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Old 12-11-2008, 06:44 PM   #1
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Default what does "pinch" mean?????

ive heard alot of people say this term what does it mean for instance the pinch is gone?
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Old 12-11-2008, 06:58 PM   #2
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When a P&S are new (and cold), they have a tight fit at the top of the stroke. Over time through wear and subtle annealing by the engine's heat cycling the fit between the piston and sleeve opens up. Eventually there is enough wear that the cylinder begins to experience Blow-Bye (loss of seal). Pinch is an attempt to re-establish the snug fit at the top of the stroke by compressing the sleeve radially, reducing the I.D.

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Old 12-11-2008, 06:59 PM   #3
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The sleeve on a nitro engine is tapered so it is smaller at the top. This is how it seals on the piston to create compression. When an engone is new, the piston needs to seat itself. Until the engine is broken in you will have what is called mechanical pinch. If you can easily turn the motor over by hand and get very little resistance it means the machanical pinch is gone. When it wears further you are at the end of the road and the engine will become unreliable and you will get random flame outs.
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