QR50 - Assembling the Transmission

Now that we have the engine back together, we can look at the transmission.

The QR50 has an automatic (centrifugal) clutch, with a chain drive to a reduction gear set up at the rear wheel - it is single speed, and the transmission acts as a single sided swinging arm. The wheel is mounted on a single sided axle, running in a sealed ball bearing in the inner case (that forms half of the crankcase) and a plain bearing in the outer cover. We can see it fitted to the bare case here: 

First we need to set up the drive chain tensioner and rubbing strips. It's a good idea to use some Loctite 243 in here to ensure we don't have a bolt come loose:

Here's the complete assembly:

I've mounted the clutch drum. This has the engine sprocket welded behind it - I have fitted the drive chain and the intermediate sprocket, which drives the rear axle. the chain is pretty loose and I think we may need a new one

Here's a close up of the tensioner. As you can see there is not much travel left - the upper path is the slack side of the chain as this engine runs backwards, by which I mean it rotates in the opposite direction to the wheels.

Here's the crankshaft taper and it's woodruff key:

Here's the clutch and final drive pinion in position. You can see the two pawls which are driven by the kickstart:

The case needs etch primer and a lick of satin matt black:

These are both U-Pol paints

Here's the kickstart quadrant and it's spring in position in the outer case:

The outer case has a couple of neat features. You see that cast slot linking the interior to the screw hole - that's the drain. there is another similar arrangement for the level plug:

Here's the drain plug from the outside:

Outer cover fully assembled with Threebond 1215 used for sealing. Unfortunately, it appears that the kickstart relies upon a fibre gasket for the proper axial clearance - the kickstart does not return with the case screws tight. It all has to come apart again to have the OEM gasket fitted, which does the trick. I fitted it with 1215 one side and grease the other, so it will come off easily next time - and maybe preserve the gasket for re-use.

This is the brake cable bracket:

I've fitted the rear wheel and centre stand to have a look-see; I've also assembled the rear brake.

I can see now that the centre stand is misshapen - possibly bent, or the wrong stand. It needs to come forward 10 - 15° or so which will stabilise the bike and allow the spring to work to retain the stand in the up position. I think I will also straighten that kickstart...

Gear Selecter

While putting the kickstart & gear selector case together after the spring incident, I noticed that the selector pawl persistently fell off as I replaced the outer case. With the SQ4, I don't remember having a problem reassembling this complex bit of the gearbox.

Something was wrong.

Pulling the selector apart, I realised that the pin on which the pawl pivots was loose, and that the small springs were not moving freely. Investigation found the pin bursting out of the spring box casting:

In the absence of a better solution, I fixed the pin with Araldite Steel and scraped the spring box enough for the small springs to move freely. At the same time, I searched eBay and put an advertisement on the AOMCC forum:

I never tested the repaired spring box, though I am sure it would have worked (but not for how long). I now have this fine replacement, very kindly provided by Guy Hovey after a plea on the AOMCC forum:

So this is now in place, the box back together and selecting gears again.

The way this works is interesting.

1. Movement of the gear lever is transferred into the interior of the box, moving a short internal lever (item 37 in the picture below), which has a pin fixed at one end
2. The pin moves up or down against the small springs, moving the pawl (item 36) about its pivot until they become coil bound. The pawl is now engaged with the selector quadrant.
3. More gear lever movement then moves the whole spring box against the large springs, which are resisted by the pin screwed into the outer case, behind the gear lever.
4. The pawl moves the selector quadrant (item 35), changing gear
5. When pressure on the gear lever is released, the pawl moves back over the teeth cut in the quadrant and returns to the central relaxed position.

The mechanism works in both directions, in the same fashion.

Gear Indicator

Burman gearboxes fitted to bikes of the 1950's are often provided with a gear position indicator, or neutral indicator which is always handy as there may be more than one neutral position in the box... (!)

The indicator is nothing fancy, no dial on the headlamp nacelle or coloured lamp to wink at you; it's a simple pointer or or rotating bobbin with a fixed indicator on the kickstart case. The CP gearbox fitted to the W/NG has a pointer made from sheet steel fixed to the gearbox cover, with a moving pointer, also sheet steel, fixed to the gearbox camshaft. When the two pointers align, you have found neutral in its normal position between 1st and 2nd gear. It will also tell you if you have found the other neutral that can appear in these boxes, between 2nd and 3rd when the bike is going nowhere and the clutch is engaged, and the gear indicator is pointing forwards...

The W/NG gear indicator was incomplete - it had the fixed pointer, but the moving one was missing. As found:

With an appeal on the AOMCC forum, I soon found what was supposed to be fitted to W/NGs of this era. A kind member, Ian Scott, found an original indicator in 'patinated' condition that was just right:

This was easily fitted. The nut shown here retaining the indicator should, according to the factory parts list be a domed nut; this one has obviously been there for years and I have no intention of changing it:

Looks the part doesn't it!