Spark plug holds torque, but allows blow-by...help!

[vtdave]

I went to change the plugs on my 4.0 for the first time and found that the old #6 plug was cross-threaded by the prior owner. Doh.

I used a thread chaser to "clean" the #6 hole, and now the new spark plug holds the specified torque.

The problem is that the #6 plug hole now allows a small amount of exhaust gas blow-by. I can't even smell it in the engine bay unless I put my nose right by the #6 hole. It creates a faint smell in the cabin at stoplights when the climate control fan is on, which annoys my wife.

Any suggestions? I've already tried re-seating the plug and using a different plug. No change. I don't want to do a heli-coil because the plug actually holds torque just fine, and there's no guarantee the heli-coil will hold long-term.

So far I've read about coating the threads in teflon tape (it melts), anti-seize lube (probably a bad sealant), and RTV (?). Help!

Thanks,
Dave

 

[BlownFiveLiter]

It's probably going to need a Timesert placed into the spark plug hole, to repair the threads. Chasing them was probably the best course of action, but unfortunately, it seems to have been permanently damaged, so this seems to be the next step. I've never personally done a Timesert, so I'll let someone else comment on whether it's something you may want to tackle yourself, or farm out to a shop.

 

[vtdave]

I think TimeSert will be Plan B (just because there's always risk inherent in tapping the head...could cause problems as well as solving them).

I'm going to pop-in an Iridium plug, coat the threads with Deacon 8875, and hope it lasts the life of the car, which is already at 126k mi

Deacon 8875 looks to be an impressive product. It seals up to 1800F and according to tech I spoke with on the phone is impervious to gasoline. They use this stuff on pipe joints in oil refineries in high compression applications. Scroll most of the way down:

Deacon Industries - High Temp Sealants

I'll post an update in a week or two.

Dave

 

[streethorse]

Im thinkin that 8875 stuff will probably make it imposible to ever remove
the spark plug without damage..... Id try a different option.

 

[vtdave]

This is the really cool thing about 8875 (if it performs as advertised). It can resist extreme temperature and pressure, but is designed to release @ 42ft-lbs of twisting force, or so I was told by the guy on the phone. Granted that's higher than the torque spec for the V6 plugs, but it's pretty reasonable. It's a thread "sealer" but not "locker."

I know this repair will either be brilliant or horrible. I guess I'm willing to take the risk because both my Mustang and F150 are paid off, and...well...if I have to get a 2012 5.0, so be it.