Classic British Bike Magazine

Norton Commando

Excess vibes. Poor maintenance. Cracked frame.

 

Norton Commando front Isolastic mount. This vernier kit from Mick Hemmings worked for us. The trick, we hear, is to tighten it for minimal movement. But don't lock it up or you'll pay for it.

The Problem

 

My first 750cc Norton Commando taught me an expensive lesson about the importance of keeping the three Isolastic engine mounts adjusted properly. I'd owned the bike for around six months, and I hadn't done much else to it except change the oil once, replace the plugs, wipe it twice with a rag and ride the hell out of it. I put around 3,000 miles on that bike and kept meaning to adjust the rubber mounts.

 

I knew that these mounts needed to be checked and shimmed every so often to get the very best out of the Commando. But part of the reason I'd avoided doing this was laziness. Not laziness with the spanners, you understand, but mental laziness. I just couldn't get it together to read up on exactly what needed to be done.

 

Norton Commando Isolastics; the way ahead according to this Norton brochure

 

Two's company, but a threesome is better—according to Norton's PR department. Sexism aside, the system worked and still works decades on. But until you've tried a Commando, you won't really appreciate the usefulness of a little rubber in the right place.

 

 

In any case, when you ride a bike every day, you slowly adjust to the changes in performance and handling and braking, and I didn't notice how far it had slipped. It was only when I rode another Commando that I realised how bad mine had become. So it finally got the attention it needed.

 

However, my first attempts at adjusting the Isolastics were pitiful. I didn't understand the shimming issues. A lot of corrosion had got into the mounts. And I was working in lousy lock-up garage conditions.

 

The Norton Commando Isolastic frame; a revolution in motorcycle chassis technology

But I spent hours trying to adjust the shims correctly. My manual advised me to tighten the shims to somewhere between 0.010" - 0.020" (ten thousandths of an inch to twenty thousandths of an inch). Sounds simple in principle, but if you're a bit cack-handed and unused to the finer points of engineering, it can be tricky (and most of my adjusting is usually done with a curse and a sledge hammer). Note: For more on Isolastic clearances see Sump visitor feedback below.

 

I took the bike for a ride after I'd fiddled with it, but didn't much like it. So I adjusted it some more. There are three mounts; front, rear and top. The rear one was the awkward one, however. In the end, after various attempts, I was as satisfied as I figured I was going to get. A month later, the frame developed a nasty crack right above that mount.

 

Stupid. Stupid. Stupid.

 

Norton Commando Isolastic kit from Mick Hemmings

 

Mick Hemmings will sell you this vernier adjuster kit for your Norton Commando Isolastic engine mounts. These are easy to fit and adjust. Yes, there are other kits around, but the Hemmings set-up hit the right spot for us.

 

 

 

The Solution

 

This involved a complete strip down. Various other problems came to light, none of which were related to the Isolastics. The engine came out. Swinging arm too. A friend welded up the frame and did a fantastic job. I spent about two days cleaning it all up. Another friend got his spray kit out and used a tiny air brush thingy to paint over the damage. He used a two pack paint and got a near perfect colour match (tip: All black isn't the same black).

 

When it was done, the repair was almost invisible. Four days later, the bike was reassembled. But this time I junked the original shimming set-up and used a vernier kit from Mick Hemmings. This kit is basically a reversion to the original design concept. But in production, Norton skimped on this idea and went for the shims. By the time the error was recognised and addressed, a lot of bikes had been built.

 

And broken.

 

 

Norton Commando Isolastic head steady from Old Britt

 

Old Britts in Seattle will sell you this kit to replace the Norton Commando top engine mount. The standard one works. But this, we hear, works better. Looks like a lot of metal and rubber. But if you want to quell those vibes, this is the kind of hammer you need.

 

 

The Hemmings adjusters went on without any problem. And they were easy to set for the correct clearances. They're made from stainless steel, by the way. These were fitted front and rear, but I simply replaced the top mount with a standard one. Will upgrade that to a new one soon, probably using the kit from Old Britts in Seatlle, USA. They're a helpful bunch and have a good reputation. I like to buy from businesses around the world and stay in touch with the different biking communities. Here's a web address for Old Britts: www.oldbritts.com

 

Norton Commando Isolastics

 

Conclusion

 

The best advice seems to be to get the mounts as tight as possible without locking them rigid. That was my mistake when I tried it before. I took the mounts for granted and forgot that they needed intelligent adjustment (not my strong suit).

 

Also, you might have to play around a little to suit your riding taste. In other words, the mounts don't have to have the same settings. You might prefer it tighter here or there. It might be different for two-up riding.

 

But can you fit this system to your Triumph? Ho, ho, ho (but in the end, that's exactly what Meriden did with the AV project).

 

 

Sump visitor feedback

 

"The isolastics are critical on any Norton Commando, especially clearance - should be in the region of 0.004” - 0.006”, suggested clearances were a bit wide which would result in excessive lateral movement of the rear wheel and result in poor handling/steering.

"Problems arise at achieving an equal gap around the iso collar and thrust washer due to the front and rear iso brackets not being parallel to each other in the horizontal or vertical plane which means the iso gap is tapered and is why there is no choice but to adjust to the bigger gap to achieve tolerable vibration limits - but the handling suffers.

"They are also not horizontal or vertical to the steering head axis and subsequently neither is the swing-arm axis. To make matters worse the rear wheel is offset typically by approx. 0.25” (that’s a quarter of one inch!) toward the drive side away from the frame centre line. To check remove the mudguard and have a look at the position of the rear wheel in the seat loop and you’ll also be able to see that the rear wheel is not inline with the frame spine."
Simon Ratcliff.
www.parallelengineering.co.uk

Mobile: 07794 637159

— Big End

 

 

Think you know better? Good, drop us a line at Sump and we'll include your thoughts/experience.

 

 

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