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help needed please. i have removed cracked cylinder head from my bike an discovered slight damage to top of piston the only numbers to identify the piston size is inside above were the gudgeon pin goes its printed sciam 785 does this number indicate the size of piston cheers

I have just looked at my old pistons, one of which has a crack near the gudgeon pin . Same number as you have. On the top face there is a 4 ( on the “ pointy” part) which I think is + 0.4 mm
Lots of info here- you’ll likely need to translate it.
https://www.lamaneta.org/forolamaneta/v ... hp?t=19810

No, that is just a casting number (or forging number) If the piston is over sized it's usually identified by a stamping in the top faces. Or measure your cylinder bore to determine the size you need, it can be done accurately enough with vernier calipers. If yours is a nicasil cylinder (has the name GILARDONI cast into it externally between the fins) then they were graded A, B, C for selective assembly. The difference in sizes is very small then and almost impossible to detect without some very accurate equipment.

thanks for four help lads looks like i need a cylinder head and a piston

Hello ,

One way to know the size of the piston is to measure the inside cylinder with a caliper . Standart is 62mm when 62.2mm it is the first oversized and than if 62.4mm that's the second oversized and the last one is 62.6mm .


Christian , 123 .

i am not a newby to morinis but i did stop riding bikes specifically morinis 40 year ago so i am a bit rusty .my question is if piston is standard size will it have its size stamped on it so as to identify it i cant find any numbers on piston cheers

Most likely to have STD stamped, if anything. Measuring the bore size and getting a piston to fit that is the way forward.

Hello ,

As I said in my former message if you measure the Ø of the cylinder you have the answer .
If the Ø is 62mm the answer is std .
If the Ø is 62.2 the answer is +0.2 .
If the Ø is 62.4 the answer is +04 .
And if the Ø is 62.6 the answer is +0.6mm

This cylinder is for a standard piston . std .

And here it is a piston 0.2 to fit in a cylinder with a bore of 62.2mm
Christian , 123 .

Here is the head of the piston + 0.20mm .
Christian , 123 .

i will have to borrow vernier gage i only have rudimentary tools at moment checked piston all over an there is no numbers on crown of piston tried to post pic of piston but it wont upload

Usually on Morini V twin pistons head when nothing is stamped it is standard piston 62mm Ø


Christian ,123 .

Pictire of Daves Piston

Yep, most likely standard as Christian said. I have a pair of new standard pistons that I will try to get a picture of soon.