In the late 1960s, Norton was struggling. Faced with the dilemma of keeping its outdated big twin, the 750cc Atlas, viable in a rapidly changing market, the cash-poor company needed to do something but couldn't afford a clean-sheet approach. Fortunately, that something came when Dr. Stefan Bauer, ex of Rolls-Royce, developed Norton's Isolastic engine mounting system for the new for 1968 Commando.
In the Isolastic system, the engine, transmission and swingarm are mounted together in a subframe. The assembled powertrain is hung off the main frame, but instead of being rigidly bolted to the frame it's mounted via two cross-frame tubes — one at the front of the engine and one at the rear of the subframe — with a head steady attached to the top of the cylinder head and bolted to the frame. Bolts holding the powertrain assembly to the main frame pass through rubber buffers in the tubes, isolating the engine from the frame.
This allows the engine to “float” on the vertical plane, with lateral movement controlled by shims in the mounts. Properly set up, the system effectively eliminates vibration, and it launched a new era for Norton with the resulting success of the 750 and 850 Commando.
Until the final MkIII of 1975, the cylinder head was fixed to the top frame tube by triangular plates fixed to the frame on rubber buffers, one on each side, and bolted to a head steady mount on the cylinder head. This was adequate to muffle vibration at the engine where the cylinder head attaches to the frame. However, the buffers weren't really designed to be load-carrying, so the weight of the engine was always on the two main Isolastic mounts.
For the MkIII, Norton added a paired spring rated to carry the weight of the engine and transmission assembly, about 140 pounds. The spring attaches at the frame on a simple plate, and at the engine on a trunnion mounted to a plate on the existing boxed head steady mount. The spring, mounted in line with the angle of the cylinder, carries the weight of the engine, taking the load off the Isolastic mounts. The result is better Isolastic working life and better sensitivity to adjustment.
|A MkIII-style head mount with spring plate and trunnion (left) and a pre-MkIII mount.|
Fortunately, owners of older 850s (and 750s if an 850-style head steady mount is used) can upgrade to the MkIII style by installing the necessary MkIII head steady bracket, trunnion, spring and spring retainer.
This is a fairly simple upgrade that is well within the capacity of the average weekend warrior. Modifications are limited to drilling two holes in the existing boxed head mount, and if you want to maintain the originality of your Commando you can modify a used head mount, preserving your original for posterity or future sale.
We conducted our upgrade on Q&A guru Keith Fellenstein's 1974 850 Commando, and even taking photos it took us less than two hours. To get the most out of the upgrade, we suggest re-checking your Isolastics when done, re-shimming as necessary. As ever, we strongly suggest having a good shop manual on hand for reference to critical parts and torque specs.
Our Norton with the pre-MkIII-style head steady. 750 owners will need a later 850-style boxed mount for this upgrade. Although not clearly visible here, we removed the ignition coils for access. Refer to photo #8 to remove the center bolt on the mount.
Here are the pieces you'll need, with Norton part numbers. From left: spring retainer (06-5585); spring bracket with trunnion and nylock nut (06-5458, 06-5456, 06-7892); head steady spring (06-5454). You'll also need two 1/4 x 5/8-inch bracket bolts, plus corresponding nuts and washers.
With the mount removed, make a mark 1/8-inch to the right of center (viewed facing forward) on the base of the mounting plate. Mark the center of the spring bracket and line it up with the mark on the plate, flush with the bottom of the head steady.
Mark the head steady where the two holes on the spring plate line up. Transfer the marks with a center punch, then drill through the mount face with a 1/4-inch drill bit.
Next, securely bolt the spring mounting plate to the cylinder head steady. We used blue Loctite on the bolts to ensure they stay tight.
Now, attach the spring retainer to the frame at the ignition coil mount bracket using the two rear most bolts holding the ignition coil mount.
Hook the spring on the rear tab on the spring bracket as shown. Loosely attach the head mount to the cylinder head, followed by the side plates to the rubber buffers. Don't forget the long bolt spacers that go inside the head mount. Next, center the head mount and then tighten the side plates, followed by the head mount.
The center head mount bolt is a tight fit, requiring a 7/32-inch Allen head wrench. Most Allen wrenches don't reach, so we cut 1 inch off a 7/32-inch Allen head wrench and then used a 7/32-inch socket to loosen and tighten the bolt with the stub. Quick and easy.
Finally, with the bike supported off the centerstand so as not to load the Isolastics, set the tension on the head steady spring by adjusting the nut on the trunnion until the spring coil is extended to approximately 1.5 inches. There should be approximately 0.5 inches of thread showing below the trunnion. Check the Isolastics for proper adjustment and you're done.