Need help with an old engine - Page 5 - RC Groups
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Feb 18, 2012, 09:29 PM
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DL8698's Avatar
Originally Posted by jfacky
I would use some machinest dye on the piston and "run" it up and down the liner a few times. It will show you the high spot by being rubbed off. Then use a jewlers file or emery board to take down the high spots


I have taken out the liner and piston. I find that there may be a very slight high spot in 2 places which i am working on now to remove.

I have a question, regarding the piston diameter. Does the bottom edge of the piston skirt have identical size as the top of the piston? I seem to see the top of the piston go into the liner very easily and smoothly, while the bottom of the skirt at about 3-4 mm from the bottom edge starts to get a bit of friction.

I'm not sure if this is due solely to the high spots which I will try to remove, or is the piston meant to have a bit of interference near the bottom edge, for crankcase compression, maybe?

Should I keep working on the high spots till the piston slide in smoothly, just as it does with the upper part of the piston, or should I stop once I've removed the high spots. BTW, the piston seems to be a real soft material, cos I'm causing quite a bit of scoring marks with my jeweller file and 600 emery paper.
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Feb 19, 2012, 02:21 AM
What goes up must come down
jfacky's Avatar
on glow motors, The liner is tapered towards the top and the piston is the same diameter top to bottom. on larger displacement engines (automotive ect) the piston is tapered larger at the top and narrower at the skirt.

It's ok t remove a little bit more than needed to stop the piston from rubbing as long as its not from the top sides, just the bottom of the skirt.
And yes it could do much more damage to the liner than is already there.


Last edited by jfacky; Feb 19, 2012 at 02:30 AM. Reason: more damage
Feb 19, 2012, 10:54 PM
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DL8698's Avatar
I managed to start the engine after just a few turns, but this time I had the glow nice and lighted although not really all that bright. But as I was trying it out on my front yard and probably waking up my late sleeping neighbours, I only ran her for a few seconds, just to see if she will even fire up at all, with my 20+ years old methanol.

Later I will go the flying field and give her a good, real rich run in to seat the piston properly again.

Thanks for everyone's help and advice.

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