MG MGA Twin Cam Workshop Manual page 118

Driven plates
It is important that neither oil nor grease should contact the clutch facings.
Lubrication of the splines of the driven plate is provided at assembly only, when CS881 graphite
grease or zinc-based Keenol is used.
It is essential to install a complete driven plate assembly when renewal of the friction surfaces is
required. If the facings have worn to such an extent as to warrant renewal, then slight wear will have
taken place on the splines, and also on the torque reaction springs and their seating. The question of
balance and concentricity is also involved. Under no circumstances is it satisfactory to repair or rectify
faults in clutch driven plate centres, and we do not countenance this as manufacturers.
Condition of clutch facings in service
It is natural to assume that a rough surface will give a higher frictional value against slipping than a
polished one, but this is not necessarily correct. A roughened surface consists of small hills and dales,
only the "high spots' of which make contact. As the amount of useful friction for the purpose of taking
up the drive is dependent upon the area in actual contact, it is obvious that a perfectly smooth face is
required to transmit the maximum amount of power for a given surface area.
Since non-metallic facings of the moulded asbestos type have been introduced in service the polished
surface is common, but it must not be confused with the glazed surface which is sometimes
encountered due to conditions to be detailed subsequently. The ideally smooth or polished condition
will therefore provide proper surface contact, but a glazed surface entirely alters the frictional value of
the facing and will result in excessive clutch slip. These two conditions might be simply illustrated by
comparison between a piece of smoothly finished wood and one with a varnished surface; in the
former the contact is made directly by the original material, whereas in the latter instance a film of dry
varnish is interposed between the contact surfaces and actual contact is made by the varnish.
If the clutch has been in use for some little time under satisfactory conditions the surface of the facings
assumes a high polish through which the grain of the material can be seen clearly. This polished
facing is of light colour when in perfect condition.
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