Spark Plug Threading Issue

[Jester808]

Hello everyone

I have a bit of an issue, since I first bought my Legacy Touring Wagon a few months ago really. I noticed when talking to the Seller he was adamant on me not changing the Spark Plugs since he said he had just done it, he mentioned it several times. I found it odd, but wrote it off (I really wanted this Legacy). Well, I went ahead and changed them out since I had to do the Spark Plug Wires anyway, so I figured I would purchase NGK everything and just put new ones in, couldn't hurt (he had used Autolite's instead of NGKs anyway ).

When I changed them out I noticed how untight Cylinder 1 Spark Plug was, and the Boot, they were very loose. Upon closer inspection I noticed the Threads inside were all but stripped. The Spark Plug was hanging on by one incomplete Thread. The Legacy ran great though throughout. Even when I went to the NGKs, I just made it hang on that one Thread as best I could, and it holds up fine under all conditions.

The inherent action of the Engines vibration however had made it work itself on out. Today on my way to work I pulled out in front of traffic, got on it slightly (3k rpms, 8.5lbs. boost), then BOOM! The Spark Plug for Cylinder 1 shot out, dented my hood, and tore the boot to poop. I limped it to work on 3 Cylinders .

I spent my lunch hour going to Autozone, buying a Spark Plug and another Ignition Wire Set, then putting it back together. It runs like it did before, just fine really, but I'm looking for a more permanent solution. What options do I have to repair it? I noticed Autozone carries several different re-threading sleeves in metric sizes. Would those be an option? And then, if so, what's the proper preocedure?

Any help would be greatly appreciated, and sorry for the long story, I just wanted to be thorough

-Alexander

[kimokalihi]

I would never drive it like that. I suppose you could helicoil it. You buy the kit and it comes with a tap and new threads and an insert tool. It tells you what size drill bit to use and you drill the hole out larger (very carefully and straight) and then tap it. Go in a little and then back it out a bit and back in a little further and back it out a bit. Don't just keep going in all the way through. This will ensure it's a clean job and it frees up the metal shavings. Metal shavings is an issue though. I would be worried about getting them into the cylinder and scratching up the walls or doing damage to the valve seats. Ideally you should remove the cylinder head first.

There could be better methods, I'm no expert. Talk to a machine shop and see what they suggest.

[Jester808]

Well that sucks

Thanks Kimo .

[WRX2FFU]

You can helicoil, it's gonna be tough to get to with the motor in the car. I don't think you'll have to pull the head if you vacum out the cylinder when your done.

[bry]

i just recently did this repair on a '90 and it is very easy to do on Gen 1 legacy. previous owner had changed out plugs and somehow managed to strip threads from cylinder 4. BEFORE INSTALLING NEW PLUGS, COAT THREADS WITH ANIT-SIEZE!!!!

the fix is to go to o-reily's and buy the heli-coil set. take in your old plug and match it up. note that you will be using a heli-coil that is shorter than the thread length of your plug. also note that the set i used had drill and tap integrated into one tool....

coat the tap with grease so the cuttings will stick to the tap. tap the thread in at least three increments, wiping/blowing clean the tap and regreasing in between. this means you tap shallow first, then a little deeper, then just short of all the way thru. leave a short ridge at the bottom of the threads in case the insert were to turn farther in (i.e. during spark plug install), it would never extend into the cylinder. also, in between the increments, use a Q-tip dipped in grease to swab around the newly cut threads which will pick up any loose chips.

next, install the coil onto the insertion tool. screw it in until the top of the insert is flush with the top of the plug bore. be careful not to screw it in too far as it can be a pain to unscrew an insert. with the thread insert flush, use the swaging tool to flare the top. OPTIONAL: clean the new threads with alcohol, then apply red loctite to the insert OD before installing. may not help much since the heads get hot and loctite red breaks down at high temps, but it might add a little resistance to keep the insert in place the next time you remove the plug.

finally, grab your "fingers on a cable tool" (does anyone know what i'm talking about?). install a Q-tip into the end of it and smear grease on the cotton swab. poke this arrangement through the spark plug hole and dab it on to any aluminum chips that can be seen. keep doing this until you don't see anymore.

for the final cleaning, use compressed air to blow out the sparkplug hole.

that's it, done. re-install plug with anti-sieze on the threads....

[Jester808]

^ That is an awesome right up, thank you! I'm a Machinist, so I've done helocoil before, shouldn't be too much of a problem, especially being Cylinder 1.

[bry]

appreciate it! i also used to be a machinist back in the day....

[kimokalihi]

I wish I was a machinist. Nice writeup. Picked up some tips from your method.

[Legacy777]

Just be careful to minimize getting metal bits in the cylinder.....