By Jason Ballamy 
Part two
M4. C4n

The next job is the main base. On the full size this is a single casting with the water cavities cast in place, the model uses three parts - the base and cylinder jacket which are both supplied as casting and the liner which comes as a length of Dura-Bar continuous cast iron. The base is a bit awkward to hold firmly so after some initial marking out to see what needed to come off where, I mounted it the right way up on the mill with packers and feeler gauges to get it sitting true and stop any distortion when bolted down. I took a very light skim off the oiler hole and edges of the  bearing housings which gave me three nice ‘legs’ to mount it by.

I noted the difference in height when machining the three points so it was a simple matter to arrange suitable packing so the underside could be milled flat and true. At the same setting the four mounting holes were drilled and the ones under the cylinder reverse spot faced with a home made cutter.
The ones at the crankshaft end were easily done in the conventional way.
And a quick test fit on the sub-base. You may also be able to see a vertical line that is cast into the side of the bearing support to indicate where the crankshaft should come and just to the side of that another cast dot also to indicate where the centre should be!
The base was then bolted to a machining plate and mounted at 30deg to allow the bearing housings to be machined to height, I took this as the vertical cast line and my scribed line at the correct height from the bottom of the casting. you can now see the groove where the bearing caps locate.
The last bit of metal under the timing gear stud hole had to be removed with a fly cutter and then the mounting holes drilled & tapped.
The bearing caps were then bolted in place with temporary cap screws and along with the housings machined back to finished width.
And the final photo in this sequence shows how I had to mount the casting. As I don't have a tilting table or mill head my usual pair of angle plates were set at the required angle and some additional clamping added at the lower end, i looks a bit bodged but was surprisingly rigid. The machining plate makes it easy to clock the casting true.
The next job on the base was to machine the flange for the water jacket to butt up against. The casting was not disturbed from the machining plate and transferred to the lathe. I know the exact centre height above the cross slide is 3.389" so it was a simple matter to subtract the casting and plate thickness from that to arrive at what packing was needed. The side of the machining plate was clocked true to the lathe axis ensuring the face to be machined is at right angles to the bearing faces.
The flange was then fly cut to the correct distance from the bearing centreline.