Hard vs Soft Isolastic Rubber (2009)

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Stephen Hill

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I need to overhaul the isolastics on my 1968 Fastback and have a question about different rubber hardnesses. I have found comments on the web saying that the rubber that comes with the vernier style isolastics is harder than the pre-vernier style rubbers. And that this results in more vibration if you use these rubbers in a 750, compared to an 850. I understand the pros and cons of the vernier in terms of adjustment. But does going to vernier isolastics mean harder rubber and more vibration on a 750? Is this a good reason to avoid the vernier style? Is there such a thing as 750 rubbers versus 850 rubbers?

Stephen Hill
1968 Fastback
1970 Mercury
1971 Roadster
1974 Interstate
 
Good questions, Stephen!
My 750's always transmitted less vibration with the same iso clearance settings than my 850s.
The only possible reason I can think of is that the 850's have heavier pistons.
In theory, it should not make any difference if the iso rubber's end caps have shims or adjustable threaded adjusters.
In reality, I don't know because I converted my 73 to threaded adjusters years ago and can't remember.

I do know that put in all new rubber donuts about eight years ago and the bike with same iso clearances does indeed vibrate more so.
I suspect I was sold the "harder" donuts.
If I was you, I would call old britts and mick hemmings and ask them specifically if the sets the sell now are considered "soft".
I would want the soft ones, if that is an option, and I will check on this next time I replace mine in a few years.

I can't see why there would be rubbers specific for a 750 and then an 850 because there was no change in the cradle specs they go in to, to my memory.
 
From the info I have gleaned, I think the early rubbers are softer than the newer ones. This is from Phil at Fair Spares, "All the early isolastic parts I sell are genuine Norton and made in the softer rubber." If you want the soft rubber and a vernier setup, get the Mick Hemmings adjuster for around 60pounds and the rubbers from OldBritts or Phil and you will have a vernier setup with the soft rubbers. Just make sure you order the early part numbers.

Dave
69 S project
 
My MK1 850 had the original iso setup, so I bought the Norvil stainless MK3 setup and fitted it. I was shocked. I now call it the "Atlas conversion". I couldn't believe how my cuddly bear 850 had turned into this vibrating lump. It was smoother over 3000rpm, but getting there was a chore. The other Nortons I've ridden locally are MUCH smoother. Jim Comstock told me he fixed the problem by drilling holes in the outer rubbers (on a milling machine for accuracy) and this fixed the problem. When I get around to needing a bit of work on the cradle, I'll address the problem by using this method as, of course, on these conversions the rubbers are bonded to the tubes. In hindsight, the answer is to use the the original bits, with new soft rubbers and one of the vernier adjuster setups which attach to the original pre-vernier isos. Well, that's my take on it. Probably cheaper than any other route as well.
 
I have the Mick Hemmings Iso adjusters and I like them. I got them to replace the MkIII's that I got to replace the standard issue '73 shims. You can keep the rubber of your choice and they cost much less than MkIII's.
 
Reading this thread has just stopped me in my tracks!

I was about to order the Norvil MkIII conversion for mine, but now I'll be going the Hemmings route.
I just had a look at my old original isos (fronts rotten and rears still have the remains of the through bolt seized into them :oops: ) ... I wouldn't have bothered checking if I hadn't read this:
The small buffers (i.e. in-between the 'proper' bushes) are a very hard rubber, but the big suspension bushes are definitely much softer.

Looking at the images of the other conversion kits it looks like they're all made from the same compound, which seems to be consistent with what's been said, in other words, hard!
I just had a look at the RGM ones and they're listed as 'heavy duty'....Hmmmm.

Once again, the Forum to the rescue :mrgreen:
 
At risk of sounding a bit thick :? do I understand correctly that if I was to fit genuine Norton early type seperate iso rubber bushes, available from Mick Hemmings (and presumably any other geniune Norton spares outlet) and Micks patented vernier conversion kit that works with the earlier type bushes, that I am likely to have a softer vibration experience than the vernier adjustable Norvil bonded bushes that are presently fitted in my Norton?

I perceive the advantage being that I will have the softer smoooooth rubbers but with vernier adjustment :?:

Can this be so :?: And are all of the genuine Norton seperate iso bushes softer than the bonded Norvil type :?
 
The next question is whether the Hemmings adjusters fit the Mk111 tubes as well (they are equal lengths on each side). Anyone know ?
 
Fullauto said:
. Jim Comstock told me he fixed the problem by drilling holes in the outer rubbers (on a milling machine for accuracy) and this fixed the problem.

Any idea on the size and number of holes?
Click to expand...
 
It's my understanding that the Hemmings kit will fit any setup. It just replaces one of the front and rear abutments with an adjustable one, but maybe someone that has one can speak to that.

Dave
69 S
 
79X100 wrote;
The next question is whether the Hemmings adjusters fit the Mk111 tubes as well
Click to expand...

I'd wondered about thee front isolastic mounting as well. As far as I know though, the rear isolastic mounting on the cradle is the same.......isn't it :?:
 
This is what I've found out today...

My understanding (note!) is that the Hemmings vernier adjusters replace the pre-MkIII shim assemblies, retaining the original front mount, and are not MkIII type.
The MkIII conversion (complete assembly) from RGM requires the front mount to be skimmed back, whereas the Norvil item is a straight fit to the earlier front mount without any mods.

Hemmings also lists the MkIII conversion.

I noticed that the RGM early iso bushes are a different part number to the parts book, and stated to be Heavy Duty; whereas Hemmings and Norvil list theirs under the original P/N.

What I couldn't find was a price for the Hemmings verniers on their own :roll:

A final thought....would the weight of the starter motor justify the change in compound? Evidently 'yes' :? but are MkIIIs any rougher or smoother than the earlier bikes?
 
Hemmings adjusters were £52.00/pair in the 2005 catalogue. Can't find them listed in the 2009 price list.

Dave
 
rbt11548 said:
Fullauto said:
. Jim Comstock told me he fixed the problem by drilling holes in the outer rubbers (on a milling machine for accuracy) and this fixed the problem.

Any idea on the size and number of holes?
Click to expand...
Click to expand...

'Fraid not. Jim had done it so long ago that he couldn't remember.
 
DogT said:
Hemmings adjusters were £52.00/pair in the 2005 catalogue. Can't find them listed in the 2009 price list.

Dave
Click to expand...
The 2009 list has them at the top of page 32. Front 06.7116, rear 06.7117. Available in steel and stainless. I'd recommend steel. The stainless ones gall if you use them dry and if you use antiseize they won't stay put with the setscrews supplied.

As to the question of them being adaptable to MkIII's, it helps to get a picture of what they look like.
They are replacements for the collars, #25 and #43. One for the front and one for the back. I can't get to my bike tonight, but I'll get a pic tomarow.
http://www.oldbritts.com/1973_g13.html
They are two piece units. The flat section has a threaded hole in its center. The inner section is threaded on the outside and fits between the frame and the metal tube of the bush. It is stationary. The flat is moved in and out by turning it one way or the other, vernier. There are set screws on a shoulder of the flat to lock the adjustment. They are made to be the approximate dimensions of the one piece collar, so they are meant for the Pre MkIII . But there is no reason why you can't use it on a MkIII after you you take up the slack created by the shortened tube on the right side of the front mount. I would think around two or three of the washers, #15 would do it, (on the right side). You would need to get one standard collar for the front and back plus Pre MKIII bushes and buffers.
 
Yes, the M111s are inherently smoother. The primary chain has tensioner so it cannot whip, hence one good thing towards smoother.
The entire bike is the heaviest commando ever, the sheer extra weight may also disguise felt vibration.
In my opinon, by design, the mark 3s should be and seemingly are, the smoothest.

Get the rubber donuts from whomever says they have the softest available, I would trust fred at old britts if he says he sells the softest.
 
bpatton said:
DogT said:
Hemmings adjusters were £52.00/pair in the 2005 catalogue. Can't find them listed in the 2009 price list.

Dave
Click to expand...
The 2009 list has them at the top of page 32. Front 06.7116, rear 06.7117. Available in steel and stainless. I'd recommend steel. The stainless ones gall if you use them dry and if you use antiseize they won't stay put with the setscrews supplied.
Click to expand...

Not sure if we're looking at the same thing here - bearing in mind they've gone from £52 a pair to over £60 EACH in 4 years :shock:
I think these may be the MKIII 'full kits'...I'll call Mick to get the full story.
 
B+Bogus said:
bpatton said:
DogT said:
Hemmings adjusters were £52.00/pair in the 2005 catalogue. Can't find them listed in the 2009 price list.

Dave
Click to expand...
The 2009 list has them at the top of page 32. Front 06.7116, rear 06.7117. Available in steel and stainless. I'd recommend steel. The stainless ones gall if you use them dry and if you use antiseize they won't stay put with the setscrews supplied.
Click to expand...

Not sure if we're looking at the same thing here - bearing in mind they've gone from £52 a pair to over £60 EACH in 4 years :shock:
I think these may be the MKIII 'full kits'...I'll call Mick to get the full story.
Click to expand...
You're right. Looking at the Norvil site 06 7116, 06 7117 are MkIII Iso's.
 
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