Wear Evaluation of the Camshaft Follower Bore

Lycoming showing camshaft, cam lobe, cam follower and crankcase bore

Closeup of follower bore in crankcase

One overlooked engine inspection is the camshaft follower bore in the crankcase.  The camshaft follower "tappet" is installed into a bore in the crankcase where it travels in and out while rotating. A worn bore disrupts the proper movement of the follower resulting in premature follower face and camshaft lobe wear. Typical clearance is .001 to .004 inch so the allowable wear tolerance is small. The engine does not have to be taken completely apart to do this inspection, it can be done with a cylinder removed.

For our evaluation we will use wear limits based on evidence of proper or improper function rather than the published tolerance limit. Although, we cannot ignore tolerance limits, the limit of tolerance is established based on the concept that when within tolerance limits, the part is expected to function normally. When the bore is outside of the expected tolerance limits, then the follower is not expected to function properly.^1. If we see evidence of out-of-tolerance then we can measure the bore to quantify our evaluation.


We will examine wear surfaces for evidence of out-of-axis movement. A properly functioning camshaft follower should rotate and travel along a longitudinal (length-wise) axis. 

Normal -- Camshaft follower showing circular non-contact area (black to gray area)

Picture above shows normal circular black line that defines the edge of the lobe contact for a properly rotating camshaft follower. Compare with the follower below where the black area is uneven because the  follower wobbles (out-of-axis movement) around in the bore.

Worn bore -- Camshaft follower showing  uneven non-contact area (black area)

Here the black (non-contact) area is thicker on the right and thinner on the bottom left. The follower is wobbling latterly in the bore due to excessive clearance. The wear occurs in the aluminum bore rather than the follower itself so that an extensive crankcase repair is required to restore proper follower function. Just replacing the follower will not restore proper function.

Continental camshaft follower showing proper rotation on left and no rotation on right.

The Continental followers above show normal rotation on the left and no rotation on the right. Lack of rotation has several possible causes:
1. No tapper on camshaft lobe caused from excessive wear or improper shape
2. Deposit build-up in bore sticking the follower in position
3. Scratches or scored bore that prevents rotation

Video - How to inspect the Lycoming camshaft follower

1. If the follower functions properly outside of the tolerance limits then this might be evidence that the tolerance limit need to be expanded. Excessively tight tolerance are a sign that the design might be fragile whereas wide tolerance limits might indicate a more robust design.