The machining of flat surfaces on motor cycle parts is almost always carried out on milling machines.
There are basically two type of milling machines: vertical and horizontal, and the work that can be carried out on each of them is quite specialised.
Both types had 3-axis movement, although the vertical height was normally only used for initial setting-up.
In the example of the crankcase shown here, the two main joint faces would have been machined on a vertical mill using a fly cutter, whilst the mounting lug faces, being semi-hidden, would need to be worked on a horizontal mill, with side and face cutters.
Here's just a few of the different cutter types used on both sorts of machines.
The milling department at AMC was situated on the ground floor, between the press shop and the gearcutters and was equipped with a large quantity of both types of mills, in separate groups.