[Infinite Monkeys]

Hi,
Just wondering if anyone has any personal experience with using a backtap to fix a cross thread spark plug hole. I have cast iron heads, not aluminum. I'm concerned that it may not work as well on iron, still not a big deal, I can still pull the head, but compression on all other cylinders is good, so I'm assuming compression on this is also good, just the plug threads need attention.
Or should I just pull the head and tap it from the inside out with a regular tap?
Guess I don't need to explain what I'm trying to fix. It wasn't me, I bought it this way.

Here's a link to what I'm referring to:

Un-Oops That Cross Threaded Spark Plug Hole | Toolmonger

Here's a youtube video:

(try and ignore the awful music)

Thanks.

 

[Infinite Monkeys]

It's a 2 valve, 1993 FI, 460. I'll look into the timecert. don't know about those.
My guess is that it's just the first few threads. I can't even start the plug. However, the plug was in there pretty tight when I removed it, so it's entirely possible there may not be much left.
I think you are right, pulling the head is the right thing to do.
If I do pull the head, can I then tap from the inside out? I haven't done this before.
Thanks

 

[cobalt327]

It's a 2 valve, 1993 FI, 460. I'll look into the timecert. don't know about those.
My guess is that it's just the first few threads. I can't even start the plug. However, the plug was in there pretty tight when I removed it, so it's entirely possible there may not be much left.
I think you are right, pulling the head is the right thing to do.
If I do pull the head, can I then tap from the inside out? I haven't done this before.
Thanks

Inside out is fine, but I would use a well lubricated thread chaser first to see if it will reform/untweak the threads w/o removing metal. Only if that didn't work would I use a thread cutting tap.

 

[cdminter59]

Backtap for cross threaded spark plugs

Another tool to use is a spark plug thread chaser. Most Auto Parts stores carry them. Craftsman Spark Plug Hole Thread Chaser - Tools - Mechanics & Auto Tools - Automotive Specialty Tools. I think 14mm is your spark plug thread size. Take an air hose and blow all around the spark plug hole to clean the area. Put a little dab of axle grease on the thread chaser to keep metal and dirt out of the cylinder. Use a shop vac to suck anything out of the hole when done.

 

[cobalt327]

If that tool is anything like the Lisle tool I have (they both look identical- like the same photo! Photo below is from a Lisle ad), be sure to dress the ends of the threads where the flutes cut across them (arrows below) if there's any raggedness to them. Hopefully the tool you get doesn't need anything, but do check it first.

 

[Infinite Monkeys]

Can the chaser be used from the outside.

If not I'll pull the head and first try chasing then the cutter from the inside.

Thanks guys.
Much appreciated.

 

[cobalt327]

Can the chaser be used from the outside.

If not I'll pull the head and first try chasing then the cutter from the inside.

Thanks guys.
Much appreciated.

First, the best way to do this w/the head on is w/the internal chaser, as you know.

The cheaper chasers can be used from the outside. The threads of the chaser should be slightly tapered to allow it to start even w/buggered threads. The rubber O-ring is there to hold the chaser in a socket w/o falling out. If you're going to go in from the outside, the tapered threads are important because you generally only get one shot at it- either it'll work or it'll cross thread, and if you use a chaser w/an abrupt start to the threads (see below), starting the tool w/o cross threading is hard. If you're careful you can add the taper yourself.

You might find the double ended chasers are too bulky. I have a long single thread chaser here but I cannot seem to find it online. There are single ended thread chasers (like K-D p/n 3379) that can be used w/a 3/8" extension and ratchet or a 5/8" plug socket. It is shown below.

Good luck.

 

[cobalt327]

The threads in a cast iron head will be trashed if someone is stupid enough to continue to thread the chaser in even though it is taking too much torque. Most guys will have sense enough to stop as soon as they feel any undue resistance. At that point, the other options should be explored.

The internal chaser is used from the inside out, w/the head on the engine.

 

[cobalt327]

Infinite Monkeys, if you end up pulling the head, look closely at the threads at the end (chamber side) of the plug hole. These threads at the very end are often folded over or otherwise in poor shape and might be filled w/carbon, and will need to be cleaned and/or dressed. This is yet another reason to use a tapered chaser/tap.

You can do a reasonably good job of cleaning the threads even on an assembled engine by chucking a brass 12 gauge shotgun bristle brush into a drill. Spin the brush in the hole using carb spray to clean the threads.

 

[Infinite Monkeys]

First, I want to say thank you for all the advise. Because of help from people like you, I have tackled projects I would never have and always succeeded !!!!!
I just bought a single sided, taped chaser, the kind you use a 3/8" rachet on and will be pulling the head to do all the work. I will also replace the valve seals and reseat the valves.
My plan is to first try chasing it, if that does not work, I'll tap it and lastly put in a time cert. I will chase and tap from the inside. To me it's not worth destroying a head for a days work.
Again,
Thank you.