Norton Commando - Isolastic Mounting System                         

The interesting thing about the Commando is its frame and engine mounting system; called Isolastic. Giving up the highly acclaimed featherbed frame, a new rubber mounted engine/transmission cradle was designed to greatly reduced the Atlas derived engine's vibration. Because of the mounting system's design, the swing arm was also mounted to the cradle. Otherwise any lateral twisting would miss-align the drive sprockets; resulting in the chain being stressed against them. This was all part of the quick development Norton was engaged in to compete against the Japanese invasion. The Commando's debut in 1968 was a hit despite steering head fractures on 1968 and 1969 models.

Any slop in the mounting system would cause a reduction in handling performance. The earlier models required adjusting the tight clearance between frame and mounts via shims; a tedious process. The later Mark III models had a threaded adjuster. Converting an older model to a newer mount requires machining the front mount down a bit, and replacing the Isolastic innards....all easily done. Remarkably, a Commando with Isolastics works very well for brisk British riding....much better than the 1982 Honda Sabre rubber mount nightmare I wrote about elsewhere.

Photo below shows the guts of an older style front Isolastic mount. Notice the shims.

Here is a Mark III front mount conversion. The front engine mount has been machined to the correct size (0.25" extension on either side of plates); sand blasted for painting. Notice the large threaded adjuster caps along with some washers and rubber boots.

The whole set up lays out as shown below. The cradle holding the separate engine and transmission is held to the frame via the rear Isolastic mount. The back of the engine bolts onto the cradle. The front Isolastic mount holds the front of the engine to the front of the frame. The entire weight of the power/transmission unit rests on these two points. There is a rubber mounted head steady on top, but that just keeps the cylinder head from flopping around.

 

 In addition, the swing arm mounts to the rear of the cradle.  Photo below shows close up of where the swing arm fits on the cradle. More on the swing arm in another section.

Photo below shows comparison of rear standard (bottom) and Mark III Isolastic components (top). Note the threaded tube for the threaded adjusters that replace the spacers and shims on the standard mount.

Pushing out the old rubber is pretty easy if you dislodge them a bit from the tubing wall with a blunt screwdriver. The new rubber will be over 1/8" larger in diameter than the inside diameter of the tube. At first it seems impossible to push it in without damaging the rubber mount.

Its amazing what a bit of lube will do. I applied a thin layer of silicone grease around the rubber ring, and it took no effort at all to work it into the tube. I started at a slight angle, working one edge in first. I would not recommend a water based lube. That will cause rusting inside the sealed mount.

Here you can see how the transmission sits in the middle of the cradle and the rear engine mounts attach to it at the front.

 

Note the nice custom head steady and Mikuni carb.

The most time consuming part of any rebuild is the polishing. I lowered the bike by 1.5" front and rear; note Progressive shock in rear.

Solid Mounting

Here are photos of a solid mounted Norton engine in a flat track race frame by Redline. Photos are from the Ashland, Ohio vintage flat track races in July of 2009. The added rigidity of solid mounting provides great control, especially at the high turning and drifting speeds these guys travel on in the dirt. The proof was in the pudding....Dave Atherton was unbeatable in his class.

Here is Skip talking with Dave....you'd be surprised how many dirt track racers have gray in their hair.