OK, this procedure is so easy I'll just go open up my monovalve and get the washer and measure it!

The washer is 1 MM/.04'' thick, it's 30 MM/1.180" O.D., and the hole is 18 MM/.710" I. D.

It appears to be plated steel as a magnet will hold to it, but I suppose it could be a magnetic stainless of some sort.



P. S. no i don't know where you could get one, I would try a good old fashioned hardware store, or an industrial fastener place. Or you could visit a pick and pull boneyard and get one and start your collection of other small pieces and parts! Maybe the dealer or a foreign auto parts place, now that you know the proper specs, just take your vernier calipers with you and measure till you find the right one!