Ariel Owners Motorcycle Club

I'm confused by the way NH pistons are advertised for sale, what is the difference between pre and post '55 pistons and who are the best vendors?
All advice gratefully received,
David.

There are a number of WNG 350 pistons offered online, which have three compression rings instead of the two compression rings and an oil ring of later NH pistons. Using the WNG piston with the later type oil pump, with a larger feed plunger can give problems with too much oil reaching the combustion chamber. There have been previous posts on this topic, and no doubt others can add additional information.
I am not sure when the oil ring pistons were introduced, but post 55 NHs had the enclosed pushrod barrel, as opposed to seperate external pushrod tubes, although this ought to make little difference to the piston type. I think the later oil pump was introduced around 1950.

Thanks Pete, the piston in it had three compression rings and a very thick layer of coke on the top.
Regards, David.

1956-on had a bigger 'dome' (higher compression ratio), earlier are lower compression. Late '40s had the oil control ring instead of the lower compression ring. (The 1955 Hepolite catalogue in the online library suggests 3 compression rings were used up to 1939).

They're all pretty much interchangeable with the possible exception of having to accommodate with differences in oiling and compression ratio.

SG

I turned a WNG piston 3rd compression ring groove to an oil control ring groove.

(Could not find a high comp NH piston at the time).

Been running successfully for a few years now.