Modern Vespa

Can a 125cc vintage cylinder (VNB etc) be machined to use a 150cc piston? The cranks of a VNB and a VBB/Sprint etc are identical, the stroke is also the same, so why not? The only thing that needs to be replaced is the top.

Or do I miss something?

You're overlooking breathing of the engine. Carb, exhaust, the whole airflow system. That might not handle 20% more volume of air. On the other hand it might handle it, depends on the design.

When oversizing a 2 stroke it cuts into the ports. With the usual sloping roof on each port, this lowers the port timing. Need to do some measurements to decide if this is what you want.

yes, in theory you could do that. but why be sadistic? you will have to change the head or least machine the head for a 57mm piston.

it just doesn't make sense when there are several 2-port, 177 top ends available in the market. or frankly, just buy a complete 150cc cylinder kit and run the original carb and exhaust.

Thank you all for the comments; from my side it was just a thought.

Johnathan, which 177 2-port kit do you recommend?

From memory I know that Pinasco make one. Couple of guys down my way used them on their 150 Supers but dont know their reliability etc.

Pinasco and Parmakit I think are the only two options for 177 2 port motors. I could be wrong though. Unless you blocked the third port on other cylinders.

I'm finding the Parmakit cylinder as discontinued. As mentioned, I think the DR 177 can be used with the third port covered.

Stock 150 p and c is probably the least fuss.

Here are a couple:

https://www.sip-scootershop.com/en/product/racing-cylinder-pinasco-177-cc_80520000?usrc=Pinasco%20177


https://www.sip-scootershop.com/en/product/racing-cylinder-pinasco-177-cc_80510000?usrc=Pinasco%20177

https://www.sip-scootershop.com/en/product/racing-cylinder-parmakit-sprint-gt-super-177-cc_80527000?usrc=Parmakit%20177

Doesnt seem to be any reviews on the parmakit version, would love to know more. I guess it were me and had to choose, id go for the aluminium pinasco accepting that if it seized that would be it and no rebore option. Horses for courses, as I like to thrash about the aluminium kit sounds like the right one purely from its ability to take a lot more heat.