The Commando is surprisingly tolerant of loose Isolastic
mounts and handling problems usually have their root elsewhere.
However it doesn't do to let them get too sloppy.
In a perfect world, Norton would have fitted the Mk3 850 Isolastic mounting, which was in the original design, to all models. Spoiling the ship for a ha'p'orth of tar as ever, the early Isolastic is such a pain to adjust that most don't bother until it becomes desperate. And it doesn't help that the shims go rusty and fall to bits.
A conversion to the Mk3 Isolastic is easy enough but it requires a strip down to replace the rear unit and it is not that cheap. Galloping over the horizon to the rescue is our friend Mick Hemmings with a simple conversion to a threaded adjuster for the early Isolastic units. It consists of a special end cap which incorporates a threaded adjuster at its centre and its made in stainless so it doesn't rust or sieze. Simply withdraw the Isolastic mounting bolt, front or rear, slide out the end cap and slide in the new one. Vernier Isolastic adjuster kit. While you're at it you might as well throw away any shims and replace the PTFE thrust washers with modern nylon ones. After that you won't mind checking the Isolastic every time you change the oil.
The workshop manual calls for a clearance of 0.010" (0.25mm); for a more comfortable ride you might let this go up to 0.020" (0.50mm) or for better handling at the expense of extra low speed vibration try 0.005" (0.12mm). On any Commando that's been "used" the clearance is often not the same at the front and back or top and bottom of the adjuster, so check all the way around. Remember to push the engine over in the frame away from the side you're checking and torque the mounting bolts to 40lbft. Whatever you do don't over tighten the Isolastics. The frame is not designed for a rigidly mounted engine and it will break. If one Isolastic unit is tight and the other is free then the frame will break very quickly indeed.
For racing, it is possible to tighten the Isolastics so there is effectively no measurable clearance. Do this by adjusting back from a tight setting until the thrust washers can be rotated by pushing them with the end of a screwdriver.
The engine essentially rotates about the rear Isolastic mounting. Consequently, the front unit moves more, wears more and has a greater influence on the transmission of vibration. Any play in the rear mounting is doubled at the rear wheel, play in the front mounting is simply matched in the rear wheel. Therefore it is possible to tolerate more play in the front mounting than the rear and get a more comfortable ride. Try varying the clearances a little and see what suits you best.