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Old 06-20-2008, 11:51 PM
RLorange RLorange is offline
Motorized Bicycle Elite Member
Join Date: Jun 2008
Location: Australia
Posts: 127
Exclamation New Cylinder is taller; ports bigger

I just purchased and installed a new cylinder and rings for my 70cc engine because the original was poorly cast with bubbles in the side which started to leak combustion gases directly through the cylinder wall! Strange problem I know.

The new cylinder comes from a differenc factory and is a much cleaner cast but I noticed straight away that both the exhaust and inlet ports are much bigger and now are a major mismatch for the manifold and muffler.

Also the whole cylinder sits around 1mm taller. I can tell because of the stain which occurs at the very top of the inside marking the maximum rise of the first piston ring.

Question: How much of a difference would 1-1.5mm make to compression? The engine is really piss-weak I havn't broken it in yet. Would it be worth laything off the extra to boost compression? maybe a bit extra would be good too!
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