The ignition cam which rotates with the
distributor shaft bears on a heel or rubbing block attached to the moving point arm.
The rubbing block must be softer than steel to avoid scoring the metal of the cam and the
only lubrication is the thin smear of grease applied when the points are serviced. Various
synthetic materials can be used for the block but it still wears down over some thousands
of miles, decreasing the points gap at maximum opening. This retards the ignition timing
which saps engine performance. In extreme cases the points may close almost immediately
after opening which quenches the spark before it has a chance to ignite the fuel mixture
properly.
The rubbing block will eventually wear the
ignition cam over high mileages, possibly leading to inconsistent points timing and lift
between cylinders and accelerated wear of the block on new point sets. The sideways
pressure of the rubbing block also encourages wear of the distributor shaft bearings.
Wobbly bearings means a wobbly ignition cam and inconsistent points operation.
At high engine RPM, the moving point arm
accelerates faster under spring pressure when closing and will bounce a little when the
moving point meets the fixed point. This interferes with the flow of primary current in
the coil and reduces the energy available for the next spark. The fitting of "high
performance points" with a stronger spring alleviates the problem but increases the
rubbing block wear and the sideways pressure on the distributor bearings.
The actual time the points spend closed decreases
as engine RPM rises. In a typical four cylinder ignition, the points are closed about 50%
of the time. With a typical coil, the time needed for the coil current to reach its
maximum value (for maximum magnetic flux and maximum spark energy) is around 15
milliseconds. If sparks are needed less than 30 milliseconds apart, the spark energy will
be less than the maximum possible. Sparks spaced 30 milliseconds apart translates to a
spark rate of 33 sparks per second, which is only 990 RPM for a four cylinder engine. That
engine will be barely off idle before the spark energy begins to fade away.
The contact faces of the points are made of a
tungsten alloy to combat the erosion induced by switching several amps thousands of times
a minute. The contacts will wear eventually, usually by transferring metal from one face
to another, leaving a pit on one side and a pip on the other. It is impossible to gap
points in that condition with a feeler gauge. Filing the pip can extend the life of the
assembly but some skill is needed to ensure that the two faces remain square to each
other.
Most coil makers offer a "sports coil"
replacement for their standard coil. These have thicker wire in the primary winding so
that heavier coil current can flow giving higher energy. These coils go some way towards
alleviating the drop off of energy at higher RPM. The trade-off is that the coils run
hotter and the heavier current wears out the points contact material faster.
