my mod 19 is 82mm bore does anyone know of an alternative piston i can fit to my bike
There are alternatives, but it depends on the year. My 1956 19S is using a Matchless G80 piston, reference 9991. G80 standard bore is 82.5mm, equivalent to 19S bore +20. Gudgeon pin diameter and piston crown height are the same. The skirt however will need to be shortened to clear the flywheels. The G80 piston is flat top, giving a useful compression increase. Earlier Model 19 pistons had a different crown height. Again I think there is another Matchless piston that can be used but it will require a substantial compression plate at the base of the barrel. Sorry, I don't know the reference number for that one.
mine is 1954 i will check this out,is there a balance issue with this piston, thanks for the info pete
There should no significant difference between your '54 engine and mine. According to my notes the weight of the Matchless piston (after shortening the skirt) was 476g against 447g for the original Norton item. I calculated that would reduce the balance factor from the original 51% down to 49%. Most of the extra weight is due to the Matchless gudgeon pin having a parallel bore for the full length instead of tapered at either end. Machining the pin removed another 20g, making the finished assembly 456g, just 9g heavier than original and giving a balance factor of 50.5%. This is probably insignificant, but I decided to rebalance the crank anyway and ended up close to 64%. The vibrations really smooth out from about 50mph upwards.
Hope this helps, Ian McD
It might be worth trying John Harding.
Very knowledgeable and helpful
It was John who originally suggested the Matchless piston.