Sprite Distributor Bearing Modification

by David Vizard

Originally published in the Austin-Healey Magazine, March 1991


This Tech Tip was borrowed from the February issue of "Healey Trails" the newsletter of the North Texas Austin Healey Club. Member Dennis Button excerpted it from "Tuning BL's A-Series Engine" by David Vizard.


Being fed up with replacing distributors, l mentioned to a retired engineer friend. John Blackwell, the possibility of fitting roller bearings into a distributor to cure the problem. John looked into a number of ways of modifying these distributors and decided upon the following as being most feasible.


First of all. it was agreed that the roller bearing would be required in the top bearing only, the one directly under the advance/retard system, as this was the one that took most of the load resulting from the vibrations imposed by opening and closing the points. The bearing used was an Ena or Torrington bearing, type number NA 4901 his is a roller race with a 24 mm outer diameter and a 12 mm bore. The bore is just too small to fit over the shaft of the Lucas distributor, this being 0.04798 inch diameter, so it is necessary to grind the bore of the inner race to fit the distributor shaft. To get everything in the relevant position on the shaft, a spacing washer has to be made up 5/8 inch O.D., 0.480 inch I.D. by 0.070 inch thick. This is fitted prior to the installation of the inner race onto the distributor shaft. The inner race is 'Loctited' in place on the distributor shaft, then the nylon thrust bearing and shim are bored to fit over the inner race. Next the stripped down distributor housing is mounted in a lathe and trued up on the original bore of the bearings. From here on the distributor case is machined as in the diagram. Note that the bore is stepped: this is so that the inner race, which is longer than the outer race, is clear of the distributor housing.


The last operation before assembly is to drill a hole in the boss and tap it quarter Whit (or equivalent U.S. course thread) 0.300 inch deep and 0.25 inch down from the thrust face. Using an 1/8 inch bit, drill a hole from the bottom of the tapped hole into the bearing housing. This drilling allows the fitting of an oil nipple to supply lubricant to the bearing. Assuming that care has been taken in the modification and final assembly, distributor bearing problems should be a thing of the past.


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