The MGA and early MGB 3-main bearing engine flywheel bolt is a shoulder bolt. The shoulder is nominal 13/32" diameter, and light press fit in the crankshaft flange. The threaded end is 3/8-24-UNF, and the bolt is hardened for high strength. In 1950's British terms it is called "high tensile". That means a little better than ANSI grade 5, but not as hard or strong as ANSI grade 8. Torque spec is 35-40 Lb-Ft.

[You might notice the good original cylinder head studs have "22" stamped on the top end as a grade identifier. Hang onto those forever, and don't ever change them.]

The shoulder on the bolt is precision ground for exact size. Fit for the shoulder in the crankshaft should be FN1 (Force Interference class-1 fit), which is a light press fit.
The hole should be 0.000/+0.0004 (standard reamer tolerance) 0.40625 / 0.40665
The shoulder should be +0.0005/+0.0008 (always oversize) 0.40675 / 0.40705
Interference is then +0.0001 / +0.0008