Modern Vespa

Hello friends, I ordered rings and was checking the gaps, when I noticed an inconsistency from the gap where the rings are in the middle of the bore just above the ports, vs the gap near the top end of the cylinder.

This suggests to me the cylinder is 'worn in'. Is that the case or is this 'normal'? According to some posts, ring gap should be measured closer to the middle of the bore, but if the rings travel to the top and they expand due to heat, I imagine that the ring ends will be forced together and cause a 'collision'.

The middle of the bore one ring measures at 0,35, but near the top end, it measures ~0,05. Quite a difference

Should I replace the cylinder and get a new kit? (piston, rings, cylinder) or can I get the bore machined, then upsize the piston and rings?


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What cylinder?

I will state this- check with manufacture of the piston first for ring gap and bore clearance.

That being said-
I would have zero confidence with an end gap of 0.05. Ring gap on top should be .25-.45, I usually shoot for .3 for standard LF. At 0.05mm that thing is on the very edge of expanding larger than the cylinder bore. You can get dead accurate by using 0.004 inch x bore (inch) = piston ring end gap. Though manufacture will give you their recommendation.

Cylinder bore piston clearance will very depending on manufacture as above. Generally, .003-004 inch . . . But I see Vespa factory books stating much larger.

If the cylinder is conical instead of a true cylinder, would the compression suffer from gas escaping the ring gap before it closes as it reaches the top?

Meaning, if the bore is a true cylinder, and the gap is consistent, in a heated engine the gap would be optimally reduced to between 0,2 - 0,3 the entire stroke which means less blowby...

Another way of asking the question:
If I want consistent compression, I need a perfectly cylindrical bore, right?

Tighter ring measure on top may exceed ring reach/travel which could explain what you're seeing use the piston to drop or raise the rings to where they land and measure again

What cylinder is it?

I'm not sure how to answer that question… From what I understand it is a top port, 2port cylinder for 150cc (see my scooter in profile)

I see it is a Spanish bike. Does this cylinder take GS150 pistons are something different?

57mm piston. Seems standard... though I have not been able to decipher the numbers stamped on the piston. Are those the original size marks of the piston? I know of Italian Vespa's there are different indicators, it would follow that these are indicators as well..

I didnt answer your one question, this scoot most resembles an SS180 or GS160 though the engine is a 150.

Engine: 502M *142024*