Hi Darley, your reasoning seems logical, as I posed the same question about a year ago. The answer I was given(and I can accept) is that the whole assembly is a 'system' which strenghtens the total spindle/bearing assembly. By having the distance piece in there is is like multiplying the area of total load bearing surface. This way they might have been able to do with a smaller diameter spindle. That last statement is alittle weak for me as I don't see why , if spindle was of questionable strength, why not just make a bigger spindle. Only the brits would know! yet assuming that the spindles were not purpose built for just the Healey, maybe they could do with some strength enhancements. Now with the shims, the ideal is to get the proper number and thickness of shims in there to be able to wrench up the castle nut snug, where it makes the whole assembly snug yet does not put a preload on the bearing rollers. In this way you have a snug and smoothly rotating assembly, with no fear of overtighting on the bearing and 'NO' end play at all to cause looseness.
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Dave C.