'82 RS Airhead starter
 

It's a Bosch, the part I'm curious about is the lump right next to the pinion itself that looks like a ball race, it has part no. 1 006 209 401 on it. I've Googled this number and found it shown as Freewheel Gear, starter which makes me think that it might possibly have a clutch inside. Mine is locked solid, it doesn't turn either way relative to the pinion. Should it? It's always worked fine on the bike as far as I know, although it has been off the road for a couple of years by now. :)

I checked my Clymer manual and it says to check many things - I expect those are the common failure modes - but there was no mention of the free wheel gear getting stuck - but to check for missing / damaged gear teeth. The bearing gets no mention at all.

Ah, thank you Jay, so that lump really is a one-way clutch then. I only wondered because the thing is absolutely locked tight without any play whatsoever in either direction. I was thinking it was unlikely to have seized this solid in the last couple of years while it stood standing, but now it seems obvious there has to be something like that in there to make sure the starter isn't driven by the engine once it starts.

I'm trying to convince myself that I would have worked that out for myself when I was younger. Sigh ...

Thank you. bier

It is as you say a free wheel gear, one way clutch, sprang clutch etc. It’s purpose is to transmit torque to the pinion adjacent to it, which then drives the flywheel ring gear. When the engine fires and the pinion remains engaged, it will try to drive the starter, which may damage it. The clutch however will slip or free wheel when driven from the engine, thus preventing damage to the starter by overrevving it. It should slip in one direction if you try to turn the pinion by hand.

They are usually very tight and do not slip easily by hand power alone. put the armature shaft in a vise and try to turn the pinion gear in either direction with a pipe wrench or channel lock pliers..

The inmates below have given you good advice.
The old Bosch starter is remarkable in its longevity and parts availability but before I threw money at an old Bosch unit I would look at the newer Valeo or if you have the funds the ultimate Nippon Denso. Both of these alternatives are much lighter and more powerful and have the advantage of not requiring the front cover removal to pull the starter. Your choice, just my 2c worth.
And yes that clutch will be firm so a vice will be handy and don’t wash that bendix clutch assembly in solvent, it’s pre lubed and needs to stay that way.