For an M8-1.25 helicoil, you need to drill a 21/64" hole where the previous tapped hole was. Note that the drill is not included in either kit. To ensure I didn't drill too deep, I measured the depth of the existing tapped hole and transferred the depth to my drill bit, using a little teflon tape.
Use masking tape if you want, I just had the teflon tape around from doing some plumbing for my compressor.
Use a 6" combination square to ensure you're drilling straight into the old tapped hole (see photo below for alignment of tap). If you've got one of those drills with the bubble on the back to help you drill a horizontal hole, ignore the bubble. The engine is slanted back a few degrees, so a horizontal hole would be a bad idea.
Mark the tap at a point where there are sufficient full threads to allow the coil to be fully inserted.
Mount the tap in a tap handle -- trying to turn a tap with wrenches or pliers is idiocy.
start tap and check using a small combination square that it is perpendicular to the gasket surface (perpendicular in both directions)
once tap is straight and true, turn it clockwise one turn and then counterclockwise one half turn (to break chips). Repeat the back-and-forth until the tape marker is flush with the head surface.
After drilling and after tapping, give the hole a good squirt of compressed air. Cleanliness is next to boostliness!
Note the wadded paper towels jammed in the exhaust ports to keep the chips out of the ports. That orange thing in the right centre of the picture, for the benefit of our SoCal friends, is called a block heater.
