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Stuck Contra - What is your fix?

Old 12-25-2010, 07:14 AM
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RocketRob
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Default Stuck Contra - What is your fix?

I was flying (trying) my new/old converted (Freeflight to RC) .020" Answer with my .25 schlosser yesterday for a maiden. This is my high time engine pulled from the littlest Lancer plane. It worked well in the lancer inverted and I rarely moved the contra due to the full cowl and inverted mounting.

Any way, I got her running and while trying to back off a bit the contra was stuck. I tried richening the mixture to where she would barely run and no joy, then in the richened state tried bumping the contra screw in and out to break it loose. This was unsucessfull. Stopped the engine and carb primed it and no go either.[&o]

I thought I recall reading a thread on a PAW stuck contra and someone suggested a real "pearl" of an idea while running but for the life of me and my diminishing grey matter can not remember or can I find the thread.

So what do you guyz do when running and she wont back off?
Old 12-25-2010, 08:30 AM
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Default RE: Stuck Contra - What is your fix?

RocketRob, I have used the following for stuck contras on rear induction motors. Start the motor and from the rear with one hand back off the compression a bit. With the other hand pulse choke the intake at a rapid rate. I started using this in the late 1050's with a rear drum induction Super Tigre G.30 2.5 cc diesel. Also I have used it on Mills and other rear induction diesels. As the engine warms up less choking required. I would not use on a large front induction diesel but have used in on small front induction ones. Be carefull of the prop with front induction diesels. I do this even toda
y with my Oliver Jaguar (Panther and GB diesels as I do not have to change the compression or needle valve settings. Jack
Old 12-25-2010, 11:45 AM
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JohnAV8R
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Default RE: Stuck Contra - What is your fix?

RocketRob
see post #1480
http://www.rcgroups.com/forums/showt...054975&page=97
and the suggestions that followed.
If this doesn't work you may have to take the engine apart and use a small wooden dowel in place of the piston and tap the contra piston out.
John
Old 12-25-2010, 11:59 AM
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Default RE: Stuck Contra - What is your fix?

On most PAW engines I have it will move up once the engine has cooled down properly.
If it has actually seized, then I guess it will have to be taken apart.
Old 12-25-2010, 02:50 PM
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Default RE: Stuck Contra - What is your fix?

The technique with a traditional diesel with a contra piston in the liner is this.

Stop the engine.

Disconnect the fuel line or cramp the fuel line or empty the tank.

Back off the compression screw.

Port prime against a closed piston.

Flick hard against compression.

Motor should fire and run a "micro" burst.

Continue untill the contra moves back hard against the comp screw.

This almost always works.

Ralph

ORIGINAL: RocketRob

I was flying (trying) my new/old converted (Freeflight to RC) .020" Answer with my .25 schlosser yesterday for a maiden. This is my high time engine pulled from the littlest Lancer plane. It worked well in the lancer inverted and I rarely moved the contra due to the full cowl and inverted mounting.

Any way, I got her running and while trying to back off a bit the contra was stuck. I tried richening the mixture to where she would barely run and no joy, then in the richened state tried bumping the contra screw in and out to break it loose. This was unsucessfull. Stopped the engine and carb primed it and no go either.[&o]

I thought I recall reading a thread on a PAW stuck contra and someone suggested a real "pearl" of an idea while running but for the life of me and my diminishing grey matter can not remember or can I find the thread.

So what do you guyz do when running and she wont back off?
Old 12-25-2010, 03:43 PM
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Default RE: Stuck Contra - What is your fix?

This worked on my MVVS 15 D after a few runs no longer an issue> It would just"pop up" then loosen till miss, than in maybe a quarter turn in for that nice diesel hum , have 3 but only had the issue with one of them martin
Old 12-25-2010, 05:22 PM
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Default RE: Stuck Contra - What is your fix?


ORIGINAL: RocketRob
So what do you guyz do when running and she wont back off?
Hi ,
If the engine is up torunningtemperature then the contra will be the hottest part of the engine and so will have expanded the most.
So what you want to do is reverse that situation by making the hottest part of the engine the liner, then it will expand away from the contra piston.

Simply start with a cold engine, use a heat gun on the liner, fins and all, to heat it up so that you have trouble grasping it, then back off the comp screw and inject a small amount of oil into the combustion chamber.
Slowly and carefully turn the crank and the relatively cold contra should move along the hot liner.

There is a link provided in this thread about the use of an ultra sonic cleaner, just be very wary about using high frequency sound waves if your engine has ball races still installed in it - and I believe that allSchlosser's have. The bearings have moving parts and will wear rapidly against each other when subjected toultrasonicwaves.

The crock pot method will also work but it is a cleaning method and if your engine is not gummed up or dirty then do not bother.

Schlosser's are precision made engines so my bet is that there is a little bit of carbon above the contra that is stopping normal compression from pushing it past that.

Up the comp by using something lesscompressible and reversing the expansion rates - that should work for you.



Old 12-25-2010, 08:05 PM
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Default RE: Stuck Contra - What is your fix?

As the engine has been run relatively recently,and was used in an inverted position, it probably only has a thick ring of (fairly pliable) castor oil running around the edge of the contra. On an engine that small, its not advisable to apply excess pressure to the crank-pin or rod, as breakages may occur. Try this - remove the compression lever from the cylinder muff, and find a piece of hard wooden dowel (or alloy rod) that fits down the hole - about an inch long should do it. Warm the engine up in an oven until its just too hot to pick up with your bare hand, insert the rod down onto the top of the contra-piston, and give it a sharp tap with a SMALL hammer. It should move easily. BOB
Old 12-25-2010, 08:42 PM
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Default RE: Stuck Contra - What is your fix?


ORIGINAL: 74SEVEN7

Try this - remove the compression lever from the cylinder muff, and find a piece of hard wooden dowel (or alloy rod) that fits down the hole - about an inch long should do it. Warm the engine up in an oven until its just too hot to pick up with your bare hand, insert the rod down onto the top of the contra-piston, and give it a sharp tap with a SMALL hammer. It should move easily. BOB
Wouldn't tapping the contra through the compression screw hole drive the contra piston down further and thus make the issue worse?

I don't advocate stressing the rod or crank in any way either but simplyplacingenough oil into the chamber to convince the contra to be pushed up and the key to this is expanding the liner away from the contra and then using the natural mechanism of compression to do the rest.

I hate taking apart precision machinery and avoid it at all costs. Oil gumming is easily solved by heat, 200C in an oven for 10 minutes normally frees things up for me if its bad and the only 'problem' that I have come across is carbon build up. It can get crystalline and hardenoughtoscratchthe bore if you drive a contra down past its normal range of operation.

Old 12-25-2010, 09:34 PM
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Default RE: Stuck Contra - What is your fix?

The main aim with a stuck contra-piston is to MOVE the thing, either up or down. Once this is done and lubricant can reach the infinitesimal gap twixt the cylinder walls & the contra, then it will again move smoothly. Tapping the contra down has 2 advantages, 1) the bore in a Diesel is usually tapered inwards towards the top, thus as the contra comes down, it gets looser, and 2) trying to force a stuck contra-piston up by using the other mechanical parts of the engine puts IMMENSE pressure on the rod, crank-pin & crank-web. As this particular engine would not be stuck as tight as say, a 50 year old Diesel which is NIB, it should be possible to free it up quite easily by tapping the contra down, then squirting some oil down the compression adjuster hole so's the contra will then be sliding back up on a lubricated surface. Speaking of old Diesel's that have lain in their box for decades un-run, I have seen the thread in the alloy head stripped out, merely by forcing the adjuster to turn whilst trying to move a stuck contra-piston. This gives you some idea of the forces involved in trying to get a contra-piston to move ! BOB
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Old 12-26-2010, 05:56 AM
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Default RE: Stuck Contra - What is your fix?

Guys, Thank you for the great information!
The poor little engine was burried deep within a cowl, inverted with the compression screw replaced with an internally wrenching socket screw for acess with a allen key.

I flew this plane every time I went flying for the last 2 years till it met it's demise, - trying to share the same airspace with a POS foam Corsair or AT6 or whatever.

It had a soldered up brass fuel tank of around 1/3 oz and would fly for 15+ minutes. It was rode hard and put away wet and stuffed into the ground on more than one occasion. I rarely even adjusted the comp screw preferring to let it run a bit under compressed as our weather conditions vary minimally through out the year. The contra piston will go down with the aid of the screw but wants to stay there.

So upon rekitting, decided to bring the little engine out into the light of day.
Old 12-26-2010, 10:17 AM
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Default RE: Stuck Contra - What is your fix?

Well I tightened up the comp screw to just break the contra loose, took out the comp screw and injected a drop of fuel inside the top and replaced the screw backing it off 1/4 turn. Primed and flipped and was reassurred with the slap of the contra piston slapping up against the backed off comp screw while it ran off the prime indoors. Worked her back down back and forth and seems to have removed the castor/carbon gum.

Thanks for everyone's help! I'll get the plane back outside and run it up to operating temps and fiddle with her.
Old 12-26-2010, 03:27 PM
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Default RE: Stuck Contra - What is your fix?


ORIGINAL: 74SEVEN7

The main aim with a stuck contra-piston is to MOVE the thing, either up or down. Once this is done and lubricant can reach the infinitesimal gap twixt the cylinder walls & the contra, then it will again move smoothly. Tapping the contra down has 2 advantages, 1) the bore in a Diesel is usually tapered inwards towards the top, thus as the contra comes down, it gets looser, and 2) trying to force a stuck contra-piston up by using the other mechanical parts of the engine puts IMMENSE pressure on the rod, crank-pin & crank-web. As this particular engine would not be stuck as tight as say, a 50 year old Diesel which is NIB, it should be possible to free it up quite easily by tapping the contra down, then squirting some oil down the compression adjuster hole so's the contra will then be sliding back up on a lubricated surface. Speaking of old Diesel's that have lain in their box for decades un-run, I have seen the thread in the alloy head stripped out, merely by forcing the adjuster to turn whilst trying to move a stuck contra-piston. This gives you some idea of the forces involved in trying to get a contra-piston to move ! BOB
Well done Bob! The above seems to have solved this problem nicely.


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