Part two - large bores by Jason Ballamy

Before the bearings housings could be bored I needed the bearing caps so these had their ends cleaned up but left over length and then the mating surface was milled down to size.

They were then flipped over and the stud holes drilled and spotfaced, the wick oiler tube hole drilled, and a large counterbore cut to hold the oil.
It was around this time that I confirmed my earlier suspicions that the whole casting was a bit narrow, 0.100" or thereabouts. There was not enough room inside the crankcase for the crank to turn  let alone the flanges of the bearings. This shows the cross section, with what I actually had marked in red.
A plan of action was decided upon and the caps held in place with caphead screws for now. The casting was set up on the lathe cross slide and a long series 1/4" drill put through first. before opening up to 9/16 in a few steps.

A 1/2" boring bar was then used to take the bore up to just over 18mm.

This allowed me to just squeeze in a between centres bar to take the bores to the final 3/4".

I made use of the side access hole to change the bit to a side cutting one and counter bored the insides to take most of the bearing flanges so I could keep the crank as wide as possible.

The final job was to face back the bearing caps to match the narrow crankcase using a flycutter in the lathe, fit an angled bit and cut a decent chamfer - probably larger than needed looking at it now. I also marked the caps with punch marks.

The cylinder liner had been roughed out a bit earlier and allowed to settle. The bar in the end is just incase it came loose as there was a lot sticking out the chuck and the bore was too large for my centre.
It was then finish bored and turned, you may be able to see a couple of changes in the pattern on the outside this is because the top nearest the chuck is a couple of thou larger than the bottom and the area between the two smaller still. This allows it to go most of the way into the crankcase before final pressing in with some Loctite for good measure.
Next time - a large crankshaft from solid.

Part 1  part 2  part 3 Part 4  Part 5  Part 6  Part 7  Part 8  Part 9  Part 10  Part 11