On off topics I was talking about stripped spark plug threads on a Subaru engine, here's the thread chasers I bought and used.

I've just been using my old regular taps for many years. Most of my taps will be resharpened a few times. It only takes a minute or two to file the cutting edges with the right file. Taps eventually become thread chasers when I get new ones. I've never had any issue with a proper size tap or die removing more material than the original that made the threads. A decent tap and die set will not make threads any larger unless forced. Some of the cheap sets might be built to cut a looser "fit" tolerance. All bets are off when using cheap tools.

I have and have used a spark plug hole thread chaser. Saved my bacon and a head replacement.
I have thread files both std and metric
I recently bought a 5/8 18 bolt thread chaser because there was not enough room to get all the threads with a file. Very handy.

That is a wild looking coil pack. Could not figure what that was at first.
How in the world do you get those in there with only 2" of clearance, and how in the world did the spark plug back out with that on top of it. Weird.
So how did the chaser work? How do you keep little metal filings from getting in the cylinder? And how do you get it started straight and in the right threads? Is it like starting a bolt, you turn it backwards slowly till it sets in the threads, then turn forward slowly and monitor the resistance? Curious minds want to know...
Thanks for the info...

How in the world do you get those in there with only 2" of clearance

You have just enough room to persuade the plug and coil back in.

and how in the world did the spark plug back out with that on top of it?

I've been rack'n my brain ever since this happened, Just don't know, Now that I think about it I did not have to unbolt the coil pack- it was just kind'a bounce'n around in there, I'm not sure if the plug just backed out? or was never tightened ?, It's a conspiracy I know it is, it's got be !
ROFL

And how do you get it started straight and in the right threads?

Ok ,this was easy, what ya got'a do is- Insert the tap/chaser with the extension in the spark plug hole, Then stick your tongue 1" out of the left side of your mouth and bite very lightly, then close your left eye and sight down the spark plug hole and start turning the chaser and "Bingo" perfect threads LOL
My options were kind'a limited, That's pretty much how I did it and everything worked out perfect.

As far as filings ?, I don't think there were any or very little, there was very little resistance with the tap/chaser , I blew out the cylinder with air when I was done, that's about all I could do in this situation .

Later,dave

Thanks for the info Dave. That's where I go wrong. I keep closing my right eye.
I was doing some reading and they say to pack the void in the chaser with heavy grease and it will catch any filings.
Glad to hear you got er done..

HiYa Dave-
Thread chasers are good tools to have. A good set of regular taps on hand are a must as well, both INCH and METRIC sizes. Most, if not ALL spark plug threads are METRIC. One must be cautious to ensure they have the correct thread size and PITCH size or risk boogerin' up the mating part. Our N's have spark plugs have thread size at 14mm x 1.25mm pitch. Some think a 9/16-20 tap will work but that'll just wallow out the plug holes. For an OD thread, a good thread file will work too. Be careful.

Tim Daley(MI)