Norton Upgrade: MkIII Head Steady


Norton Upgrade: MkIII Head Steady 

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 Faced with the dilemma of keeping its outdated big twin, the 750cc Atlas, viable in a rapidly changing market, the cash-poor company needed to do something but couldn't afford a clean-sheet approach.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.

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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.

Head steady changes

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.

3/18/2021 4:31:17 PM

In reference to the instructions given for the 6th photo: Braze or weld either 1/4" NF nuts or 1/4" NF x 5/8" long bolts onto the back side of the two holes you just drilled. This is how the original Mk III head steady was made. This allows easier removal of the spring mounting plate in future. You need to remove this plate to access the center cylinder head mounting bolt. Using loose nuts on the back side to secure the spring mounting plate will cause you grief if you ever need to remove the cylinder head. Why? Because you will need to retorque the head bolts multiple times after the head removal. Much easier to remove and replace that spring plate using this method.

8/21/2013 10:26:16 AM

Definitely recheck the isolastic shims after this mod. I remember dong this to my 74 850 MKII or III. My bike was a British "Dock Strike" smuggled in through Ireland to Berliner Motor Corp when we could not get anything else that year. To be honest, I should never have sold my Norton, Dunstall rear sets(touring), Lucas/Dunstall Halogen he light, Clip-ons, German Magura quick levers, second disc brake and the head steady changes. There were many more little changes as I worked for the dealer and we were the West Coast Dunstall Dist. At the 500 mile service, the first week of ownership, the engine Seized at idle. Not only did this require a "Complete teardown" under warranty, but Dunstall parts were returned to their respective proper positions. Turned out the cause of the stoppage was from a piece of metal restricting the oil flow to the rod bearing on the right side. Could ot have asked for a better time to have that failure if it had to happen at all. Freeway speeds in California would have been devastating. The base price of my British Norton (later referred to as Interstate or MKII) was 2100.00. I paid 1900.00 and invested every pay check for the next year in the Motorcycle. Dunstall Touring fairing, Smiths/Dunstall angled instrument bracket that angled the tach and speedo steeper under the fairing. The counter shaft sprocket was 21 instead of the American 19T, with the rear remaining @ 48T. I eventually had to sell the Norton for lack of ability to pay for it. Worst thing about working in the Motorcycle industry was the pay. $1.00 per hour at most So Cal shops in the 60's. A fella could go to work almost any where else and start at 3-5.00 per hour and GM paid $13.56 to start... The tow most regretted losses were the 1974 CZ 250 desert sled and my 1974 Norton 850 Commando MKII or III. Just don't remember, but not electric start...That came out in the '75 model year.

8/17/2013 12:34:50 PM

i installed an Alton electric starter kit on my 1973 Norton 850 Commando and with the updated shock absorber, couldn't be happier,everything is working very well.Thank You, Alton !!!

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