Numbers puzzle

[Pharisee]

I never understood the gudgeon pin position discussions...Surely this makes no difference to the swept volume (capacity) of the cylinder ?

It doesn't... The swept volume is only dependent on the bore and the stroke. Moving the gudgeon pin relative to the piston crown won't affect the capacity but it may well affect the compression ratio. Assuming nothing else changes, moving the pin down, relative to the crown will increase the compression ratio, while moving it up will have the opposite effect.

[Seven13]

I guess the mystery won't be solved, I did mention Frank Cooper and a whisper that he was involved with the larger motor size on the twins and perhaps the 648. Perhaps there is a US enthusiast that might have more info in this regard. As pointed out the motor now has the updated barrels and head and is no doubt a 646! Many thanks for the input from club members.

[Duncan]

The 1958 piston change was due to the stroke of the crankshaft changing, its all explained here: https://www.jampot.com/article_read.asp?id=22 and here: https://www.jampot.com/article_read.asp?id=23