Norton Commando Sprocket Change

Post date: Dec 25, 2011 12:49:10 PM

Degree of difficulty: *** (three stars), Time: 1 1/2 hours for an expert, if no unforeseen problems.

This description is based on a 1972/3 750. It is not very different for 70 to 74. Very early bikes may be different, I have never seen inside one. MKIIIs are very different. There are two Belville washers in front of the rotor on a 74 MKIIA.

Tools... Clutch compresser, sprocket puller, torque wrench, 1 1/2AF ring or tube spanner, various sockets including 15/16AF. No Whitworth needed.

Parts... crankcase to primary gasket needed, clutch circlip, inner primary seal and clutch tab washer optional.


Find a calm frame of mind.

Remove the left foot peg (1/2AF socket and ext), do not hang it from the stoplight wires.

Undo the centre nut (3/4AF?) on the primary case and remove the outer, you will need to make arrangements for the approximately 180cc of oil in there. There is no drain plug.

Remove the stator, if the connection in the wires is hard to get back out of the hole in the inner, suspend it from somewhere higher up with a coathanger (a coathanger must be used once in every job; local custom). Do not allow it to hang from its wires or twist as the insulation gets very brittle from the heat. They can be re-insulated if broken, but it is fiddly.

Take the rotor off. (15/16AF) To do this you need to put the bike in 4th and replace the footpeg so you can use the brake to lock the wheel/chain/gearbox/clutch/primary chain/rotor. Behind the rotor there is a key, a spacer (note the way it goes on, with the recess out) and a couple of shims.

Ist special tool... you need a puller with two bolts (5/16 UNC?, about 100mm long) to screw into the front sprocket and one pointed centre one to bear against the crank. Get the sprocket started, but don't try to pull it right off, it needs to come off along with the clutch and chain.

Second tool... remove the clutch adjustment nut and screw, attach a clutch compresser, take up the spring pressure until the spring plate will rotate easily, remove the big circlip with a screwdriver and take off the spring. (Good opportunity to wash the plates, do it).

Undo the tab washer and the clutch nut (spark plug spanner fits). Slide the clutch, primary chain and front sprocket off as one, threading the stator through it if you have it hanging. Easier with three arms. Take off the shims and spacer behind the clutch, noting the orientation of the spacer (recess towards the circlip).

Take the woodruff key from the crankshaft, then remove the inner primary cover by undoing the three 7/16AF bolts. They have tab washers.

Remove the tab washer from the countershaft sprocket. (2BA screw) Lock the rear wheel and remove the nut (1 1/2AF), needs to be a tube spanner or a ring with cranked ring. Note that this is one of the few reverse threads on the bike.

Change the sprocket.

When putting it back together, it is sometimes hard to get the 2BA screw to line up with the holes in the sprocket. I have often had to loosen the nut slightly to get the screw in. I Loctite the screw.

This is a good time to change the felt seal in the inner primary, and the circlip on the mainshaft. I would not think of doing this job without changing them. I used to. The felt seal can be tricky, the secret is to compact it forcefully sideways at the same time as stuffing it in with a small screwdriver.

If there was more oil in the primary than you put in, it probably means the crankcase seal has gone. This is a good chance to change it. Get the seal out by drilling a couple of holes in the thin metal ring and hooking it out. put a little sealer around the outside of the new one.

Watch you don't lose the washers on the centre shaft that holds the primary covers, they are an adjustment.

When you have replaced the stator, you need to check the air gap between it and the rotor. 8 thou is OK all around. If it is tighter atone point, loosen the nuts, push the stator in the direction it needs to move and retighten. RE-check. Do not leave the bike with insufficient clearance here, it will wear a hole in your pocket very quickly.

The rotor needs to go to 70 ft lbs and the clutch nut something similar, although I only use 50 lbs here as 70ft lbs once sheered a circlip for me.

Things you can check on the way through if you have time and inclination are, the alignment of the inner primary, the alignment of the two primary sprockets. You can also flat the primary covers and renew the seal if sealing is an issue. The condition of the clutch hub and plates can be inspected also. It is worth looking to see whether there is oil coming down the pushrod from the gearbox. It smells different and can sometimes be seen radiating away from the tab washer. There are aftermarket fixes for this.

Put 180cc of oil in the primary.