spark plug hole stripped

Two fixes: TIG welding or lots of cutting.

1. TIG welding.

Check out the pics here, they ripped a bolt hole in half here and weld in a glob of metal:

www.artsautomotive.com/CylinderHeadWelding.htm

Welding can be as strong or stronger than original metal.

2. Alternate drill press method:

This is one way threads can be repaired on machinery (usually steel though). The fact that the head is soft aluminum poses a challenge.

Also, the cost of an end mill if you don't do other machining and can't borrow one will make a new heads appealling.

It looks like a lot of steps, but welding has lots of cleaning and grinding involved too...

Main tools:
- big drill press
- drill bits
- a *short* end mill narrow enough to fit where your spark plug socket does. A non-ferrous grinding bit might do the this job, albeit slowly and without the cool chips.
- tap and die wider than spark plug
- spark plug sized tap. You may also prefer to use a thread repair kit if the threads don't look good.

Parts: round aluminum bar stock with similar thermal expansion to the heads, skinny aluminum pin to lock repair threads in place.

Drill out a Medium hole 1.5 to 2 mm larger than the spark plug threads.

Drill a Wide Shallow portion on the same center as the Medium one. This leaves a shoulder to keep the repair metal from screwing down into the cylinder. If you can't make this as wide as the damage without fear of weakening the head excessively, I recommend getting it welded.

Tap the Wide Shallow section.

Make a plug:
- From round bar stock as similar in metal to the head as possible. Art's above suggests 356 cast alloy.

- Medium width at the bottom to mate flush with the inside surface of the head to avoid affecting combustion. Make it a little long at the bottom to mill flush later.

- The rest of the repair plug is the same width as the Wide Shallow portion of the hole and threaded to screw in there.

- The top of the repair plug has two flat sides and sticks out enough above the fins to grab hard with a wrench or vice grips. Keep it short enough for the drill press to clear it with the end mill installed but without losing grip.

- Don't install the repair plug yet.

- Centering a skinny bit the size of your key pin on the edge of the Wide Shallow hole. Carefully clamp the head in the drill vise. You center this first because when you mill / grind the metal all oozes together, and you won't be able to see where to drill for the pin.

- Raise the drill.

- Take the skinny bit out and put the end mill or grinder in.

- Mill / grind straight down until flush. You will end up with a crescent moon sticking up since you are centered on the edge of the repair plug.

- Put the skinny bit in and drill a shallow hole for your key pin.

- Drive the key pin in to lock the threads in place.

- Grind pin and crescent flush.

- Flip head upside down.

- Line up and drill spark plug sized hole in the center of length of excess you have sticking down.

- Grind off excess.

- Mill / otherwise make inside surface identical to other spark plug holes inside cylinders.

- Tap hole for spark plug

The drill press method is mostly here to show another option. I think it might work ok on some older bikes that are built heavier.