18G191B - Bevel pinion and Differential setting gauge
Indespensible in adjusting and setting 'A', 'B', and 'C' type standard axles.

There are two stepped gauge blocks here. The first step block is for setting position of the input pinion relative to the carrier bearings. The distance from the face of the pinion to the centre of the crown wheel axis is 2.395". The OD of the crown wheel carrier bearing is 3.1496" (80-mm). The radius is therefore 3.1496/2 = 1.5748". If this dimension is subtracted from the 2.395" you get .820". This is the hight measurement of the setting tool referred to in the Workshop Manual as gauge block surface B. (The perpendicular measurement from the head of the pinion to the edge of the bore in which the carrier bearings are clamped).

The second step block is for measuring offset of the carrier bearing inner race relative to the outer race. Inner and outer bearing races are same width and accurately ground. But there is some internal clearance in the ball bearing assembly. When you lay the bearing on its side and support the outer race only, the inner race will settle down a bit due to the internal clearances. The dial indicator is used to measure this bearing offset. In the carrier assembly shims must be adjusted to compensate for this offset. The gauge block height at surface B is equal to the design thickness of the bearing race, which is 0.7087" (18mm).

These dimensions are for the Austin B-series rear axle. Gauge block surfaces A and C are similar heights for the A-Series and C-Series rear axles. I do not have those dimensions.