isolastic question

I think I read somewhere a long time ago that without the inner smaller rubbers, the outers would destroy themselves over time, coming unbound from the center tube and all hell would break loose, as well as the motor/trans/swingarm unit.

I was totally WRONG! Earlier in this thread I said that I couldn't see how the buffers (small rubbers) did anything, stating that I didn't think they could ever hit. As others have noted, that's wrong, Wrong, WRONG!

I was examining all the old parts closely today (all new parts, metal and rubber arrived this AM) and I saw that the old front mount tube caps and the front mounting collar show evidence of the mounting collar striking the tube cap which will happen if the engine moves approx 5mm in any direction from centered. Interestingly to me, when mounted, the buffers are between 4 and 5mm from the inner surface of the tube. So the point at which the buffers hit the tube is only about 1mm from the point where there is metal to metal contact between the tube cap/mounting collar. Seems like slightly larger dia buffers might be a good thing but then again, new mounting bushings should be bit more firm than the old ones. The bushings were deformed "downward" slightly so that the engine sat a tiny bit lower in the mount than it will with the new components...I assume.

ALSO...I discovered that the collars (that hold the shims) were not true at all, one of them having a "bowed" surface that rides against the PTFE washer. This meant that the clearance measurement was much wider toward the outside of the diameter, where you would normally measure it, than it did in the center. So this made the washer wear more in the middle and also meant that the clearance was narrower than measured - I had it set at .010 but if I had shoved the feeler gauge all the way to the bolt, it would have been around .005.

I'll check the trueness of the new collars and, if they are not true I'll surface them on the plate glass/wet or dry paper that should have been listed in the shop manual with a Norton part number!

MexicoMike said:

Having sorted out the engine stuff, I disassembled the front isolastic mount. I shimmed it a while back to get the .010 clearance but I had never taken it completely apart until today. SO... I''m trying to understand the setup.

There are two large rubber bushings at the outside which press fit into the mount and two smaller ones at the inside. The two inner bushings are too small to actually bear on any part of the mount itself. I can't see what possible use these inner bushings provide. They just sort of "float" there with at least 1/4 inch or more of clearance between them and the mount. Why are they there? is There NO WAY the system can flex enough to bring those bushings in contact with the inner tube surface. I don't understand what purpose they serve. ???

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.From a chap who makes them.....upon heavy take up the outer would not handle the pressure, and in the event of total collaspe the inners provide a safety net......but who really knows :?:

Last night I checked the new collars for trueness; although better than the worn, "bowed" collars, they were not true either though they were better than the old ones. They were slightly dished in comparison to the old ones but a couple of minutes of glass plate/wet or dry (320/400/600) corrected that and made them nice and smooth where they bear against the PTFE washer. As delivered they are a bit rough (I won't launch into my rant re the non-quality of AN parts).

I agree with the recommendation to use a dial indicator to measure clearance so I will use that method from now on. OTOH, when I had my '71 Commando back in the day - sold it in '78 - I raced it as well as rode it nearly every day and I don't recall adjusting the isos (changed any shims) more than once during the whole time I owned it so it's not like there is really much need for "constant" maintenance.

One thing I found on the mount tube was that there was thick original paint on portions of the edge of the tube where the collar/shims sit. This also contributed to measurements that were different depending on where you stuck the feeler gauge. This was another case of glass plate/sandpaper to surface the outer edge. Of course, whether the outer edge is actually square with the mount itself is a different issue, one that I have no way to determine. I suspect one could make themselves crazy worrying about the squareness/trueness of the various mount components!

And one more thing re the compression of the donuts - I was able to mount the donuts on the mount bolt and push the assembly by hand into the table, compressing the large donuts enough for the buffers to hit the table. So again, contrary to what I thought, the buffers can hit the inner tube rather easily - and only 5mm of movement will cause that to happen.

Do the bores for the isos need to be smooth or can I leave them wire brush rough ? Dont forget someone is selling MKIII isos with the bore a little to big for the engine mounting stud or front engine plate bolt. I got a pair ill have to put a sleeve in. Phil

I derusted the inside bore of the mount, smoothed it with a rotating "sleeve" of 400 paper and painted the interior to forestall future rust. Can't imagine that a wire brush surface would matter much. As long as there are no big pits/roughness where the large donuts bear, I don't see any way it could adversely affect the operation. With roughness/pits I can understand how it could reduce the life of the donuts as they work in there. OTOH, it seems that the smoother the better since the donuts "move" in the sense that as they compress, the outer surface of the donut slides a bit back and forth against the mount tube. But I'm no expert on such interfaces so maybe it makes no difference at all!

grandpaul said:

A bit more vibration, yes.

Better handling with managable vibes, I believe, is obtainable.

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When I did the MKIII upgrade to my '74 I added 2 small buffers to the rear and I regret it. A bit more vibration and no noticeable improvement in handling. But then, I'm not a racer.

MexicoMike said:

Yeah the lower pics look just like mine though the inner tube, inner surface of the mount and the circlips were heavily rusted. Maybe the inner donuts could actually hit the tube but it would take a lot of motion and the outer donuts are pretty stiff. Frankly, I can't see how the headsteady could allow enough movement for those inners to hit but I guess it must happen. As noted, Norton must have had a reason for setting it up that way. But I was surprised, I expected to see 4 donuts the same size.

I'll order new internal parts from Old Britts; I had contemplated going to a vernier adjuster but I had a '71 Commando that I sold in '78 and the whole time I owned it, it only needed adjustment it once. It was also a race bike as well as a daily rider so it got a fair bit of use.

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Personaly ..I consider the Iso's a cheap and nasty anti vibe system...When i worked on a "Pop bank" this " Bit of rubber " was always a make shift....Torque Arm/motor mounts....and "This chunk of rubber" was used to mount pumps.....these Isolastic's are from the stone age...and need reviewing......Rubber is self cureing...getting harder by the day. Soft when new in summer...Hard as rock's ..when old and winter....Very cheap and NASTY. ...John Lansdowne Eng

MexicoMike said:

I sold my 996 Duck and kept the Commando pretty much shows my opinion of the bike!

I think the isos work very well and have no complaints about them at all. It would have been nice had the vernier adjusters been on Commandos from the beginning but, OTOH, the need to actually adjust the isos is rather seldom and I don't consider it much of an issue to do it "manually."

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100% with you on this! I sold an 851 to buy my 850 - no regrets yet, although it's still not roadworthy. Always wanted one since I rode a 750 years ago when I had a T150. Just so much more relaxed and long-legged than any other Brit I'd owned...maybe because anything else Brit above 5000 rpm felt like it was being thrashed?!

Back on-topic... I had all the options available when I replaced my isos - a popular mod is to skim the front mount and fit a MkIII assembly, but when I looked into it it was evident that the MkIII isos are a harder compound than the originals, so I went with the Mick Hemmings vernier kit and a new set of 'correct' rubbers.
Having seen how some things can vibrate (helicopters), I can quite easily envisage the inner bushes coming to play on occasion, especially when the main rubbers start degenerating.