The Story of a Norton Commando Racing Motorcycle Rebuild
Part 6
February 2003
Of course I was still not out of the woods with the camshaft. You see, because it's a non-stock cam, it is very important to"degree-in" the cam shaft. This involves attaching a degree wheel to the crankshaft and a micrometer dial gaugue to the pistons. The absolute Top Dead Center (TDC) is found, and then the specifications that come with the camshaft on a printed card are used to set the camshafts precisely. For example, 40 thousands of lift should occur at 36 degrees before TDC.
I experimented with a Vernier camshaft sprocket, which allows you to achieve 1.5 degrees of change, which is much finer than you can get with the standard camshaft sproket driving the timing chain. Fortunately I did not need to use the Vernier camshaft to the the excellent manufacturing tolerances of the fine Megacycle camshaft.
By this time, the engine head had been finished by Rick at Rabers. Once the camshaft was degreed, I needed to check the clearance of the pistons to the valves in the freshened head. The only real way to do this is to put clay on the top of the pistons, assemble the head with pushrods, and turn the engine over a few revolutions. Then you take the head off, and check the thickness of the clay to ensure that there is at least 80 thousands of an inch clearance.
In the above picture you can see where the intake valve indented the clay. I cut the clay with a razor, and then measured the clearance. Vertical clearance was fine, but horizontal clearance between the edge of the valve and the edge of the cutout in the piston were only 0.040 inch. Remeber how the intake valve whacked the piston in the original engine? I can assure you that I didn't want to repeat that engineering blunder, so I got out my trusty dremel tool, and machined the valve cutout valves in the pistons to increase the clearance to a safe distance. What fun!
At this point, I could put the timing side of the engine together, and then start to put the bicycle parts together.