The MGA With An Attitude
CAM WEAR - CM-102 - page 1 of 3

Here we have some pictures and explanations of cam wear. This is not a full definitive description of all modes of cam wear or failure, but is intended to help you identify what may be acceptable and what is not when deciding whether to reuse an existing cam and lifter set.

This first picture was taken under fluorescent lighting with no flash and gives a pretty good idea of the texture and color of the parts involved. This is the rear end of the cam from a MGA 1500 engine with over 200,000 miles on it. The tappets which were run on the last two cam lobes are also shown for comparison. More pictures follow for additional comparison.

During this discussion it should be noted that the cam lobe and tappet on the right are fairly badly worn, while the cam lobe and tappet next to it exhibit very little wear. The cam is positioned so you are looking directly across the top of the last cam lobe, and at the bottom of the tappet that runs against it. Notice that the tappet is badly pitted. This pitting comprises about 20% of the surface area while the other 80% of the surface is still intact and smooth. The pitting is typical of an effect called spalling, where small chunks of metal are lost from the surface as opposed to being worn away by friction. This happens when repeated impacting of the cam lobe loosens a particle of metal along the grain lines and causes it to flake off as a result of long term fatigue of the metal. In the early stages this spalling of the tappet is not particularly detrimental to the cam, but as the pits get more numerous and larger, the flat area of the surface gets smaller, and the pressure on the oil film increases. With too much pressure there is metal to metal contact and the wear increases. After the original hard surface of the cam lobe has been worn away, the tappet will be running against the softer base metal of the cam, and much accelerated wear of the cam lobe will ensue.

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