Modern Vespa

Good afternoon,

I am hoping to do my first engine strip and rebuild of a Vespa 100 Sport (V9B1M). During this I am hoping to replace the piston but I am unsure which I need as after searching on the net I have seen a number of different sizes e.g. 57mm, 57.4mm and 49.6mm, 49.2mm. Is there a way I can measure the existing piston?

Can someone please help and let me know of anything else I need to consider when replacing the piston such as rebore etc?

Many, many thanks in advance.

Can some one please help and let me know of any thing else I need to consider when replacing the piston such as rebore etc?

Many, many thanks in advance.

i think if you remove the head & clean head of piston well it will have the size stamped on piston crown. hope this helps

Why are you planning on replacing the piston? Is it seized?

I'm only asking so I can better answer your questions.

The standard bore on a 100cc Vespa is 49cc.

The reason you see different sizes is that each oversize is .2 larger, so 1st over size would be 49.2, 2nd oversize would be 49.4, 3rd oversize 49.6 etc

You have oversizes because as the piston and cylinder wear it makes the cylinder bore larger. Also, if you seize, you may need to bore (shave off material from the cylinder walls in a uniform manner) out the cylinder to get rid of grooves/score marks from the seize. The bore job is done to standard parameters to ensure the good performance of the bike, and so that piston manufacturers know what size to make their pistons to be standard with all the other piston makers, speaking in a general manner

57mm is the standard size of a 150cc bike, so definitely not applicable to a 100 sport engine.

As Motorsport says, why are you replacing the piston? The reason for you replacement may effect what we tell you about finding out what piston size you need.

Evening gents,

cheers for the replies they have both taught me a lot. I was hoping to change the piston as most things I have read about full engine rebuilds most people replace the piston, or is that not the case? I did eventually get into the piston and its actually looks like it may need changing. Please see below picture and let me know your thought?

Help/advice very much appreciated!!

It looks seized a little bit side of your piston.


If your cylinder is not seized you can replace stock size Piston ring and piston or Both at once for better condition.

But Best way for getting power back like new stock is reboring with right size piston

The one thing you should remember when you think of rebore, The number of rebore you can and size of piston with the cylinder is limited.

When you rebore a cylinder it's inlet port angle is changed a little bit

it can cause over heat problem.


I got some pic you might be intersested
It's from janpaness tuner

It looks like you might just need new rings. Piston replacement is not standard procedure during rebuilds. One of my concerns is what your cylinder looks like. If it looks like your piston does (scratched up), you should get the next oversize piston and re-bore the cylinder. Or buy a 135cc cylinder kit for a little more.