DP:

There is a reference in one shop manual to the depth of the centering bore on the front of the flywheel.

Flywheels for manual transmissions have a bore depth of 5mm.
Those for auto transmissions have a bore depth of 6.5mm.

That difference of 1.5mm might appear in the crank hub; the hub of the crank for manual trans' could be 1.5mm shorter. What to do? A spacer ring of 1.5mm thickness would put the face of the crank hub in the same place.

[The same situation appears with M116/117 engines; two different hub thicknesses, and a spacer.]