Belt Primary ID & Introduction

A

Alta-Rock

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Mar 19, 2025
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Hi all,
I'll take the second point first: my name is Ted. I've had BSA and Triumph twins and triples but never a Norton. I'm slowly recovering from losing everything in the Eaton fire in Southern California this past January. I'm in a rental house with a garage so can't help but think about finding projects. Still, I'm not psyched about re-buying all my tools. I ended up grabbing most of by burned tools before the lot was cleared but they are probably useless as they've lost their temper. This isn't a plea for free stuff- just sharing. Everything is moving slowly. There is a certain freedom when you're parted ways with everything you own.

The reason I'm here today is I'm working on a friend's (also new to Nortons) '73 850 Commando (5/73 303xxx) with a shredded primary belt. The problem is we can't identify it. It's orange so that might narrow things down-- RGM? I emailed RGM and haven't heard back.
I've read through a couple of threads but haven't found anything definitive. Maybe someone can spot something definitive from my picture? Thank you!
 
Hi Ted,
Very sorry to hear of your fire loss. We watched that on the news each day & it looked terrifying.
As for the belt, it looks like an AT10 oil resistant type, as used by RGM.
Hope this helps.
 
It sure does look like the Synchroflex AT10 Gen III belt that RGM uses. It should have the spec printed on it somewhere. RGM is not real quick on email. Sometimes they never respond at all.

I have an RGM belt clutch and use the Red Synchroflex AT10 Gen III belt. I've not busted mine yet, so can't tell if the one pictured was too loose or too tight. By the way, I actually worry about that damn belt coming apart the whole time I'm riding my Norton. Saving money on a belt clutch is not all it's cracked up to be.

If your friend has money to spend, the Colorado Norton Works (cNw) belt clutch is superior to anything RGM has come up with. I would swap mine out for a cNw kit, except my belt length is shorter than a Commando uses and Matt doesn't have a shorter belt available in his spec.

Belt Primary ID & Introduction
 
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Usually when a belt shreds it is running too tight. Keep it just loose enough to not skip over teeth when the bike is cold when putting the boots on the kickstart. The clutch basket expands quite a bit when it heats up. If the belt is set correctly, they last a long time.
 
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Thanks for the help!
This was a case of someone not getting the reverse shift pattern memo, so I think he started off in 3rd or most likely 4th. Not something I would hold against anyone- I had a '62 BSA I was always getting crossed up shifting.
There was an oil bath in there which was a little surprising.
 
CanukNortonNut said:
Usually when a belt shreds it is running too tight. Just loose enough to not skip over teeth when the bike is cold when putting the boots on the kickstart. The clutch basket expands quite a bit when it heats up. If the belt is set correctly, they last a long time.
Click to expand...

Did it shred or slip then bunch up and snap due to the oil being in the primary?

Mine has seen some high revolution abuse and has not shredded yet, but I don't care much for the AT10 belt.
 
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Alta-Rock said:
Right. Thank you. I kicked it around and it does- Synchroflex GEN III 028 AT 10/890. It measures closer to 30mm, though.
Click to expand...

Max width you can use depends on the crank gear pulley width usually. You sure it's not 28mm? It could be 30mm. I'm not sure what the 028 refers to.
 
If it was in an oil bath the chances are high the crank seal behind the rotor is leaking.
Not so uncommon issue.
 
Ok... now I see the OP first picture. I was like ashman before I made the comment and did not see the picture so I was just reading the description. I am still sticking to my best guess. You should be able to turn the top run on the belt 90 degrees if set right on a cold engine before start up.
IMHO. Welcome to the forum.

Cheers,
 
Salut Ted,

My full support to you and your friend in those so tough times!

Yes, it looks like a RGM belt drive. Pay attention to the RH flange (engine case side). The 4 screws have to be secured with Loctite, otherwise they may come loose. I know that because it happened to me (loose screws starting to machine the case) !

Laurent
 
A 90 degree twist is not a correct adjustment for an AT10 belt on and RGM belt clutch on a bike that spins up quickly. I don't own a stock engine, so no clue what a mild cam heavy crank engine is like anymore. I'd be surprised if it actually worked on a stock engine unless it doesn't get ridden.

The instructions that come with the belt mention a good starting point. For the most part the adjustment is trial and error and it is less than a 90 degree twist that may work for rubber belts. The AT10 is stiff slippery polyurethane, and 90 degree twist adjustment will slip under throttle. It does on my bike. 70 degrees might be closer, but it's been a while since I last checked it. I use an up and down slack measurement as defined in the instructions.
 
robs ss said:
It's your meds Ash
Click to expand...
Looks like I am not the only one who only see a white blank without a pic from the OP so must be on the same meds then, I can see the other pic from Schwany OK, with the RGM belt they can be also run in oil as well no oil, all depends if running a sealed bearing in the clutch housing or not.
 

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