So, being a novice, I have to voice an opinion...

I have done a fair amount of reading, and yes, the bore of non-ringed engines is "conical" in that it is slightly narower at the compression end of the stroke.

The reasoning for this is *not* to get better compression/seal, but because a running engine is hotter at the top than at the bottom (the combustion happens at the top of the stroke, and cools throughout the descent). Thus, the top end of the cylinder head expands more than the lower end, and thus the heat of the combustion expands the con-shape into a cylinder, or as near as dammit!

The intention is to make the "conical" cylinder a cylinder with expansion.

Then again, I may be wrong.

gus