New Cylinder is taller; ports bigger

[RLorange]

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!

 

[stude13]

it would be best to return everything and call your credit card co. like my dad always said--you pays your nickle and you takes your ride.

 

[RLorange]

LOL, Pretty bad luck with the bubble-casting I reckon. My supplier does have a 3 month waranty but the leak only started after 8 months of hammering my engine daily.

A bloke in Australia took the cylinder onto a metal lathe and shaved 3mm off the head increasing compression BIG TIME. Unfortunately combined with a expansion chamber muffler he blew one of the head bolts stripping it out off the engine block !!!

He said the compression was boosted so much that it barely ticked over even pedaling fast!

3mm might be too much...

 

[microbore]

1mm to 1.5mm makes a big difference in compression. Before you put any labor into decking the cylinder or milling the head, check for obvious problems in port timing. With the piston at bottom, the transfer ports should be fully exposed and the bottom of the exhaust port should be flush with the top edge of the piston. If the ports are below the piston, don't waste your time.

You may be able to get back some of that 1mm by using thinner cylinder and head gaskets (beer cans are a good source of thin gasket material).

Either way, just make sure that you are leaving about 1mm of space between the piston and head at TDC when you're done.

 

[TurboNegro54]

post a side by side picture of each cylinder and the ports to compare. this might be a good thing if it comes with bigger ports, worst comes to worst you can just mill the head ir put a smaller head gasket on it.

 

[flybytaco]

about 99% of theese engines absolutely do not expose full ports never seen it