Thanks Guys

The crank comes as a casting:

The first thing is to mount it up in a three jaw and get it mounted parallel - remembering that the circular bosses are actually tapered so we need to check against something parallel:

Then we can face the central boss:

And as expected the little dimple in the centre of the boss was not central

We need to drill deep to make sure our subsequent drilling follows the centre and not that dimple:

Having drilled and reamed it to size it can now be mounted on the hole on an expanding mandrel and the bottom faces both machined flat:

Now we can take the mounting mandrel and casting over to the milling machine, having clamped it square, find the centre of the hole with an edge finder:

Then use a pointer to move up to the correct throw distance then tap the casting to centre it on the little end:

Now it can be drilled and reamed:

We have the holes:

Now we can take the two bosses to distance:

You can see the hole drilled from the rear came out in the centre of the big end boss from this shot while the crank was rotating on the mandrel:

Mounted onto the crankshaft we find:

It hits the side of the hole in the bottom casting

Jo