The Gearbox

The first problem I had assembling the gearbox was that the primary shaft (Table 8,#54) was not the correct one! I sent this one off to Jerry Kimberlin who very generously machined it down to the correct specs including welding on the tang at the end. Here are the correct dimensions in case anyone needs to recreate a shaft. The trueness of the shaft needs to be checked on V blocks and cannot have a radial runout greater than 0.05mm.

The second problem was the bush inside the shaft (Table 8, #59) was not the right size and had to be remachined which again, Jerry did. Here are the dimensions if anyone wants to make one out of 660 bronze. I have also shown the dimensions of the ring that sits in between the direct gear and the bearing. This is a critical dimension and needs to be set for each individual gearbox (more on this later). The spacer ring (Table 8,#55) is assembled with the flat side against the bearing as shown in the photo below.

The next step is inserting the direct gear and the spacer ring into the RH cranckase output bearing after cleaning the small oil hole with a blast of compressed air. The spacer ring is assembled with the flat portion of the ring facing the bearing. The gear is a tight sliding fit and should not need a lot of force to either insert or remove.

Assembling the gearbox:

All the assembly is done on the LH side of the crankcase. I assembled it without the crankshaft as you have to open and close the cases several times to check if the spacer ring behind the direct gear is the right fit. I first inserted the clutch hub (Table 7 #10) from the outside into the LH side upper bearing. As specified by the manual, I made sure to support the inner race with a short tube while tapping the hub into the bearing. I then inserted into the hub, the tapered end of the mainshaft with the flat side of the spacer ring facing the bearing. After tightening the nut on the outside of the clutch hub, I rechecked the alignment of the mainshaft (max run-out is 0.05mm) and made sure there was no lateral movement between the shaft, the hub and the bearing. I next assembled the gear selector quadrant making sure that the "4" punched in the casting matched the "4" marked on the tooth arc. Next to go in was the secondary shaft with the large gear closest to the LH casting. There are two spacer rings that go on either end of the shaft-the thinnest (15.2 mm ID x 22mm OD x 1.2mm thick) one goes on the side closest to the direct gear (RH) while the thicker one (15.2 mm ID x 22mm OD x 2.3mm thick) goes on the clutch side (LH).

Now, to assemble the shift drum. The gearbox is assembled in 4th gear. The shifter was properly lock wired and ready for assembly. I rotated the 4th gear shifter (the one opposite end to the gear side of the drum - RH side in the photos) close to the top of the slot in the drum. I then rotated the drum till the lower shifter (LH in the photos used for 1st and 2nd gear) aligned with the first and slid the twin first and second gears into the LH lower clevis fork and the single gear into the upper RH clevis fork. It is easier to rotate the drum with the stationary part of the cylinder clamped in soft vice jaws. The whole assembly then drops into the LH case with the bottom gear teeth mating with the teeth on the shift quadrant.

Now for the most critical part of the assembly - ensuring the right amount of end-float between the direct gear and the end of the mainshaft splines. I first inserted the direct gear onto the mainshaft and slid it till it almost touched the right hand end of the mainshaft splines. I had a washer that was 0.29mm thick that I slid onto the shaft and rested on the RH side end of the splines. I then rested the direct gear on this washer. The specified range is 0.20mm to 0.30mm; I used 0.29 since I had a washer that thick. I next measured the distance from the front face of the direct gear hub to the RH end of the mainshaft. I measured this 4 times and averaged the measurements at 49.71mm. I removed the gear (keeping the washer still on the shaft) and with the metal ring (#61) 1.48mm thick in between the direct gear and the output bearing, installed the gear into the bearing and then installed the RH crankcase and tightened up the screws all round the case. This effectively "compresses" the whole gearbox assembly to its operating clearance. I now measured the distance from the face of the direct gear boss to the end of the mainshaft four times. In the blog post "Engine-the Crankcase" you can see the mainshaft protruding from the direct gear outside the gearbox bearing boss. The portion of the mainshaft you see protruding in the picture from the gear boss is the distance to measure. The average external measurement was 50.18 resulting in a 0.47mm decrease in clearance. Since the external measurement had increased it meant that the internal clearance had decreased. If the internal clearance has decreased a thinner spacer ring needs to be used. Since this was the thinnest ring from Jerry's collection, I ground it down to a thickness of 1.40mm resulting in an external distance of 50.10mm. The difference between the internal measurement (49.71) and the external (50.10) or 0.31 is just outside the optimal max of 0.30mm but within the 1.0mm max tolerance range - I would have had to make a whole new ring to get the 0.30mm. This was quite a painful exercise and the reason why you do not have the heavy crankshaft in the crankcase as the cases required quite a bit of manipulation.