Dirty Threads ?

I pulled spark plugs to do some checking, and had trouble getting a few of them back in. Not cross threaded, as they had made a few turns. I see multiple types of thread chasers on amazon. Any suggestions on the best one ?

 

I don't know what all the styles of thread chasers are, but for ID threads I use this type (not this brand) all the time.

Use oil (don't run them dry) and clean them off after each use, and they last for a long time. I chase the threads of just about any old part that I plan on re-using, and sometimes even new parts, or parts that have been painted. I also don't usually use them with power tools, use them with a hand wrench. And if there's a tough spot, back it in and out a few times until it clears.

If you have compressed air, that's a good way to get the chasers mostly clean after using them, with a narrow outlet air gun. then wipe with a towel to get any remaining junk off.

For spark plug holes, I think I would also blast out the hole with air once you're done, incase any junk fell in.

https://www.summitracing.com/parts/...2OoW6_SiBEKvBBKzBum5H9M6uiZjBK1SxWe7fhSNA5bzc

 

Dirty threads call for thread chases.
If thay don't work get the lube but keep the sensitive bits covered or the rubber on top of the plug won't seal well and you'll have other issues.

Also make sure it's in the right hole.
Might have a different thread in one of the holes if someone else has been in there too rough and needed some tlc with a helicoil. But remember not to shove anything in the hole that doesn't belong in it. You could loose your tool and need to take it in for help.

 

I've had sockets rubbing the header flange that seem to make the plugs hard to turn back in. Be sure that you're using the proper socket and clearance the header flange if that applies. You may already be well past this point. I learned by turning the spark plugs back in with my fingers and making it much easier.

 

I normally spin any threaded item in reverse by hand so the beginning can “find” the thread. I did this in this case as well. One of them stripped bad enough where it will need some work