Center maib bearing journal original size

[Dave Mann]

What was the original size of the center main bearing journal?

[Chris KC]

Dave, is it the diameter of the bearing housing that you want? Or of the shaft?

[Dave Mann]

Shaft

[Chris KC]

As far as I'm aware, no works drawing survives for the three-bearing shaft.

The two-bearing shaft drawing applied a tolerance 1.312" - 1.312" +1/2 thou on the big end journals, and I suggest that's the best clue available to the dimensioning of the centre main -
i.e. it would most likely have been 1.7500" - 1.7505".

If re-grinding the shaft to an undersize, I would suggest it's more important to consider the shaft size the bearing shells were designed for - 
The Reliant types were designed for a shaft 1.7500" - 1.7505" (n.b. this lends weight to the assumption above, given that early Reliants used Austin parts and designs).
The Mini variants were designed for 1.7505" - 1.7510".
1/2 thou difference isn't much, but may as well get it right.

In any case, whoever grinds the shaft should take into account the shells which will be used and grind accordingly. The relevant data can be found in the bearing manufacturers' catalogues. If they don't have them, a polite call to one of the better-established recon shops (e.g. Ivor Searle) will usually get answers.