Piston Sizes?

[RoadRunner]

Hi,
Bought a set of A7 cast iron barrels complete with pistons but?.the pistons are very different. Trying to work out the bore size assuming these pistons were run in these barrels?

In the photos below:
The left piston has the BSA part number 67-1445 and ½ N/M OS plus some smaller figures too indistinct to make out. Weight 9 grams.

The right piston has the number 11603 and something like 0.2 (?), plus ?Made in England? ? it also has a larger cutaway beneath the oil scrapper ring. Weight 8 grams.

I would expect to see +30 or the likes of ? not come across these markings before; am I correct in assuming they are both +20?

With the weights being different presumably they shouldn?t be used together?
Does the larger cut away beneath oil ring improve performance being lighter and oil flow or some other reason?
What does the  N/M stand for??

Any info would be appreciated to improve my little knowledge  
Cheers
Road Runner

[trevinoz]

Road Runner,
                      I would say one is 1/2 mm os and the other .020" os which is close enough to the same.
However, they are completely different pistons so I wouldn't use them together.
Probably one was replaced after a failure of some description to keep the bike on the road at little cost.
Trev.

[muskrat]

G'day Road Runner,
                          I agree with Trev. The one on the right is similar to the split skirt pistons that can run a much smaller bore clearance, 1 1/2 thou as opposed to 3 thou.
Cheers

[RoadRunner]

Thanks Trev & Muskrat,

Appreciate the info - will give me something to look for at the next autojumble - the barrels/piston are a spare set I couldn't resist buying at the price offered since the barrels on my bike have fins welded back (a back up set!) Anyway it'll give me an excuse to go piston matching which should be fun!

Many thanks

RR (getting there albiet slowly....)

[Goldy]

Roadrunner. I have a pair of used A7 pistons 66mm plus 20thou. Let me know if your interested. All the best Goldy