Fixing an RGV250 VJ21 gearbox and input retainer plate

There’s a small weak link that a lot of people are not aware of with the 1988-1990 RGV250 VJ21, currently the bike to have in Period 6 250 Production historic racing here in Aus. That weak link is the bearing retainer plate for the input shaft bearing on the gearbox. I put together this comparison with the parts diagrams available from CMSNL to show the point:

The VJ21 gearbox on the left has the input shaft bearing (10) retained by 2 small plates (31). On the other hand, the VJ22 gearbox uses a large plate (also 31) to retain it. The screws (32×2 for VJ21, 32 and 33 for VJ22) all mount up to the same holes in the gearbox plate so there’s no huge change required to swap in this plate.

So what happens when they fail? In short, carnage. Happened on a bike I built up and it snapped 2nd into 5th into 6th, busted a few gears and generally was horrendous. Bike finished the race but pulling the clutch pulled the entire input shaft in and out.

The failed small retaining tabs…

… and how that looks with the gearbox still in the motor:

The upgraded VJ22 bearing retaining plate on a later gearbox:

The carnage that happened when the input shaft was free to move about:

Not pretty. Fortunately the cassette gearbox meant it was about 15 minutes to pull it out, another half an hour to swap gears around on the shafts I had, and 15 minutes to throw it back in… still a right pain though.