Well a bit more done today, and you will all be saying in the beginning, 'what is that daft old bugger up to now'.
So I may as well explain things a little. Being from an engineering background, you always try to look ahead a little. Well with this post I am certainly doing that. A couple of hours spent making a few fixtures can save many hours over the build of maybe not an engine this small, but anything a bit larger and more complicated, and you will have trouble progressing very far at all without them, so I am taking the opportunity to save myself a little time, and keep things more accurate than I could ever have hoped for if I was just holding the castings in the vice.
First off, I grabbed a bit of my favourite stuff, 12mm thick ali jig plate. Then I machined it square on all sides, and it ended up about 3/4" over the size of the main base casting.
The parallel sticking up the side (there is another one in the opposite direction on the other side) is to help keep things from flopping about and vibrating while being machined, because it is sticking up so far above the vice jaws.
Almost any material can be used for the holding plate, as long as it is sturdy and accurate enough to do the job.

A cutout was put into one side, you will see why a little later.
You will also notice the odd hole here and there. When this stuff comes out of a factory as scrap, it has invariably already been used as a jig plate, you just gotta take what you can get.

So this is the basic holding plate made, all nice and square and flat.
I suppose you can guess what fits into the cutaway now.

So the next job is to find out where to drill the holes in the plate, and this is one of the very few times I will hold a basic casting as large as this in a vice. All because the are usually no straight sides on the casting, so making holding in a vice very unstable. I could have clamped it down to the bed, but that would have meant me removing either my vice or RT, and because I am only doing very light machining on it, I take the chance.

I have already decided that the top faces of these bolt hole bosses are going to be my first start datum points, so what I am doing, by eye, is finding the centre of each of the four bosses and spotting it with a ball nosed cutter.

This shot shows just how 'bent' the casting is, look at what should be a straight face on the side left hand top edge, and the same on the right hand bottom edge. Now you can see why castings can't be treated like normal bits of metal. They can be bent like a bananas at times, and what you should be trying to do, is not to take all the bends out, but get them looking a little more presentable.

So now having the four 'holes' spotted, I can measure up and come to a 'mean' position for the holes. By drilling smaller holes in the holding plate, than the larger ones in the base, it will give a bit of 'fiddle factor later on.

The small holes were precision drilled in the plate, and the larger hold down bolt holes were drilled by eye. On the underside of the casting, where it won't be seen, I put recesses for the screws that will be used to hold it down for initial machining.
Also I made four 1" long upstands that were threaded all the way thru. The upstands were first screwed to the holding plate.

The plate, tapped down onto parallels, had the four upstands all skimmed on the top faces to ensure they were all the same length

The cast base was then screwed downwards onto the four upstands, the screws went into the recesses while the sticky up bit of the casting went thru the cutout in the holding plate.

The whole lot was now gently tapped down onto parallels and everything given a final tighten up.
Then the gnasher came along and took off the hard skin, and the highs and hollows on the base. It took a cut of 0.020" to clean it all up

The fine sweeper then came on the scene, and with a 0.003" cut, took out all the rough machining marks and left behind a nice smooth, perfectly level surface.

I now have a main datum base I can actually work with.

Everything was cleaned down, the casting taken off and the upstands removed. The casting was then bolted, using all the same drilled holes, onto the holding plate.

Once it is mounted back onto the mill, by using a DTI, I can swipe down all the edges and standing up parts, and by gently tapping, I can get the casting into it's optimum position to have any machining done on it.
This will ensure, by using the holding plate as a reference, I will be able to locate all holes and ensure that they are all drilled in the right places and all parallel to each other. Almost an impossible job if you were trying to hold the raw castings by themselves.
This plate will also be reused for the other parts that need machining, so it will not be wasted.
I now hope this goes some way to explaining what the daft old bugger is up to.
John