750 K1 grumbling gearbox

[750rideruk]

My 72 UK K1 has developed a grumbling sound from the gearbox while idling. I know about the primary chain noise so it is not that sounds a bit like a bearing. Bike has done 70K miles any ideas? sounds like it could be expensive.

[Bryanj]

If its a bearing you wont know till you split it, i think they are all available from Yamiya in Japan(the ones with grooves are "specials" and not available from bearing suppliers)

[Trigger]

The output bearing is the first one to go on early 750's. Honda redesigned the crank cases to take a larger bearing on the K2 onwards.
50 year old bearings and 70,000 miles ! It is time for a full change of all the gearbox bearings and as Bryan has pointed out that it is lucky that they are all still available