Same part number as the tappet cover caps, as Mark says. They are an odd size, 30.8 x 3.2mm. These are not usually available off the shelf, special order to have them made and usually a minimum of 500 before they'll do them. Most sellers sell ones which they say are correct but are in fact just close enough (in their opinion) to work. I've personally had some which were so bad I refused to fit them in a customers bike, so bad I removed 4 old tappet cover rings and used them instead, it's easy to replace the tappet cover ones, not so much the manifold ones and if they leak it causes running problems. If you have a genuine Honda top end set you'll get 12 of them in there, I'd advise using them ONLY in the manifold and using close enough ones for the tappet covers, that way you get 3 sets of manifold ones, enough for the lifetime of the bike normally. Most sellers provide something like 31 x 3.0mm.
