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Point Wear and Timing

September 23, 2010 | Troubleshooting & Repair

By Pat Tobin


Q: Are there better ignition points to use than others for a 356? Should I set point gap before or after setting my dynamic timing?

A: In Vic Skirmants's column in the current Registry (vol23 #5 --ed.), he quotes a friend as reporting that the current (!) replacement Bosch points for our distributors "use a white nylon rubbing block" and that the nylon blocks "tend to expand with use. This not only increases your dwell, but also tends to advance the timing..."

I have a little different take on this. The "normal" (older) rubbing blocks are fiber, and even when running with a light coat of grease, they wear slowly. However, points which are in a conventional (non-electronic) ignition system also "wear;" actually the surfaces are burned and eroded away by the passage of current and the heat and other effects that deterioration which result from that current. So the wear of the rubbing block and that of the point surfaces themselves sort of balance out, and the point gap stays somewhat uniform throughout the life of the points.

If the points "wear" but the rubbing block doesn't, the gap will become greater. I think that rather than the rubbing block swelling with use, it is actually the points which are eroding while the nylon rubbing block doesn't wear. That would cause a wider point gap, which would lessen, not increase, dwell, but the timing would be advanced as stated. On the other hand, If the rubbing block wore but the points
didn't, the point gap would become less. (This often happens with cheap points; rarely with Bosch.)

Both these scenarios are possible. The electronic systems which utilize points (Delta, Permatune etc.) pass only a tiny trickle of current through the points, so there is no erosion and loss of material from the point surfaces. So if points with a fiber rubbing block are used with an electronic ignition system, the rubbing block will wear but the points won't. The point gap will slowly close with ordinary wear.
Dwell will be increased (not a factor, because electronic systems don't care about dwell) and timing will become retarded. On the other hand, if points with a nylon rubbing block are used in a conventional ignitionsystem, the point surfaces will "wear" but the rubbing block will not. With normal use, the point gap will become greater, resulting in less dwell (which a coil in a non-electronic system does care about) and timing will become advanced.

Some years ago Bosch points had nylon rubbing blocks. This was about the time I installed my first electronic ignition system, the Delta. This was a great combination--the nylon rubbing block didn't wear and neither did the point surfaces. I sometimes wondered what would eventually wear out - the point spring? But after a couple of years or so the Bosch points were back to fiber rubbing blocks, a big disappointment to me.

Bottom line: if you are running an electronic ignition system that uses the points, grab a set of the Bosch points with the nylon rubbing block and live happily ever after. If you run the nylon-block points with a conventional ignition, expect to have to re-set the points periodically--the gap will tend to open which advances the timing.

Last but not least--point gap should always be checked before setting timing! The gap affects the timing a lot (because it changes the point on the cam which first contacts the rubbing block and opens the points). As a matter of fact, once the point gap and timing have both been set correctly, there should be no need to adjust the timing again! The only thing that changes timing in normal use is change of point gap. Just re-adjust the gap and the timing will come back to where it was when the gap was originally sent. Of course, in a conventional ignition system, replace the points when the contact surfaces become thin and ratty. This will never happen if the points are switching an electronic unit.