McCoy 35 bottom of piston exposed

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McCoy 35 bottom of piston exposed

Postby Itmustbeso » Wed Sep 16, 2009 9:35 am

I just purchased a used McCoy 35 off Ebay(looked good in the picture!).
I noticed that there is about a .040" gap between the bottom of the piston
and the black piston liner when the piston is TDC. You can actually see a little bit of the connecting rod. I have never seen this
on my other McCoy 35, Fox 35, Stallion. Is this normal or did they replace the piston or shave the bottom off.

Thanks for your advice.
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Postby chiefss » Wed Sep 16, 2009 10:50 am

It's called sub piston induction and lets a bit more air into the lower case. Several manufacturers used it.
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Postby walt » Wed Sep 16, 2009 12:42 pm

I just looked at a couple of my 35s and there's no sub piston induction on whatever models they were. Somewhere between the first model and the Lightning Bolts. The black cylinder port edge extends approx. 1/32nd inch above the exhaust floor. Possibly some models had sub piston induction, but your cylinder may be in backwards also.
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McCoy 35 going back

Postby Itmustbeso » Wed Sep 16, 2009 12:53 pm

Thanks for the info. I took a careful look at the motor and the needle valve is about to break off and the cylinder/piston look scored. It's
going back.

Harry
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Postby Jim Thomerson » Wed Sep 16, 2009 1:34 pm

I'd bet your liner is in backwards. The piston and head may be backwards as well. I've seen this in a couple of used engines I bought cheap. Turned things around right and they were fine.
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sub-piston induction

Postby loucrane » Thu Oct 01, 2009 1:07 am

Jim may wel be right, although several older engines used this to supplement the amount of air "inhaled" during the intake cycle.

It worked, to a degree. Today, most everybody wants to run a muffler. Mufflers and sub-piston induction don't work well together. Instead of a chance to draw clean cool air in while the bottom edge of the piston is exposed, mufflers tend to trap the hot exhaust residues, filled with burned gases, near the opening.

Engines like this often run too hot, and at less power, with a muffler. Neither one is desirable for a long-lasting, powerful engine to use.

If you don't need a muffler to fly around home, sub-p induction usually doesn't do any harm. Remember, tho, many older black-fin McCoys -iron piston in steel cylinder - do better and last longer on 25% castor oil fuels...
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