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Old Jan 30, 2013, 09:01 PM
Balsadustus Producerus
Escondido, CA USA
Joined Jan 2001
1,020 Posts
Help!
Stuck contrapiston...again

All you "dieseleers"...

I recentlly bought a new MP Jet .040 diesel, mounted it in the test stand and it lit up almost immediately. What a smooth runner! However--
during the second run, I backed off the compression a little just till it burped, and then, while attempting to increase the compression the contrapiston stuck right firmly. No way to induce any movement at all. Rather than force things in a new engine, I disassembled the cylinder by pulling off the headfins, removed the cylinder and found I needed a stout wood dowel and a small hammer to remove said contrapiston. Now that it's out, it won't reinstall for whatever reason. Both the cylinder and the contrapiston are some sort of steel, or perhaps the contrapiston is a fine grade of iron. Anyway, I'm not certain how to turn the contrapiston diameter down as there is no way to chuck it in a lathe. Other idea I had was to open the upper cylinder a little but I imagine MP Jet tapered it for a reason so here I am. There is no sign of carbon deposits causing this as it has only two runs, not counting any running the factory may have done prior to delivery. No sign of scoring either in the cylinder or contrapiston. Don't believe it got overheated as the compression was low to start with and I never got around to leaning it as the compression was stuck and it produced quite a lot of oil smoke, so I think I have an engine that was set-up too tight at the factory. I'm not about to send it back to the Chech Republic so I'm open to any ideas for a fix.
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Old Jan 30, 2013, 09:15 PM
Registered User
Joined Jan 2008
1,202 Posts
What I have done is take 400 wet or dry sandpaper and carefully sand the (iron) contrapiston lateral surfaces down until the contrapiston fits to suit. Take it really slow and check the fit after every few swipes. I've successfully done this on several engines. The surface takes on kind of a polished look, with individual scratches invisible to the naked eye.
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