Spark plug only threads in halfway
 

So my dad started to change the plugs on his 87 Carrera the other day and cyl 4 game him trouble. We looked in with the scope and saw that there were nut shells in the cavity and pulled them out with tape on a screwdriver but that didnt seem to help. The plug threaded in a bit with some light difficulty so I used a toothbrush and carb cleaner to clean some gunk off the threads, and now it easily threads in 4 or 5 full turns before getting REALLY stiff. I know it's not in all the way cause the other cylinders turn like 9 or 10 times before being seated.
If I tried I could thread it in further but I really dont want to. the plug is not taking any damage or coming out with metal flakes on it after I give it another try either. Any suggestions? I searched extensively on the forums and no one else seems to have this problem :(
Thanks

Scary " Not really This happens to my ford motors all the time Why I don't know
I just put some oil on them and slowly thread them down. Your going to have to do it
Eventually. Did you see if the old plug threaded right in with no problem ?

This is those times were my 50$ harbor freight lcd scope comes in handy .

Better run a spark plug thread chaser through there before you do real damage! Grease the flutes so any crud coming off the threads will be caught and not fall into the combustion chamber. With the ignition disabled, crank the engine a bit before you install the new plug. On a DME car, I would just crank it from the starter.

The Cap'n

Oh, I know why on a few models. Which 5.4 Triton(s) do you own?

OK, here is the scoop. Something is blocking your plug from ingress (duh).

Your absolute safest way to bore in there is with a tap as per the cap'n. Do some measurements, depth, etc, spend some time figuring crank rotation and piston placement. Get the piston out of the way. Get the right extension for the tap, get the right tap (not a bottom tap) and manual driver to clean out the crud. Nail this before you start.

McMaster Carr will have the hand tools. They seem to have everything.

Grease idea to catch cooties is super. Do this, but note any build up on your plug threads down in there will be easily consumed by your engine.

You can try to drive the plug down in there after warming engine to full temp, first backing off #4 1/2 turn after warm and soaking with PB Blaster and starting fluid (or brake cleaner/carb cleaner). Let it soak for 1/2 hour. This probably will work as crud gets pretty wimpy at close to boiling point. Before attempting to drive the tap down further, warm it up to full op temp again. If you only get two more threads, repeat.

If you try the non-tap method, anything more than a stiff push back on the ratchet, set it down and get the tap. That could be a $4,000 mistake.

My gut feel is that the #4 plug was not screwed in the whole way the last time around (rodent crapola?). Carbon build up on the inside of the (bore) bottom threads through regular service resulting in the "WTF" you are experiencing.

Don't use a tap. The tool you need is a thread chaser as The Cap'n noted above.

If you don't have a thread chaser, you can make one by grinding flutes on an old spark plug. Wire wheel it before you use it.

I am all ears as to what a thread chaser is. I honestly don't know.

Is it tool you can buy?

I can conceptualize a plug with relief ground into it for not galling.

A tap may be too unforgiving, and Cap'n did say thread chaser. I just don't know what that is.

THIS IS WHAT IT LOOKS LIKE:

http://forums.pelicanparts.com/uploa...1402457790.jpg

Not as aggressive as a tap

You can get a thread chaser at any FLAPS. It is like a tap but does not cut new threads but only cleans crud out of existing threads.

Thanks guys. Sorry so long in responding. Down near Charlotte visiting my Dad.

I did call Advance Auto, they had not a clue. So looked up on O'Rielly's website and there they were. Summit Racing as well. I'll bet NAPA has them too.

Had I needed to do this and I had not seen this thread, I would have use a tap. I was a machinist for five years so maybe I would have had a wee bit better chance at not eating away at the head, but why take the risk.

Many thanks.