Updating / Replacing Isolastic Mounts

Hi folks - I recently acquired a new to me (see other thread) 1973 850 MK I. It has 12,615 original miles (allegedly). The PO had a British Bike mechanic do a full tune up just prior to my purchase (15 miles since work done). One thing he mentioned in the notes I received was the following:

"Unless you know otherwise, I expect the Isolastic motor mounts in the bike are probably original from 1973. If so, they are hardened and collapsed and should be upgraded to 1975 MKIII style mounts. CNW (Colorado Norton Works) has complete kits to replace the top motor mount and the front and rear Iso's. The last time I priced them they were $375.00 for the Head Steady, and $560.00 for the front and rear isolastic mounts. They are probably more now-a-days. Labor is about 6 hours."

A little odd that he said, if so, as in if they are original versus, I looked at them and they looked collapsed? Any thoughts? Seems pricey - bike seems to ride fine to me but what do I know.

You can always start with a Dave Taylor type headsteady and feel the improvement , cant’t remember prices when I did mine, my order of replacement was, all the ISO’s except the HS ,things were better , however the new DT head steady was a revelation once I got the spring tension dialed in , did all of it myself not that difficult , make sure to use the red grease in tubes and the softer rubber bushes .... no doubt your bike needs this upgrade .... you be very happy with ride improvement .... good luck !

There are MKIII ISO kits available from multiple sources, cheaper than cNw. I'm sure someone will post links to them. Worth while doing if you have to replace the rubbers anyway.

The top mount (head steady) is an entirely separate issue, as it affects handling more than vibration. And yes, the cNw one is expensive. But it's beautiful (I have one ready to install now). Dave Taylor makes one too that costs less. Replacing the stock upper rubber mounts is dirt cheap and simple.

Originally, the front and rear Isolastics were multiple piece with short sections of tube held together with circlips and individual rubber donuts. They went out of adjustment regularly and adjustment was by shim. A tedious process. The ones Matt sells (CNW) are one piece with the rubbers bonded to the tube. They never go out of adjustment over long distances and are use vernier adjusters which are simple to adjust. I feel the biggest difference between the old and the new is that where the old style isos allowed the engine/transmission unit to shake quite a bit up to 3000 rpm and then smooth out noticeably, the modern bonded ones can be adjusted to be very smooth right through the rev range. The CNW ones are also stainless. Money well spent.

The CNW headsteady (by Jim Comstock) works very well (over 50,000 miles on mine) whereas I'm led to believe that the DT top mount has a non sealed bearing which can wear out over a relatively few miles. The CNW headsteady is plug and play with no adjustment needed.

The iso's are primarily to isolate the drive train vibrations from the rider. Retaining a modicum of roadable handling is obviously also necessary.
For me the attraction for replacements is somewhat low, based on my research on the front iso.
The 68 commando iso rubbers have a different part# than 69 thru 74. 75's are their own system but are sometimes drafted into service on the early bikes. I suspect with less than outstanding results. This is based on continous reports that the vibes run to 3000 or more rpm. Including those who "upgrade?"

My pure stock 72 vibes are gone by 2000?. Why would I upgrade to inferior......
Some of the deficiencies such as very crusty surface of the rubber by ozone or slimy surface from oil contamination probably deserve attention. One other factor is deformation from a continuous load. This can be observed by the center support tube not being centered with no load. If this "set" is nonsymetrical it may have been from years of being on the side stand. Any iso nonsymetrical set will as a side effect shift a side load up to the top ISO, which originally was a pair of exhaust rubbers. I particularly noticed the (side stand) side load rubber set when after over a decade of my combat while using a norvil top headsteady.
I unbolted it from the top of the head and the engine was sprung to the side from the set in the rubber in the lower iso pair. I suspect this sideload into the top norvil head steady is cause for complaints as it heavily loads the teflon discs. If the rubbers were still good, on next service I will pull them out and turn them 180 degrees and go for another 50 years.

The aftermarket MK3 style stiffness has never been tested and published. There are likely several versions of rubbers on one stick with the promise of easy adjustibility. Their reported stiffness has no draw for me. The original style with individual components was unavailable for a while and NOS were scarce. Original style are now available but the durometer and physical mounting dimensions and performance has not been tested and published.

More anecdotal to follow:

Does anyone have a photo of the additional lower iso mount employed by Herb Becker and others?

RGM offers
ISOLASTIC FRONT RUBBER, SOFT
PART NO: 064673£1.40 EXCL. VAT

They also offer "heavy duty". It might be useful for somebody with a durometer to purchase some samples they arent that expensive and see how they measure up.

I have the adjustable type in my 850 Mk2 and I think they are too hard as it is at least 3k before they smooth out.

Eric Bauer said:

The PO had a British Bike mechanic

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Eric Bauer said:

One thing he mentioned in the notes I received was the following:

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Eric Bauer said:

If so, they are hardened and collapsed and should be upgraded to 1975

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How many folk get screwed by a British Bike mechanic who know little of norton or "commando". Is he getting a commision on CNW sales?

acadian said:

additional lower iso mount employed by Herb Becker and others

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A local club member with his bought-NEW (still owns it) commando ran 160,000+ mile on the originals and did not need a DT head steady, or lower ISO add-on , or "upgrade to stiff 850 rubber on a stick racing mount.
As a normal use touring bike you will be fine without every ricky racer upgrade you will be spoon fed. Are you going to track race it?

Onder said:

It might be useful for somebody with a durometer to purchase some samples they arent that expensive and see how they measure up.

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In reality the items must be assembled into the front mount and then load/deflection rate measured. The actual durometer is not the actual term for the test, The volume and amount of compression preload is all part of the system to be tested/measured.

I remember the vibration change being like a switch at 3000 rpm when my 74 was new. When I installed some MKIII verniers I was disappointed in the results. I had to open the clearances beyond .010" to make them acceptable, and that's not good for handling.

There was/is an adjustable rear iso option from Mick Hemmings that uses just a cap and the stock multi piece rubber components. This can be installed without removing the rubbers and it runs about $50 I think.
Not if you are converting front to vernier, you need to get the version made to fit the markII frame, otherwise the bracket needs cutting down.

Rion VA-11 vibrations analyzers us FFT analysis to show the frequency domain of the vibration not just el cheapo haphazard vibration amplitude.
I looked for these for over ten years until I found some on ebay. replacement cost today $9000.00 approx (pair)
I bought these for doing featherbed vibration resonance measurements related to crankshaft balance factor.
There is no reason they should not work to measure the power train lump VS coupling into a loaded (driver seated on bike) isolastic commando frame.

Tornado said:

There was/is an adjustable rear iso option from Mick Hemmings that uses just a cap and the stock multi piece rubber components. This can be installed without removing the rubbers and it runs about $50 I think.
Not if you are converting front to vernier, you need to get the version made to fit the markII frame, otherwise the bracket needs cutting down.

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Those are the ones I would use without reservation since they are for maintenance ease of setting the clearance but do not affect the rubber stiffness.

Tornado said:

There was/is an adjustable rear iso option from Mick Hemmings that uses just a cap and the stock multi piece rubber components.

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The Hemmings kit converts front and rear, however, there can't be many Hemmings kits left now.
http://www.norvil.net/norton/mhiso01.htm

Tornado said:

Not if you are converting front to vernier, you need to get the version made to fit the markII frame, otherwise the bracket needs cutting down.

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That's the pre-Mk3 front Iso. conversion kit to fit the pre-Mk3 front Iso mount tube (not the frame).
https://andover-norton.co.uk/en/shop-details/17047/isolastic-kit-front-vernier-pre-mk3-commando-

First, go to the front iso, right side, and peel back the rubber dust cover. If you see the part with several 1/8" holes drilled in it for adjusting, then the job has already been done. If so, then make the adjustment for the proper .006 - .010. Then check the rear iso on the left side to see if that has been replaced. If so adjust like the front. The rear one is much more difficult to replace so the PO may not have done it. If it is the old shim style get out your feeler gauges and see where you are at. You might try putting the bike on the center stand and have a friend hold the front wheel as steady as possible. Then crouch down and try to move the rear wheel from side to side to see where the movement is ESPECIALLY the swing arm. All these must be close and as accurate as possible because the rear wheel wiggle is amplified by slop in all three sources.
I have seen the original isos hold up for a long time and very usable. If money is limited and you are doing the work yourself you could probably still use the old set up (upon inspection) and re-shim. But if you are in that deep it makes more sense to put in the new, easily adjustable parts. I do not know about the durometer readings.

If you have a dial indicator and simple clamping set up you don't need feeler gauges and as a bonus you get a more accurate reading...I gave up feeler gauges 25 years ago for this job. Don't even have to pull the boots off.

Curiuous how you do it Dave.

Yup , I own the dial gauge but have never it used it for ISO’s .... so how bout some basic , how to , please ...

Simple typical clamping set-up
works better on center stand but not mandatory
1. clamp on end of 3/4 nut on left side through bolt
2. swivels flex and clamp in place dial indicator rod on engine case 20M3S breather elbow /200000/300000/ cam end bump
3. grab frame rail down tube & exh pipe...push-pull note +/- deflection of dial..... done

Rear clamp on rear 3/4 nut
repeat touching primary just below ISO
push pull primary note deflection,,,done

5 minutes or less, more time fetching the tools than doing the measurement

Don't rush out and buy isolastic parts until you learn something about how they function and what's there now. Two things you can do to improve the handling of your bike are improve the headsteady and swingarm clamps (Kegler clamps). Both are pretty basic operations to perform. As for the isolastic mounts, its a pain in the ass to remove them and inspect them but you might as well learn about it. Yes they might be original. However, they might be in good condition. A lot of that depends on how the bike has been stored and in what climate.

If you don't have a workshop manual, buy one or download a digital one from the tech section here. Use either Andover-Norton or Old Britts web sites to look at parts diagrams so that you have some idea about what pieces you are looking at. If you want help on these projects, ask on this forum to see if there are other enthusiasts in your area that are willing to do a garage day looking at isolastics. For the price of some beer you can learn a lot about your bike this way.