Originally Posted by jb2wheels
Hey Frisco - that drawing is excellent!! Thank you very much. I downloaded it a few months back when my 40-year-old damper seperated and had the guide made. Finally used it tonight. I have some pictures but it's late and I'm kinda lazay...
My crank is a 60s something (obviously) 327. It was not drilled and tapped but it did have a small pilot hole and countersink.
Glad to see that someone thought to have the guide manufactured. It works well and can be a very useful aid to either add threads to an existing crank that doesn't have them or even to drill and tap to repair stripped out threads that are then repaired with a heli-coil. This is for use when the crank is still installed in the engine block. The one I made the drawing is for a small block Chevrolet crank snout. The drawing is easily modified to conform to any crank that needs to have the end drilled and tapped without complete removal from the engine block.
The obvious correct thing would be to have several of the guides manufactured with various ID holes. One for a smaller pilot drill. One for the tap drill. One for the tap. If repairing stripped out threaded hole, then one for the heli-coil tap drill.
The original countersink you mentioned was done at the factory with what is called a drill/countersink (centerdrill) cutting tool. It is there so that the crank can be supported on a dead center or a live center to machine/grind/polish the crank journals.