There are some old threads on this. I have done it right first time out. If you are comfortable with a manual mill, then you can do this if you take the time. I made two simple pieces of tooling to do the alignment.
The only cavat to watch for is that a standard 'off the shelf' tap is not good enough since the tap class is too sloppy. I would suggest a tighter class bought on heres interwebs
, or better yet, going slightly undersize and follow with a roll tap to final size. Cgarr has a post on this when he educated me.
First time around took me about 8 hrs to do six with the very first setup running about 3-4 hours feeling it out how I wanted to do it.
I made a plate spiggoted to the cylinder seat that bolted the head down. Then I made a rod that was threaded for the standard plug (14x1 mm IIRC) and 3/4" on the other end to fit a standard R8 collet. I then 'floated' the head/base plate unit on the rod and adjusted the bridgeport head so that the plate was square and resting on the table. Clamp into postion. Remove rod, zero indicators. Now you need to unbolt the head and rotate 180* and rebolt. Now this is the kicker, the hole will be in the wrong spot since its a true mirror image (not a C2 rotation), so you need to translate the table. I forget the amount. Its in pelican somewhere. I had 27-28* angle on the head after bolting down just for reference. With a 3/4" end mill just touching the top of the head, youll need to plunge 2.4" deep. Then switch to a 7/8" end mill. This cut is 0.35-0.40 off the bottom so figure 2-2.04" deep. The 3/4" cutter will give your final seat for the plug crush washer so youll need to 'eyeball' out that final ~2.4" so that you have a clean, unbroken seat (you are cutting fins).
Speaking of fins, I used canning wax that i beat between the fins to support them so they didnt break while cutting. Used a rubber mallet. Scrapes out later with a probe. I have also seen the fins just machined away so you dont even have to worry about them at all.
Finish with the correct drill bits/taps.
When done use the rod from the back side of your new hole and with a standard vice, clamp at whatever wierd angle it is, then go in with a 5/8" ball end mill to cut the plug pocket within the combustion chamber.
Thats all there is to it. Spend $50 get a junk head and give it a whirl.
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