The only safe way to do it is pull the head and use a chaser tap from the good end of the threads:
What does it mean to chase threads?
A chaser tap is designed to re-form the threads, as opposed to a cutting tap, which will cut its way through, possibly removing too much thread material. This is especially important when dealing with critical-torque-load threaded holes such as cylinder head bolt holes in an engine block’s deck.
I have done it once before awkward position plug many years ago.
Remember correctly I used a little grease and eased plug in and out until it felt like was aligned, then forced it in.
Yeh, I’m sold that a thread chaser is a better option here (for me) than a tap.
I just doubt that I can get one in Taiwan, and I’m not sure I could tell I was getting one, rather than a tap, unless a side-by-side comparison was possible, which is unlikely.
IF I can get a chaser of any type in Taiwan I’ll do that, though I wont be taking the head off unless I absolutely have to.
If I have to ship-in anyway I’ll probably go for the bottom-up type, probably the Powerbuilt rather than the cheaper Sealey
Getting a chaser tap will be easy. You can also use a bottom tap as the next best option. Take your spark plug for reference. I’d bet Taiwan Luthier can source the right tap for you.
OK, whats the Taiwanese/Hakka/Japlish specific for spark plug hole thread chaser?
I havn’t listed Chinese (the language of Smoke and Mirrors Mandarins) since I think the languages of red-teethed rustic rude mechanicals are probably safer bets in this context.
What kind of machine has this problem? A car or scooter? I looked but didn’t spot the answer to this.
I understand your concern, I was young and clueless at the time (and probably very lucky), just trying to get the job done as cheaply as possible as I didn’t have much money.
My back up plan incase it went south, was to drill and re-tap to and use one of the spark plug repair bolts/kits, I just did a quick google and it looks like they are still available.
Luckily I didn’t need it as I’m not sure how good they are and also how much metal swarf would end up in the motor.
Could happen to either. This is a car (Daihatsu Skywing)
Why do you ask?
Yeh, thread insert kits like Helicoil would probably be the next step, which I’d like to avoid if possible.
Curious because I never saw what exactly you were working on. The compression should be a consideration as well. My low compression aircraft cylinders would blow out “repaired” plug holes very often
Trick for thread insertion (but I suppose it’d be applicable to thread chasing too) -
Fill the cylinder with shaving foam
Then when finished, turn the engine over and skoosh all the foam out, hopefully taking the shavings with it
I suppose that’s why they call it shaving foam
Stolen from here (mentioned but not done, while installing a Helicoil)
www.youtube.com/watch?v=B3irFj2XeCM&t=5s
Actually a bit alarming, because (having apparently done quite a few before) it fails initially, and he has to use what looks like one of the bottom-up chasers AND a stud extractor to get it to stay put.
Doesn’t look very “chabuduo-friendly”
IIRC compressions were in the 150-160 psi range and fairly even, though its quite a while since I checked them.
At least if that happens with a car you’ve got a better chance of getting your plug back, though its probably more likely to have been run over by a truck.
I’ve been in a similar situation, but I just rattle-gunned a cross-threaded new spark plug into the engine and then sold the car to a family member for a 20% premium 
Craig’s List: full service history, last serviced 10 years ago, but only 2,000 miles ago. Odometer not working.
Dunno. M14 seems to be standard, so I was assuming it was that. Spose I’d better try and check,
I’ve got a vernier caliper…er…somewhere, but I don’t have a thread-pitch gauge.
But the plugs are NGK BP5ES, which list at 14mm X 1.25mm thread pitch, confirming my assumption.
The leads on these screw onto the plug so you’re not going to lose them. Good compression reading is 80psi so getting a good hold of the plug in a high compression engine is even more important.
Aircraft cylinders have 2 plugs so they’re 2x the fun.
So all that stuff about “high octane fuel explosions” involving Messerschmits and Spitfires in the comics of my yoof was bollocks?
Another illusion shattered.
I’m pretty sure everything you read in the comics was 100%
Progress.
Hardly. I’ve had those spark plugs for as long as I’ve had the car.
I did buy some shaving foam today though.
Sort-of-interestingly, these guys actually seem to have got better results with the DIY-modded spark plug than with a conventional top-down thread chaser, perhaps because it seems rather lightly modified, with very shallow grooves perhaps in turn producing a shallow cut.
If its good enough for mangling a Mercedes, it could be good enough for screwing up a Skywing