Initially the plan was to change the crankcase seals on the Jumbo motor, but re-assembly of the gear clusters and selectors involved the exploration of some deeper mysteries.
To reassemble the crank cases, you need to move the gear cluster and selector to the LH crankcase, so that you can engage the gear change to the selector drum prior to reassembly. Though seemingly straightforward, it turns out that this is not infact the case….
After reassembling the crankcases, I checked the all the gears could be selected, that there was a neutral and the engine turned over in each gear. Prior to putting the engine back in the frame, I attached the primary drive sprocket to only notice a little later that the gear selection was now more difficult, there was no neutral and that the engine locked in bottom and top gear. Crap.
Therefore stripped the engine again and looked more closely at the gear cluster.
After speaking my mate to Graham, who’d help with the previous assembly of the crankcase, it occurred to me that the problems only occurred after I’d install the primary drive socket. Washer (25) may not have been in place. Tightening the nut had possibly moved the shaft (11in the diagram) and caused the locking to happen. Its possible that a washer was missing, or that one of the sprockets was on the shaft the wrong way round.
Also, washer (7) was missing, meaning that sprocket (9) was free to move along the shaft and didn’t always disengage from the selector sprocket (6). Further testing of the cluster is needs, with it mounted into the LH crankcase, though only with the cases put together can you really test the cluster.