll start looking for a p/l or just a piston if all else fails.
Ron

1QwkSport2.5r

You won’t find an OEM piston only. They are always supplied with a liner as a matched set. You’d have to have a machinist make the piston.

GREG DOE

Spot on. A couple of things to add. The liner has a taper that has been arrived at by a lot of trial, and error, and experimentation. Some ringless plated sport engines will have a different taper then a competition engine, and even boat motors can have a different taper. When you pinch a liner you are also changing the taper, which is not a good thing. Second I think there should be some clarity regarding the pre heating comment. "I prefer to preheat my ringless tapered bore engines prior to starting them". This would be for the first runs, and test bench break-in. Once the engine is broken in, you don't need to preheat. I use a film covering heat gun for preheating. Lastly, chrome liners can not be used straight out of the chroming tank. A new chrome liner has to be honed to fit the pistons which are ground. The piston size is repeatable, but liners will vary slightly because of variations in the honning, which was referred to as being "ground to size" in the previous post. There is a lot of "science" that goes into these little marvels of engines, and books could be written about them. Add aluminum liners, and nickel plated liners and the subject expands exponentially.

1QwkSport2.5r

I will disagree with you on the last part. I have two nearly identical engines - one was preheated before every start, and the other was never preheated. The latter has no mechanical pinch cold, whereas the former does. The longer the mechanical pinch lasts; the better/longer it performs.

Quote:

Lastly, chrome liners can not be used straight out of the chroming tank. A new chrome liner has to be honed to fit the pistons which are ground. The piston size is repeatable, but liners will vary slightly because of variations in the honning, which was referred to as being "ground to size" in the previous post. There is a lot of "science" that goes into these little marvels of engines, and books could be written about them. Add aluminum liners, and nickel plated liners and the subject expands exponentially.

For clarity, I did not say, nor was I implying one can use a chrome liner right from the tank. It needs to be cut smooth and round. In talking with Dub Jett, he has special tools for sizing his liners and pistons. . A hone alone isn’t enough; you actually have to cut the layer of hard chrome flat and round (which does not plate evenly or smoothly), thus the need for being ground first. You will use something a bit more robust than a hone alone. This is what I learned from a much more experienced fella than I. John Shannon might have some input on this as well. I was told making pistons and liners involves diamond drills, tiny hones, piston laps, and even a tiny boring bar.

Hard nickel on the other hand only needs a kiss from a hone to give it some oil retention capability. Otherwise, nickel is smooth and even, without any need for sizing it post-plating. Which is probably why some companies uses the famous single step nickel process. It’s cheap and easy.

We can agree that pinching a liner is pretty stupid. Some buggy guys rave about it, but it’s a load of crap.

GREG DOE

I'm always hesitant to reply to people who know more about a subject then I do, so I'll just concede. Interesting though, in over 50 years of knowing Dub Jett, and watching him fly control line speed, and later fly pylon, I've never seen him preheat an engine. For that matter I never seen anyone preheat an engine at a contest. Just saying!! OS went to nickel plating because of environmental constraints in Japan, not because it was cheaper.

Jesse Open

Interesting though that Thunder Tiger (Taiwan) also produced engines with nickel plated bores but we don’t seem to hear many horror stories about them . Please, I am in no way bashing O.S.. Merely pondering if being smaller numbers perhaps there was little said.

jaka54

Agree with Greg Doe!
In my 49 years in this hobby flying everything in R/C with regards to glow engines and been a pylon racing freak since I started flying 49 years ago and flying F3D pylon for many years I have not heard of one and nor myself preheated any engine.

1QwkSport2.5r

True - I have never seen anyone preheat an airplane engine either. Until I got some Jett engines myself, I hadn’t either.

I had to replace the bearings in my BSE .35 because the ball retainer started coming apart some time ago, so I had to take it apart and replace them. I had noticed the piston crown had a noticeable ridge where the top edge met the side. I have my doubts it was manufactured this way, but rather the ridge formed from being swaged into the taper. I believe this probably wouldn’t have happened, or been as pronounced as it was had I preheated before starting it. When I got my .56LX, I conducted an experiment. Before I ran it the first time, I checked the piston crown for this ridge; there was no ridge (or at least not one I could feel). I preheated the engine before starting, and ran it in. After approximately a similar amount of runtime (several gallons of fuel), I checked the piston in the 56LX to compare. Lo and behold, there was no ridge. Is it coincidence the .35 was run from cold and has a ridge around the piston crown and the .56 being preheated before running doesn’t have a ridge? Maybe. Maybe not.

Honestly, I’m not trying to argue, but rather add some perspective. The main advantage to preheating an ABx specific engine, is ease of starting and tuning. Recoil starters and shaft starters are less robust; combined with a one-way clutch bearing driving the crankpin, the stress on the starter and crankpin is already high. Add some stress from a tight pinch… well, you get the idea. Preheating the cylinder makes a world of difference in getting these engines started. Propellers being rigid make it that much easier.

Just because the aircraft guys don’t preheat their engines doesn’t make it/them wrong or unacceptable; all I’m saying is it does make life a little easier in some cases, and it has proven to me that it physically makes a difference within the engine too. Not all engines will realize the benefit though.

I would bet a dollar Dub will attest to, and I’m sure Henry Nelson and John Shannon would agree as well, tight engines are a good thing. The longer they’re tight, the better.

This thread subject is about making a piston fit tight again; it’s just going about it the wrong way. No different than using the EBIS system to “break in” an engine. Just because people do it; doesn’t make it right.

Those that have never preheated an engine, will not know how different it is compared to just fueling an engine and firing it up cold. Just because some people do it doesn’t make it wrong.

1QwkSport2.5r

I’ve had an OS and a TT liner set side by side; they look quite different to me in terms of what the surface looks like. The TT cylinder looks like the plating layer is quite a bit thicker than the OS. Visibly it just looks like a higher quality piece; and the fact that there have been few issues with the TT cylinders would confirm that, IMO.

GREG DOE

Jesse Open, I understand that there are two different types of nickel plating, but I'm not knowledgeable enough to discuss them. Mike Langlois tried educated me on this topic, and he wasn't fully informed about the differences. At one time I was mistakenly saying that the Thunder Tiger engines were chrome plated, and I was politely corrected by QwkSport. Thankfully, whatever TT used it doesn't peel!
jaka: We just learned that the 2025 pylon World Championships will be in Rothenburg, Germany next July.Will you be there? I might get there myself?