If you’re actually riding your old Norton Commando 850 — and we hope you are, because they’re one of the great bikes of the ’70s — chances are good that at some point you’ll have to replace the clutch plates. The original setup used five alternating bronze friction plates keyed to the center clutch hub, with four plain steel plates keyed to the outer clutch drum followed by a pressure plate and a single diaphragm-spring plate compressing the plates. It’s a fine setup, but eventually the plates wear. Slippage and overheating take a toll, as well: Once the steel plates start to blue, they’re toast, and both the steel and the bronze plates can warp from overheating.
The good news is, replacement clutch plates are readily available and the design of the Norton clutch makes servicing quite simple. Only one special tool, a diaphragm spring compressor, is required. You can buy the tool for $26 from donelsoncycles.com, or you can make your own if you want (go here to see how).
There are a few points to appreciate, one of them being the stacked height of the clutch plates. According to various sources, Commando 850 clutch plates should have a stacked height — the total thickness of all the plates stacked together — of approximately 1.17 inches. However, replacement clutch plates (even stock Norton items) rarely stack out to that exact specification. The height matters because of the nature of the diaphragm spring clamping the plates together. A shorter stack allows the spring plate to push farther into the clutch hub, resulting in a stronger pull at the clutch lever, while a taller stack means the spring plate is flatter, resulting in a lighter pull. That makes a taller stack desirable, but only to a point. If the stack is too tall the clamping pressure is reduced, increasing the risk of clutch slip. Back in the day, variations in stack height were routinely balanced by inserting a fifth steel “shim” plate to compensate, but shim plates are now hard to find.
So what to do? Well, as we discovered with our Barnett plates, which had a stack height 0.145 inches shorter than recommended, there’s lots of room for variation, as our installed clutch requires only moderately strong pull and shows every indication it will work just fine. Bottom line: As long as the installed assembly is below the diaphragm spring retaining clip, you’re probably fine.
As noted, we sourced all our plates from Barnett, known for quality clutch parts. The Barnett friction plates are fiber, not bronze, which brings up the age-old issue of whether to run the clutches in engine oil or, as some prefer, automatic transmission fluid. Barnett notes that their clutches are made to run in engine oil, but are also designed to work in ATF, as well.
As to cost, Barnett sells the friction plates as a set for $65.53, while the steel plates are sold individually at $8.10 each. Primary case gaskets are readily available from a variety of sources for $4-$5. As always, we recommend having a good shop manual on hand for parts identification and proper torque specs.
1. Remove the three nuts and washers securing the brake/foot peg assembly to the aluminum sideplate. Remove the brake/foot peg assembly. Place a drain pan under the primary cover. Using a 13/16in socket, remove the center primary cover securing nut. Remove the primary cover and let the oil drain.'
2. With the 13/16in socket, loosen the clutch adjuster lock nut. If it’s stuck, give the socket wrench a quick rap with a hammer. It should shock loose easily. Unscrew the clutch adjustment screw.
3. Next, install the clutch diaphragm removal tool. Screw the center bolt into the clutch to engage at least five threads. Lock the bolt with its lock nut. Next, tighten the outer nut while holding the bolt until the clutch diaphragm spins freely in the clutch hub.
4. Next, remove the wound circlip securing the diaphragm. The circlip has a slotted opening in its free end. Insert a screwdriver into the slot and pull the circlip toward the center of the clutch hub, then pull the circlip free.
5. The clutch diaphragm should now simply fall out, exposing the outer clutch pressure plate. Note the clutch adjustment rod protruding from the center of the clutch hub.
6. Next, remove the outer clutch pressure plate, examining it for any signs of scoring. If it’s badly blued from being overheated, consider replacing it as it could be warped.
7. Next, remove the clutch plates. There are five bronze friction plates and four steel plates. The plates alternate, starting with bronze and ending with bronze. On bikes with any kind of mileage, these will generally show signs of scoring and/or overheating.
8. The Commando 850 clutch works best with a stacked height of approximately 1.17in. A thicker stack equals a lighter pull, but also introduces the risk of more slip, which causes overheating and wear. Our original plates had a stacked height of 1.053in.
9. Our replacement Barnett plates had a stacked height of 1.025in. Although shorter than the stock recommendation, they worked fine and with good pull at the clutch handle. It was once standard to insert a fifth steel plate of the needed thickness to make up the difference. However, custom steel “shim” plates are no longer available.
10. Before installing the new fiber plates, soak them for a few minutes in quality engine oil or automatic transmission fluid, depending on what you plan to use in the primary case.
11. Drain the plates after soaking. Wipe off excess oil with a lint-free cloth. Install the new plates, starting with a fiber friction plate and alternating with steel plates, ending with a fiber friction plate.
12. With the new clutch plates installed, install the outer pressure plate. If reusing the original, lightly scuff the surface with a Scotch-Brite pad and then clean it with brake parts cleaner or similar before installing.
13. Next, with the removal tool still attached, place the diaphragm spring in place, then secure it with the large wound circlip. Make sure the circlip is seated in its groove, then loosen the outer nut on the tool to release the diaphragm. Remove the tool.
14. Before installing the clutch adjuster, remove the inspection cover on the right outer transmission cover. Make sure the clutch release arm is properly located and that the cable is secure as shown.
15. Screw the adjuster into the center of the diaphragm and loosely fit the locking nut. Screw the adjuster in until it just touches the release rod, then screw it back out 1/8-1/4 turn. Lock the nut in place.
16. Remove the larger primary cover rubber “O-ring” seal. Clean the groove it sits in, then install a new seal. When installing the new seal, place the bonded end joint at the highest point as shown to diminish the chance of an oil leakage from the primary cover.
17. To further discourage oil leakage from the primary, place a suitably sized O-ring over the outer primary cover stud. The O-ring will crush when the outer cover is installed.
18. Finally, fill the primary cover with 300cc of quality engine oil or, if preferred, automatic transmission fluid. Reinstall the brake lever/foot peg assembly.