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Hello,
My ac3 engine was working with the deflector nose shape piston like the oldests models but as i saw on the parts manual the ac3 piston is the same as the ac4 model with a smooth rounded head.
I am not sure if the cylinder and head are the same for both and if i can change for the ac4 piston. Maybe i am wrong but i think that if Piaggio changed the piston there is a reason, maybe better performance, am i right?

My ac1 had the deflector piston like yours but the spark plug threads came out of the head and i couldn't find a good replacement so i switched to the flat style piston and head. You can keep the same cylinder but make sure you check your ring gap clearance or you'll seize it if it's too small or lose compression if it's too large. Been running with the later style piston and head a few years now, no issues.

alex

Hi Alex, do you notice any improvement whith the late piston? Is the cylinder head the same for both or must changed as a pair?
Thanks

sorry for the late reply. no difference in performance, the head does need to be changed with the piston (you can keep the same cylinder). Replacing the original carburettor with a modern dell'orto made a big difference in the way it runs, starts very easy, idles great and runs much smoother with better response.

alex