From prolonged experience in the manufacture of engines of the overhead camshaft type, we are able to state that the life of the vertical shaft bushes which support the shaft in the cover is almost indefinite and their replacement is never in our experience necessary.
The "Oldham" couplings are, however, subject to some wear, and may after prolonged use require to be replaced to eliminate mechanical clatter. These couplings must be a light push fit in the slots in the vertical shaft, and even the slightest trace of play will give rise to noise of a much greater extent than might be anticipated from such a small degree of slackness.
Should new couplings fail to give that degree of fit required, due to slight wear in the slots, couplings oversize in width on the tongues may be obtained. These will almost certainly need the tongues "stoning" down to fit the slots.
Note that as assembled at works the engine has couplings of different centre thickness (or depth), and if one or both of the compression plates used below the cylinder are discarded in order to raise the compression ratio, the top coupling must be replaced by one similar to that used at the bottom. See also page 46.