Engine rebuild

[Terry Synnott]

Am currently reassembling a 1929 OHV engine (no.18110) after re-bore, re-grind, white metalling etc. Having difficulty understanding the logic of the 4 - 6 thou. crankshaft end float often referred to as being set between the front thrust washer and the internal surface of the nose piece to the front main bearing housing and adjusted by shimming between the housing and its nose piece. A related question is whether the front main bearing is supposed to be a ball-race or a roller-race.
On disassembly, I found a ball-race in the housing and since this was badly worn, I simply went ahead and bought a new one without thinking about it too much. On later reflection, I realise that since I also found + 25 pistons then the engine has been apart before, prior my ownership, so it conceivable that the bearing recently removed is not the original one and so finding a ball-race in situ now is not firm proof that a ball race was original equipment; a strong indication perhaps, but not proof.
On starting to reassemble I realised that with a new good quality ball-race firmly in position in the housing then the only end float I could envisage was that arising from the manufacturing tolerances of the bearing which is negligible and so I could not see what the 4 to 6 thou. was all about or indeed what was the role of the thrust washers. I discussed with Ian G. and his first reaction was that the bearing is meant to be a roller=race, in which case the purpose of the thrust washers and the end float and the design intent is clear.
At this stage I groaned quietly, loudly actually since, what with vat and postage, I had paid almost £70 for the ball-race.
I then noted that in the Sports and Vintage catalogue, Mike Downey illustrates and sells a ball-race for this position, in fact he offers 2 qualities but both are ball-races and in discussion with him he is adamant that a ball-race is correct. Mike's explanation for the thrust washers is that once the ball-race starts to wear these limit any movement and he suggest a minimum set up clearance, say 2 to 3 thou. as opposed to the 4 to 6 thou. which is often referred to. The likely principal cause of ball-race wear being the thrust generated by depressing the clutch.
So, to my questions:
i) is it meant to be a ball-race or a roller race?
ii) is it the position that the thrust washers do little until the ball-race starts to wear a bit and then their role is to limit the potential float resulting from this wear?
iii) on the other hand, have I misunderstood the whole thing?
Terry, (currently "confused of Sevenoaks" but hoping to be enlightened)

[Ian Grace]

Terry,

Further to our previous e-mail exchanges, the bearing list for the OHC Minor quotes the front race as being an MJ 1 3/8, not an MRJ 1 3/8, so it appears that the original race was indeed a ball race, not a roller race.