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Old Nov 18, 2012, 07:59 PM
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Quote:
Originally Posted by Cougar429 View Post
If there is no air leak at the carb base or rear cover then likely the air is coming in past the front bearing. In a lot of cases there is an oil return passage milled into the bore behind the front bearing that can be filled with JB weld or Loctite metal epoxy to minimize the air leak.

GMS and their TT clones were notorious for this one and that was the only solution. Even a rubber sealed RS type bearing would not work that well.
Doubt there is an air leak behind the front bearing. First there is a positive pressure in the crankcase so the chance of air coming back is slim to none. Oil moves forward.That's how the front bearing is lubricated. Second, most ball bearing engines don't use a sealed bearing anyway. They usually just have a metal shield that keeps crud from getting into the bearing. Liquids and air pass right through them. As a matter of fact Super Tigre didn't even have the shields in their bearings. The seal is made by the crank to crankcase fit but it still allows the oil to move forward and lube the bearing. As a matter of fact that's how non ball bearing engines work. If that fit is poor all you get is a wet nosed engine from the oil. Same with BB engines.

I have dozens of BB engines and most use a metal shield and aren't sealed

Here are a couple pics of shielded and non shielded bearings. The shielded came from an OS. Also a pic of a Super Tigre G21. No shield, no seal. The only seal is the crank to crankcase fit.
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Old Nov 19, 2012, 12:01 AM
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datsunguy, you're correct up to a point. If you look at the power and pressure pulses inside the case there is negative pressure as the piston is travelling up on compression. This is what draws fuel and air in through the carb. The port in the crank is supposed to open as that cycle starts and close by turning past the carb port as the piston reaches TDC and starts back down again. The intent is to close the case and then force the fuel/air charge up into the combustion chamber, as well as oil through the bearings or bushings, (if they have them as some ride in the alloy casting alone).

This is normal operation and one that works well, even with open bearings that Fox and others used successfully for years. In those cases the motors were well made and likely to closer tolerances with better bearings to start with. Even so, some oil was expected to exit the front.

In the GMS and TT clones that is not the case with a poor fit between the crank and casting that creates the path for air entering when the case goes negative with the carb CLOSED. Unfortunately the combination of that poor fit and return groove increases the available area for air to feed in when the case goes negative. That and some fuel draw in through the idle bar even with the barrel closed is why I suspect the engine continues to run.

If you can come up with another explanation of where the air is entering and how it mixes with enough fuel to keep the motor running I would love to hear it. Perhaps I could be wrong, but my own experience with runon points in the direction I stated.
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Old Nov 19, 2012, 05:44 PM
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Yes there is a negative pulse, as you call it, in the crankcase but the positive is higher and the overall pressure is positive. That's why we pressurized our fuel tanks with crankcase pressure when we flew control line. Crankcase pressure was (and still is) about 4 pounds.

The air leak is probably coming from the carb itself. These were crumby engines in the first place and they had very lousy carburettors. I worked at a major hobby company as an engine repair guy at one of the stores. I found out that if you replaced the stock carb with an OS carb the crumby engines ran quite well, The original carbs leaked around the rotor so the engine wouldn't quit. You also need fuel to keep and engine running.

Oh, by the way. I have a pretty good handle on how these engines work. I've been using model airplane engines for over 50 years. I have even used pre war ignition engines. I have over 500 engines in my collection. My first engine was (and still is) an OK Cub 049X. Still have it. Used in in control line and free flight.
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Old Nov 19, 2012, 11:39 PM
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Sounds like you too have been around the block a bit and have 20 years on me, but as I stated early on I tried different carbs, including several OS and had no success, Closing the oil return and a sealed front bearing helped a LOT.

I also agree wholeheartedly about their quality. The size of that front bearing alone is an indicator where they skimp.
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Old Nov 24, 2012, 04:45 AM
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Mark,


Instead of relying on the throttle to choke the engine to death, why don't you just pinch the fuel-line...

Doing that will kill any engine within two seconds. It will happen even if your engine is ridden with of air-leaks.

This is the recommended way, since doing it with the engine at full-throttle dries all the fuel, leaving no hygroscopic methanol that might make the bearings 'go south' in storage...
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Old Nov 24, 2012, 05:27 AM
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Here we go again....

It is not the methanol that causes that: I shut down my engines ALWAYS by closing the throttle after at least a 30 to 60 sec idle, with the intent of loading them up with a nice amount of oil before shutdown. I live in cold, damp Holland, store my engines without any form of protection in a 2-month use, 2 moth no use schedule, and the particular one on the picture below has been put away like that for close to a year, in an unheated room.

It is the Nitro that does that, possibly in combination with poor quality of oil (Castor can be acidic, Nitro causes heavily corrosive by products, and lots of synthetic oil offer no or insufficient protection).

The engine in the pic runs on 2% Nitro and 15% fully synthetic, has not seen a drop of After Run Oil or even castor since I own it (over 3 years now). It has been used by a previous owner using Castor. I did only clean exterior lightly, and let the synthetic wash out the castor residue. The small spots of rust were allready there when I bought it (don't have pics of that).

If you don't believe that, just stop an engine on the throttle, and as quickly as possible open the backplate. You will find no fuel, but oil, and possibly traces of methanol, but they will evaporate before attracting sufficient moisture to do damage.
But use high amounts of Nitro, and the oil found in the crankcase will be acidic from the Nitro combustion products. With a good oil, even that does not need to be a problem.

Brgds, Bert
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Old Nov 24, 2012, 11:13 AM
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Bert,


I believe most knowledgeable engine guys know better and recommend the high-RPM fuel-line pinch for engine shutdown.

It is a fact that methanol is hygroscopic, which means it absorbs moisture from the air.
Just leave your fuel-jug open for a period of time in a high-humidity environment; and your fuel will become unusable from being water-logged.

A small amount of corrosive nitric acid is formed during the combustion process, but only a very small percentage of the combustion products do end up below the piston.

A little castor oil (or Fuchs Aerosave) in the fuel is quite enough to prevent contact between the corrosive residue and the bearings.
Methanol on the other hand does not evaporate quickly enough and it absorbs the moisture very quickly, from condensation inside the cooling engine.

This moisture is the major contributor to bearing corrosion, although I agree higher percentages of nitromethane do cause accelerated corrosion.
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Old Nov 24, 2012, 12:27 PM
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If you don't believe the pictures attached..... than what can I say?

Well.... this:
All my engines look like that on the inside, and ALL are stopped on the throttle, after a sufficient cool down run. NONE get After Run Oil or any other form of conservation. Most of my engines I bought new, and only the ones I did buy second hand show traces of rust. I need to rely on my engines, I need them for my flight school. I wouldn't knowingly do anything that promotes rusty bearings.

On helicopters a high RPM pich is physically near impossible, and helicopters are not specially known for rusty bearings.... High Nitro operated engines on the other hand, are....

I have not really much respect for anonymous "knowledgeable people"....
Knowledgeable people once said electric flight would never be possible or at least never as powerful as IC engines....
Knowledgeable people once said that in the year 2000 all power would come from nuclear plants, and these plants would never blow up.
Knowledgeable people after that, said, oil would run out before the year 2020 (some official statement back in 1990 or so).
Knowledgeable people once said, computers would never fit in a briefcase
Knowledgeable people once said humans would loose consiousness at speeds over 40 km/h.
Knowledgeable people even once said, the earth was flat....

And you know what? Methanol doesn't give a rats a$$ what knowledgeable people say.

Hygroscopy of Methanol, and the corrosive action of water in Methanol is highly overrated.
You need water in percentages very well before the decimal point, before that water becomes a problem. Any idea how long you need to leave your fuel can open to achieve that? I do ( I used to transport the stuff in bulk): under normal humidity levels (the "human comfort zone", 70~80 % rh @ 21 degrees) it takes several days for a can of fuel to absorb that much water. Even the best quality Methanol contains typically 2 grammes of water per litre, and you need over 10 to get into problems.... It basically means that for every litre of fuel you need to pass at least a full cubic metre of air through the can, and the fuel has to absorb at least 50% of the water present in the air, in order to reach those levels. Now, I usually do not put forced ventilation on my fuel cans....
Hygroscopic is not the same as magnetic: it only absorbs moisture from the small volume of air in direct contact with the methanol, and natural ventilation in a fuel container is normally very, very slow....

Your engine's crankcase will typically be void of Methanol in about 10 to 20 minutes, and after that there is no hygroscopy anymore. If the induction port is closed so the Methanol cannot easily escape, that means that air containing humidity can also not enter....

Want proof? stop a warmed up engine after 30 secs of idle, using the throttle. After stopping disconnect the fuel line, and try starting it 5 minutes later....
Do the same, now immediately after stopping turn the crankcase so the induction ports are closed....
Both cases you will not get any signs of life, but in both cases that engine stopped with a crankcase full of combustible mixture and in the second case you even did everything to prevent that mixture from escaping.... (and yes, I tried it before I wrote it)

The pinching stop to prevent corrosion from hygroscopic Methanol is a myth.
The corrosion from High Nitro content fuels is not.... the pinch method works in engines using poor oils and high Nitro, but that has nothing to do with the methanol being hygroscopic.
Another thing: in a cooling down engine (meaning it is hotter than ambient) condensation is impossible.... just plain physically impossible. Water condenses on cold surfaces, not on warm ones. In fact, any air entering that still warm crankcase, will only be able to absorb MORE moisture, making it harder for the methanol to extract it from the air.
Under normal conditions, venting through the crankcase of a stopped 2-stroke is near zero, meaning the methanol present can only absorb the moisture present in the last induced charge (being equal in volume to the engines displacement) which typically comes down to less than microgrammes.

Nitric acid combustion products will readily pass along the piston into the crankcase: on my .90 size fourstroke the amount of oil passing the piston ring is more than 2 cc per minute, on a 2 stroke, where twice as much power strokes take place, you can bet it is significantly more. That oil will contain these nitric acid combustion products, ergo, Nitric WILL be present in the crankcase.
By pinching the line at WOT, you will flush out this Nitric acid partly, but that is about it....

Just to be clear: I am not saying that the pinch stop is not preventing corrosion. All I am saying, is that the hygroscopic properties of Methanol have nothing to do with it....

Brgds, Bert
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Old Nov 24, 2012, 12:39 PM
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OK, you win, Bert.

I have an MVVS .49 that has nearly 150 hours on it.
It is still running with the original bearings.

My Saito .72 also runs with the original bearings.

I use 5% nitro in the MVVS and 15% in the Saito.


Neither of the engines has a smidgen of rust on the bearings, although there are 'gingi' stains on the crank-webs in both.

I do stop both engines with the pinch.
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Old Nov 24, 2012, 01:46 PM
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And still, Bert, the pinch will get any engine to stop.
Don't you agree?
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Old Nov 25, 2012, 03:21 PM
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RC car engines invariably need to be pinched to shut down if they dont run out of fuel in the tank first which is much more common form of running. (They dont fall out of the sky when they run out of gas)
And I am surprised at how well the bearings hold up in them.
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Old Nov 26, 2012, 11:07 AM
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You are partially correct in that fuel/oil tends to be pushed out of the front bearing when the engine is running. But some engine companies, in a attempt to reduce oil oozing out of the front bearing, went with bearings that were more sealed, developed anti-oiling spiral and striaght, along with a anti-oiling hole too.

The anti-oiling spiral groove is on the crankshaft, and it is designed to pull oil away from the front bearing. It doesn't extend all the way to the front, but stops short of it.

The anti-oiling groove was another method used. In this case they cut a shallow groove into the crankcase from the front intake port under the carb to the front bearing. This allows the suction on the intake stroke to suck oil away from the front bearing. Obviously, if the groove is cut too deep or made too large you can have problems. Plus if the front bearing seals are not working you get a air leak.

The anti-oiling hole is another method used. They drill a small hole behind the bearing extending all the way to the intake port on the crankcase under the carb. This does the same thing the anti-oiling groove does. On the intake stroke the engine can suck oil away from the front bearing. The same issues occur like with the grooves cut into the crankcase. You can get an air leak developing there.

Now there can also be the occassional problem where the hole in the crankcase is bored just a hair too large in between the front and rear bearings and this allows for either too much oil to go out of the front or a air leak in the reverse. But some racing folks were known to open up the hole slightly to reduce the oil friction drag inside there too. Since the racing folks either kill the engine (block the fuel off or flood off methods) and run wide open throttle, they didn't care for idling or mid range response with their engines. So it didn't bother them.

Here are some pics showing the two methods of reducing oil coming out of the front of a engine. These two methods can result in a air leak developing in certain conditions, namely if the front bearing seals allow air to get inside.





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Old Nov 26, 2012, 11:59 AM
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Earl, you pretty much confirm my original posting about the run on problem originating in the front. I have NEVER seen a quality built motor exhibit this at all, even Fox that has the tendency to chuck a large quantity of oil out the nose, yet a large majority of the GMS and their TT clones do.

As for what happens after the engines are shut down and the type of lube we can argue all day. This is my own experience: I make the effort on my 2-strokes to pinch the line at high throttle and then cap both the exhaust and carb inlet at the end of every session. As I run 15% nitro and a synthetic/castor mix lube I do add ARO, my own 50-50 mix of ATF and air tool oil, before buttoning them up. One thing to remember is the motor will cool and draw in air/moisture as a consequence and temp changes during storage cause the same behavior.

I guess with close to 30 years running nitro motors, including firing up some that have sat in storage for 20+ gives me at least some credibility.

As a postscript, I find most bearing issues here are magnified by their quality. With the market swamped with cheap Pac Rim options motors with them installed generally wipe a set out per season regardless of care.

'nuff said.
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Old Nov 26, 2012, 02:29 PM
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Well the classic air leak problem is usually through the carb somehow. One needs to push down on the carb hard to compress the O-ring and then at the same time, tighten up the draw bar screw. If the carb is slightly loose in the socket in the engine, it could still maybe suck some air, especially if the engine is hot. The other point is that the draw bar and screw can leak too. Then it doesn't happen often, but the throttle barrel could be too loose and let air in too. On one occassion I saw a engine that ran without the low speed mixture nneedle screwed into the side of the throttle barrel. You couldn't stop it at idle either. But that one had me stumped as to why it ran Ok otherwise.
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Old Nov 26, 2012, 03:15 PM
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I suspected a faulty carb or seal at first, as well, but double sealing even OS carbs removed from perfectly tunable engines had no effect on the behavior.
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