The first operation on the cylinder blanks is is to drill and tap the six holes for attaching the head. These holes are used by the fixtures for cutting the fins, turning the eccentric band and for the base operations to align the head to the front of the cylinder.
On the mill I use an edge finder to locate the the center of the bore, first by finding the limits on the y axis and halving that dimension and then by finding the limits on the x axis and halving that.
The bolt circle was first center drilled
and then drilled to depth,
and hand tapped.
I put in exactly 10.5 turns of the tap per hole. you can just make out the black reference mark on the left tap handle near the tap barrel. I use that mark to count out the turns to make sure all holes are tapped to required depth.
After tapping, I use a 400 grit stone to remove the burr raised by the tap, and stone the top of the cylinder to eliminate the burrs.
Rinse and repeat the above 20 times over and in no time at all you have the cylinder blanks ready for fining and outside profiling.
