In October of 1989, I was going to run, for the first time,
my freshly overhauled 1972 Norton Commando 750 Combat. I had installed
a belt drive which could run without oil in the primary, and as a matter
of principle, I was going to see my new belt drive run without the cover
on. I started the bike, made it fast idle to break-in the new Combat cam,
and shut it down after the barrels had start to warm up. I left the bike
to cool down. Upon my return, I found oil dripping out of the bottom of
the clutch drum. After pulling off the clutch diaphragm spring, I found
the source of the gear oil. The gearbox 90w oil was coming through the
transmission main shaft.
Over the years most Commando owners will have noted the "press" in magazine articles, road tests, including the Norton News about dragging (cold) or slipping (hot) clutches. The dragging symptoms include difficulty in shifting or finding neutral, to as much as pulling the bike a little with clutch pulled in. Certainly, the problem is well known. The slipping symptoms can be a very slow slipping clutch engagements or even the motor overpowering the clutch at high speed and then the engine rpms start running away.
The actual cause, of gearbox oil on the clutch plates, may not be as universally acknowledged as one of the main sources of clutch problems. A good solution seemed to be rather elusive. After market clutch plates, or pulling the clutch apart every now and then to wash the friction plates and sand blast or emery cloth the scorched and blued steel plates did not appeal to me. I wanted a dry clutch and belt drive.
In Norton News #67 pg. 12 a clutch rod seal was made mention of, and In Oct. of 1989, I set out to develop one for my bike. I made one, installed it, and it immediately and totally stopped the leak. Some other regional Norton owners having similar clutch problems wanted seals also, so I made a small batch. Since then, acceptance of the seal as a highly benefitial modification and has helped a great many Commando owners reduce or eliminate slipping clutch.. I will continue to offer a clutch rod seal and installation instructions that will allow any Commando mechanic with a minimum of fettling and fuss to eliminate the problems associated with gearbox oil migration into the clutch.
Commonly asked Questions and Answers
1. How does it work?
A standard O-ring is used to seal the clutch rod seal against the face of the main shaft. The seal holder is screwed onto the end of the main shaft and captures the OD of the O-ring and presses the O-ring against the face of the main shaft with a calculated amount of "squish". The seal holder is held in place with blue loctite (not supplied).
2. Any limitations?
a. It has been reported that after market clutch rods are being repoped in 1/4" material. The original factory rods are 6mm. The seal is designed for 6mm rods only.
b. The only other one anticipated is if the face of the main shaft has been severely beat or hammered and degrades the ability of the seal to do it's job.
3. Fettling! Why won't this just "bolt on"?
Due to the normal manufacturing variations, the shimming of the clutch basket to make it align with the crank sprocket may vary significantly. The last .140" of the main shaft threads are needed by the seal holder. If the normal clutch center nut is occupying some of this last .140", than a compromise must be achieved.
4. How long does it take to install?
Typically less than half an hour, after gaining access to the inside of the clutch.
5. How long does it last? The highest mileage one known of is over 20,000 miles. The belt drive clutch was opened up for inspection after 10 years and was almost bone dry, in fact the steel plates were a little rusty.
6. What if it wears out, do I have to buy a whole new one?
No, the O-ring is a standard 3/8"OD x 1/16" you can get at a NAPA car parts store for about 50 cents. Neoprene or buna-n.
7. A common side effect on dry belt drives that the clutches will "chirp"on engagement similar to Ducatis'.