The MGA With An Attitude
CAM WEAR - CM-102 - page 3 of 3

Here the cam has been rotated so the second lobe is up and the last lobe is pointing nearly downward. From this angle you can see that the top of the second lobe is nearly perfectly flat, but the crest of the last lobe (underneath) is definitely rounded off. The rounded profile is a dead giveaway that considerable wear has occurred. It is very difficult to see in these pictures, but the bottom of the last tappet is worn with a concave shape that matches the convex contour of the mating cam lobe. And since the tappet runs a bit off center on the cam lobe, there is also a slightly raised spot in the center of the tappet face. (How good is your imagination?)

Cam & tappets, second lobe up

You can measure the cam lobes with a 2 inch micrometer. Or in a pinch, a dial caliper may do the job. Start by measuring across the widest part of the base of the lobe to get the base circle diameter. Then measure the largest distance from the bottom of the base circle to the apex of the lobe. The difference between these two numbers is the "lift" of the cam. The base circle diameter may not be exactly the same on all cam lobes, especially on a re ground cam, so it is important to measure and calculate the lift dimension for comparison.

If all of the cam lobes are smooth, clean, and flat across the top, and the lift dimensions do not vary by more than a few thousandths of an inch, then the cam is good for continued service. If any cam lobe is worn down as much as 0.010 inch below its neighbors, figure the wear has gotten through to the softer base metal, and the cam is on its last legs, so consider replacing it, so you are not pulling the cam again sooner that you would like. If the tappets are spalling on the bottom ends, consider replacing them. A fresh set of tappets can extend the life of an otherwise serviceable cam.

If you decide to install a new cam, always install a new set of tappets along with it. Never, ever, ever put an old used tappet against a new camshaft. The old tappet can chew its way through a new cam lobe quicker than you can imagine.

If the cam is good, and the tappets all look good, and you decide to reinstall all the same parts, be sure to get the tappets installed back into the same location where they started. Switching a tappet over to a different cam lobe has the same effect as installing a used tappet on a new cam, premature failure of the cam.

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