Single row ball bearing - Fitted to the outer of the two driveside mainshaft bearings in the crankcase on all SOHC engines, both International and early Manx. Originally Norton would have fitted a slightly looser tolerance 'C3' fit bearing, but with them now being obsolete, the next best thing is still to take the same bearing - from top quality UK manufacturer RHP - but these being the standard (STD) tolerance.
Jan 2023: Sorry to say, that just like all other SOHC RHP C3 (loose tolerance) bearings, Item 0018 (C3 version) - are now obsolete and we are only able to get them in occasionally as we find them - however, this STD fit bearing (0018B) is still the same high quality RHP bearing of the correct type, we just recommend that when running the engine in, you give it a few more miles to let it bed in fully.
Like all other original 'Imperial' fit high quality RHP bearing - they are now obsolete and are difficult to obtain - however, much better to fit best quality RHP bearings when we are able to obtain them, rather than a lesser offshore manufactured bearing
By the way - it is worth pointing out - as I sometimes get asked this by customers: a 'C3' fit bearing, just means it is slightly looser tolerance than the STD fit bearing. This means (as they are normally fitted into crankcases with an interference fit), that once fitted - they will spin more freely than a STD fit bearing initially. When used on a racing bike, where less time is allowed for 'running in' this can make a difference . . . but on a road bike - using a STD bearing should not be an issue . . . I would normally just allow 500 - 750 miles of steady running in first (and actually - many old owners were probably not aware of what a 'C3' bearing is, I have stripped many engines and found replacement STD bearings already fitted!).