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Old 02-07-2012, 08:52 PM
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aussiesteve
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Default RE: New (to me) internet myth

ORIGINAL: Truckracer
OK RC, while talking about engine bores .... do quality cylinders have any bore taper? I have to say I've never measured this on any 2 stroke gasser I ever worked on though I do check ring end gap from top to bottom of stroke. I know some full scale aircraft cylinders have a bit of taper to allow for the top of the cylinder running at a higher temperature than the bottom .... theory being that when up to temperature the bore would be straight. Just curious.
Yes, in a true high performance engine such as a full sized Aero enigne, the bores do in fact have a taper when at room temperature. This is to allow it to be "round and parallel" when at operating temperature.

Generally on our RC sized Gas engines there will be no engineered taper. The cylinders are too small and the power outputs are too low to require it. Even on the best quality engines, the amount of taper would be very small even if it was intentionally added.

So the bores should not only be parallel but they should also be round. Very few (by numbers sold) actually are

This is because the majority of the engines that are bought nowadays have a very basic manufacturing technique used. Teh cylinders are molded on a mandrel, chemically cleaned, flame plated with Chrome then the final surface finish is achieved with a basic hone unit.

That technique results in bores that are neither parallel nor round. They are however cheap to produce that way and they will allow the engine to run. What they do not allow is good ring sealing so power is lost.

Roundness is difficult to measure due to the open port designs commonly used (another source of power loss).