Cylinder Block
The cylinder block forms the water jacket around the cylinder sleeves. It is a block of aluminum with most of the material machined away, then a lot of holes are drilled in addition to the bores for the cylinder sleeves. There are two side covers that mount to the sides of the block sealing the water jacket.
The block is shown in blue in the image below:
It was going to take two hours on the CNC Router to machine out all of the aluminum from the inside of the block, so I decided to use a drill bit and an end mill on the manual mill. I started by drilling 1/4″ pilot holes, then followed up with a 7/16th drill bit to remove the majority of the material. Then an end mill squared up the pocket. Finally I hit the corners on both sides with a 1/16th end mill for the small radii needed there to clear some screw heads.
On the CNC router, I machined as much as I could from the top of the block including the cylinder sleeve bores, then drilled the few holes on the bottom of the block by hand. I made the block with an extra .010″ of material on one side for some reason, and then touched off the bottom holes and the top holes on different sides of the block by accident. So the holes on the bottom are offset to one side by .010″ Fortunately I have not drilled the matching holes in the crankcase and can offset them by the same amount and no harm no foul.
An interesting feature of the Offy block is the taper in the sides. Instead of machining a custom fixture I simply clamped the block in the mill vise with a spacer so I could machine off the required .065″ from the bottom of the block and taper it to 0.0″ at the top. I used an indicator to insure my clamping was correct.
Then I ran an end mill around the edge.
Below the block sits on the crankcase in its intended position:
The block weighs just a wiff of what the squared up work piece did. I still have to drill and tap 64 holes for the 0-80 screws that secure the side covers. I will machine and drill the side covers, then match drill the holes in the block to insure good alignment. The side covers need to be flush with all sides of the block.