Commando Isolastics

[princeton]

I'm still struggling to get on top of this 74 commando 850 which I inherited.  The isolastics are posing some difficulty largely due to my lack of experience so any help would be appreciated.  I’ve read a lot about the rider experiencing a lot of vibration but hadn’t really experienced this myself.  There was however a lot of vibration on the engine itself bouncing up and down at low revs.  I inspected the front mounting and tried measuring the gap.  I wasn’t quite sure which gap I should be measuring but I assumed it was the gap between the large rounded disc and the thin washer (which I now think is a shim).  I was about to give up when I found that the main bolt was very loose.  After giving this a good old tighten, the washer (or shim?) no longer rotated around as it did on first inspection and the engine no longer bounced up and down.  I then think I discovered what everyone was talking about when it came to the rider feeling vibrations.  I was quite satisfied but have now been told to check the clearances properly or risk cracking the frame!

My questions I suppose are this:  Should I be measuring the gap between the large rounded disc and the shim?

Should I slacken off the main bolt first and then check clearances or should I leave it tightened?  What are the tolerances?  If the gap is 0.001” off should I take the bike off the road or is it still safe to ride it around, just a bit uncomfortable?

Any help would be greatly appreciated.

Many thanks.

[L.A.B.]

Describing the correct procedure for checking and adjusting the Commando Isolastics in detail would take a while to write out, but there is a Commando workshop manual (actually for 1970-73 models but the Isolastic info is the same) on the Internet where you can find the info you need (in Section F):
        http://rocbo.lautre.net/technique/norton_workshop/index.html

Firstly, before checking, the bike should be supported by the frame rails (71-on models) with the bike's weight taken off of the centre stand.

The Isolastic mountings should be tight to check clearance.

The engine should be levered over to the right to check front Isolastic clearance, and the swinging arm pushed to the left to check the rear Isolastic clearance.  

The clearances are adjusted by fitting shims of a different thickness, the shims are available from any good Norton parts supplier.

There are kits available that convert the Isolastic mountings to the later (1975 850 Mk III) type of vernier adjustment which is a lot simpler and does not require shims to be changed as a threaded adjuster ring is turned so set clearance.

It could be worth checking if your bike has been converted to this later screw type adjustment system if so ,then the adjustment info given in the workshop manual will not be correct.
Isolastic Vernier conversion kit info can be found on the Norvil website: http://www.norvilmotorcycle.co.uk/techtalk.htm

If the engine is seen to shake but the vibration isn't being felt then the Isolastics are doing their job although feeling some vibration at lower revs is to be expected.

Of course if there is anything that you don't quite understand then please do not hesitate to say?