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Isolastics - the answer to low rev vibes


I am aware that this topic seems redundant. However, I have some news (I guess), and a solution.

I went through many of the posts, but I did not find the answer to the problem: Vibrations below 3000 RPM.

There is a lot of exchange about conversions (Vernier/Hemmings/...), head steadies, correct shimming/adjustment, +/- thous, where and how to measure, ...

Still there is many Nortoners out there getting used to those nasty vibrations below 3000 RPM because they can’t fix it – which my 2 Commandos had never suffered, over 30 years ownership. A Commando should not shake, full stop, it is a civilised bike // relatively, ok. We all are investing hours / weeks trying to fix this (convert that in your currency), just as I did, over the whole Corona period. But the answer is so simple. It is NOT the gap adjustment, our bikes are very tolerant with this.

It is the rubber rings.

I just came back from a long test ride, on my 920 MkIII (in advanced Combat tune, "Verniers" at all 3 points) and: I have her back, my sweet Commie, got rid of all vibrations at any revs, as it used to be, pulling strong like always, and my grin goes from one ear to the other.

She got really shaky after I had installed a new RGM isolastic rubber unit. As I could not get used to the idea that somebody is selling something that doesn’t work, I spent weeks to study the ideal parallel installation of the engine in the frame (it is not possible, but you know that). But: Even if RGM claim they “have sold 2000 and never any complaints” – their piece simply does not fit, it is a piece of ... !

Where are these 2000 riders ? I didn't know we are so many. This post goes to them !

I should have given up when it was almost impossible to squeeze it into the tube, using heavy tools to push it really hard, special silicone oils of different makes, etc. But you know how it is, you want her back on the road ... now. But how should that work, if the rubbers are compressed so heavily, even slightly bent (see picture, it is hard to picture it but this rubber should be straight flat). The whole thing gets so stiff, how would that ever swing with the engine ? That is certainly not how the Norton engineers had designed this beautiful invention.

When I finally opened it, I understood the problem. My correction was to drill a few holes (8) with a 5mm drill around the middle circle of the outer rubbers, leaving them inside the tube. Took me 5 minutes. You can see and feel how the rubber relaxes. I mounted again (for the 10th time, at least, I can do it now in 3 minutes), with minimum play this time, just enough that the large washer can move freely. And what an experience – all the vibes gone, just as the inventors meant it to be. So smooth, I almost had forgotten. So, it is 100% waterproof that these RGM iso-rubbers are wrong, I think they are simply too big, about 2 to 3 mm in diameter.

However, this is only my personal answer, not the solution. And I do not know how long this will last. The real solution is: Buy whatever you want from RGM (they have a lot of good stuff), but don’t buy this. And, sorry to say, don’t listen to their advice (at least since recently). If they only would admit, yes, ok, we do have a problem here ... how many hours could I have saved.

Andover, they say, have better ones, and I will certainly order one, for my next, hopefully very far, isolastics opening. If they go in smoothly (with soft pressure), I will be ok. If not, send them back.


I am about to place order for conversion kits for my ‘72 750. Planning to call Norvil for advice on which kit to buy. Seems there is a new & an old type for the rear. And then specific version for front without mods needed to the tube.  Any experience with the Norvil kit anyone?

Andreas. What head steady do you have?


Interesting finding. The Norton designers kept trying softer rubber until they got it working.

I have just fitted new ANIL (Andover) rubbers to my 1976 Mk3; 50,000 miles and the first time they have been done. The old front ones had settled off-centre. The new ones went in easily with rubber grease and felt 'soft' to a fingernail test. I still have the original design of head steady.

The bike now feels 'tighter' for handling and rides much better, and the vibration profile seems about right - less flopping around at 1,000 RPM and possibly firmer for buzz at 4,000 RPM, but it's marginal. I think they have got the rubber about right. A bit reluctant to promote the anti-NOC company but they seem to pay attention to getting some things correct.



I recently fitted AN rear iso assembly and it slid in very easily with silicone grease.  The one I installed on my front a couple of years ago (Also from AN) was really difficult to get in.

Have they changed to a softer rubber?


Maybe the same as the clutch shock absorber rubbers (Dommie and early Commando).  The new ones are far too stiff to insert.  I tried to change mine some years back because the old ones were swollen (not rotten).  I took them to Russells, and Les couldn't get them in.  So I put the old ones back and trimmed the faces.  Next time I think I'll pour silicone RTV in there instead.


Norman White will only for the AN iso's as they match the original hardness specs as they had over the years got too hard. AN did not always get this right but at least they are correct now.


frontisoAndreas  good for you.

Well it seems the word slowly gets around about all these new and improved one piece iso's.

My original combat with original old style72 iso's was smooth as silk by 2000 rpm and a friend with a combat used a new "improved" combination iso. It was horrible and he took it out and gave it to me to measure. and found it was very hard/stiff. We knew this in the mid 90's but no one wants to admit the truth that they were NOT an improvement. By 2000 we were suggesting drill the rubber just as you have done.

Ones from 89 were rock hard and the ultra soft were still harder than the ones in my original MKIII

I am happy that the original rubbers that were NLA for a long while,  but now are being made again.


… that no one has discussed what clearances are being set in the Isolastics. My experience (and this surely isn't news to anyone) is that this makes a big difference to both the amount and quality of vibration experienced at different engine speeds, and of course to the handling.

Notwithstanding the workshop manual's claim that 0.010" is the ideal clearance, I have always treated this as an upper limit, instead aiming for 0.000"—interpreted as being sufficiently tight to be still able to rotate the mounting tube caps by hand without special effort.

This (both back in the 70s and with current AN iso rubbers) leads to noticeable low frequency vibration at ticker, but this goes away as soon as the machine is under way, and eliminates all handling issues.

While I'm writing, thanks to David Comeau for his fascinating chart. The maximum load applied is very nearly equal to the total dry weight of a Commando.  


Firstly, my big thanks to David for this super interesting graph, which only supports the strange variety of this vital part of our bikes. I can only repeat my recommendation to have a close check before installation.

To Julian, I would like to repeat that with a wrong isolastics unit, the clearance does not matter at all, you will not eliminate the below-3000-RPM-vibes at any clearance. I have tried any clearance you can imagine.

With my now adapted isolastics (remember, the 8 holes), I have already tried again, and I found again not much difference, as long as the engine can move freely in their mountings. So my check is, just like Julian's, whether the washer can be moved feely between the caps in all directions, at all 3 mounting points. From an engineering point, I do not try measuring +/- any thous around this rather rough / tolerant design. Actually it took me quite some shimming effort in all the three horizontal bolts of the Norvil head steady to get the head steady itself relaxed in its center position, and parallel to the engine, and with this, remove any friction out of the system.

And, gladly, even at tick over, my mudgards ar not shaking anymore since I did this adaptation. That has been a point of concern for car drivers at traffic lights in the recent past, and a bit embarassing for me. No, this is not a Harely Davidson, this is  Norton Commando !


I can remember back in 1975/76 (my memory isn’t that specific) a Gus Kuhn mechanic telling me that in a normal service they would set the ISO’s to 10 thou but would set the Interpols to 6 thou. He said that this gave the Interpol slightly more low revs vibration but tighter handling.


Interesting information on the stiffness of Isolastic rubbers. Probably goes some way to explain why the new Mk3 I rode was sooo much smoother than my Mk2A. Well that and the sprung heady steady and general overall newness.

Not found that Isolastic clearance makes that much difference to vibration or handling unless you really go to extremes. Best compromise seems to be adjusting the Isolastics to different clearances; tight at the rear, say 5 thou, loose at the front, say 10-20 thou and, if you have one, 10-30 thou at the head steady. Biggest problem to proper adjustment is, as Andreas has alluded to, is getting the mount tubes squared up.

Loose Isolastics exaggerate problems elsewhere long before they become a problem in themselves. So I suspect the reason Kuhn's were setting Interpols on the tight side was to compensate for issues caused by the fairing and the way Plod rides. That and simply so they stayed in tolerance over the miles between services.



I might be the only person here who is not an expert on isolastic rubber composition, so please forgive my ignorance. I don't have any technical insight to offer but I would like to suggest we don't be quite so disparaging to RGM without hearing their view point. 

To the Moderator: Given the strength of feeling shown in the first post we ought to offer RGM the chance to reply.




In 2016 I fitted RGM isolastics front and rear, liking the idea of the stainless steel kit and although tight as Andreas says, they fitted in the end using a PTFE Aerosol spray. Adjusted them to 0.010" then after a few miles finally to 0.006". The stainless Norvil head steady was fitted with new PTFE shims as advised in the original Les Emery instructions, centred, then tightened it all up. No extra shims but the final gap as I recall was over 0.010"

Handling was still good with no wobbles or weaves but the overall vibration levels were higher on both 0.006" - 0.010" gaps including at 4000rpm. With more attention to carb synch and equal lift it,s almost back to the old iso level. Cruising along the motorways last week following the traffic flow between 4-5000rpm was smooth and effortless. Maybe they,ve settled in over the last four years

Andreas is also right in pointing out poor RGM customer relations at times. In 2018 I bought a pair of RGM stainless 850 exhaust roses as they call them. They didn,t fit my old RH4 or the new Full Auto exhaust ports, screwed in a couple of threads then locked. They clearly measured oversize on the threads compared to the AN originals

After much debate and the usual "I,ve checked all the dimensions personally" type of dialogue I got my refund. Nasty little touch at the end was they refused to refund return postage, a legal requirement in this case and no answers to my emails pointing this out

This long time RGM customer just walked away and not dealt with them since. I was given the expensive but superb cnc stainless exhaust lockrings from Seagar Engineering as a birthday pressie last year. Fitted both heads perfectly and don,t come loose



Let me confirm my first message saying that RGM have "a lot of good stuff", so nothing at all against RGM and therefore I do not think there is any comment necessary.

However, at least the isolastics sold to me are not fit for the purpose, and it does not take an engineer (I am one) to see that. I think I have proven that, it is a fact, there is enough evidence: This set simply does not fit inside the tube, it is too large and/or too hard. Drilling 8 holes in it makes it work, but from an engineering point, and even more from a customer point, this is not a desirable track to go.

I also have negative experience with 2 recent answers, but this might be individual problems. 1) This isolastics story  2) I bought a GPS speedo which works wonderfully, expect that it always takes minutes to find signal. RGM did not give me any good support on that so I had to work around my own solution - only to find later that actually there was a standard solution that could have been communicated to me.

Finally a quick feed-back to Neill's exhaust roses: I am now using CNC'd aluminium alloy roses made by a specialised workshop in Germany

and, because they have quite similar thermal expansion like the head, they don't shake themselves lose. I use the bike almost every day / many miles, and this was a real improvement.

Wow, what a beautiful piece of high-end engineering advice, directly from the R&D department, even if "somewhat un-orthodox". I can't stop smiling imagining this exercise on a modern car. Wish I could do this to my new e-hybrid electronic turbocharged engine, cut off a piece from the bottom cover and drill some holes into the crank. But maybe not during the warranty period ...

This advice, Andreas doesn't seem to have originated with BSA but with their West Coast USA distributor Hap Alzinas who had no compunction in using the parent company's logo on a service sheet which was clearly blatant botchery. Maybe he and his dealers were in despair with the cost of making newly delivered A10s acceptable to their customers...It's not clear what the causes of the vibration were (High comp tuning kits with heavy pistons ? ) I think it's clear why the British manufacturers, by the end of the 1960s had set up their own overseas sales organisations. Off-loading large quantities to independent agents with almost zero margin and the importer responsible for warranty costs was not a recipe for customer satisfaction with increasingly high performance machines (and, dare I say it, declining quality control ?)


The usual method for expressing the hardness of rubbers and elastomers is the Shore scale. Does anyone know whether there was ever a Shore specification for the isolastic rubbers?

Hi Chris

I'm aware of rubber hardness measurement but in this case it may not be all that relevant.

Compressing in the oversize diameter and wider donuts yeilds a stiffer deflection load.

Even the early individual donuts had a factory tapered tool to slide them in .

The later one piece just went overboard yeilding over stiff iso's that do not decouple the vibrations until a much higher RPM.

I love the early soft ones. With the throttle I can control the rpm and selectively make the front wheel, then mirrors, then mufflers, then oil tank resonate at different rpms. but before 2000 it all stops and almost feels like a noisey electric motor to red line. 

The later harder one piece rubber sticks need over 3000 to stop resonating the bikes components....They decouple the impulses to the main frame poorly.

Those were the main subject of my chart measurements shown above.


I don't want to get into any supplier bashing as seen previously in this thread as I have used all three main spares suppliers of Norton spares with varying degrees of satisfaction, I now "vote with my wallet". That is not to say that bad service or poor quality parts should not be exposed.

For reasons that would take too long to explain I found myself with front isolastic bonded rubber bushes from both RGM and Andover Norton. They are different. The RGM bushes are 49.5mm outer diameter and are a parallel disc of rubber bonded onto a metal inner tube. The Andover bushes are 50.5mm diameter and have tapered sides resulting in a narrower contact with the cradle bore. My cradle bore is 47.24mm diameter. The RGM bushes do seem less pliable in radial compression but this is entirely my subjective view and may be a result of their different shape. This does not mean they will behave better or worse in service. The Andover bushes are like the ones shown in the Norton Workshop Manual 06.5146, figure F14. 

Having fitted the Andover bushes in place of the RGM I have less vibration but I believe this is probably due to the additional attention to detail I paid when assembling the Andover bushes in the cradle with the ten thou axial clearance.

I wish I could be sure which supplier is providing the absolute OEM quality and standard of spare parts. If I go to Mr Honda I invariably get the quality of the original part but pay an extortionate amount for it which is probably worth it for critical parts. Norton owners do not have this assurance even if they are prepared to pay handsomely for it.


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