Theories on this?

Last night, while I was pumping gas at a station near my house, I spotted a guy with his hood up. When I finished pumping, I pulled over to where he was and asked what was wrong with his car. He said he was just trying to put a spark plug in. I asked if it had popped out, and he said that he had. His car was a ~96 neon. He said that no matter how much he turned the plug, it never got tight in the hole (uh-oh). I took a look at the plug, and noticed two things:

1) the threads were chewed off the end of the plug
2) the gap was completely closed

I pointed out the threads and the gap, and asked if he had the old plug. He went digging through the car, and found it. It too had the gap completely closed, but the threads were intact. He also brought a new plug to try. I tried screwing in the plug by hand (using an extension and socket, but no ratchet), and it went about 1/4 of a turn and then stopped. I gave it a little twist with a ratchet, and it just came loose. It didn't grab again.

Since it was midnight and VERY cold out, I suggested that he unplug the fuel injector at that cylinder and drive home on 3cyls.

So anyway, clearly the guy had some problems with the threads in the head. He probably overtorqued or cross-threaded a plug, and messed up the threads. What I'm wondering is what happened to the gaps on those plugs - the gaps from both of the plugs that had been in the engine were completely closed, like someone had hit them with a hammer (only there was no damage that I could see). I have trouble imagining that they got hit by the piston, but I guess it's not impossible. Another wild chance is that some plug that he had in there previously is broken off halfway down the threads, and he was butting the new plugs up against the old one when he put them in.

Any other theories?

This is so weird!

Did you ask him how his car was running before he started changing the plug? I'm inclined to go with the second theory of yours. And one of my friends works in a car repair shop - things he's seen send shivers down my spine - like a guy finning his almost new BMW with motor oil... until oil started flowing out of the filler dap. Then he started it. Rest is history. One more - there was a woman who asked for a bit of oil to add to her car. She got some and left. Then the guys went outside to see waht she was doing. She was still putting the oil in ... through the dipstick tupe :lol2:

Oh, and the point of this all is that one never knows what went on with that engine... so the broken thread of the sparkplug is not so inconceivable.

I used to work as a delivery driver & counterman at an auto parts store - I've seen some beauties myself. Examples:

* guy comes in carrying an AC compressor by a cut hose. He sets it on the counter and says "I need a new alternator."

* woman buys a new lexus, drives it for a few weeks, then decides she'd better check the antifreeze. She unscrews the cap on top of the engine, and doesn't see any antifreeze in there, so she adds it until it's full... Car didn't run very long like that.

* cat in the radiator fan... I'm sure you know how that one goes

I agree with your theory of how the threads in the head got stripped as the most obvious way is cross-threading.
As to the closed gap on the plugs, did you notice if the bonnet (sorry hood) had any dents in it. The idea I'm trying to put across is that when a plug blows, and sometimes they can blow, it can only go as far as the end of the HT lead. If by chance the lead is long enough, or short enough, it can hit the under side of the hood.
During it's short flight the HT lead will be slowing and turning the plug and there is the possibility of the gap end being turned around and colliding with the hood.
Well that's my theory anyway........doe's make you wonder though.

Dunno... that's definitely another possible explanation. I looked, but I didn't see anything suspicious on the hood. It was dark and freezing cold, so I didn't look all that carefully. The head was one of the ones with the plugs at the bottom of a deep hole, so the plug wires had approx. 6" (152 mm) boots on them. The valve cover to hood clearance was almost certainly less than that.

Ivy

I read the email. I know this is going to sound stupid but was the replacement plug or indeed it's predecessor the correct plug for the car. If the previous installer had stuck in the incorrect plug - it's conceivable that the electrode may be extending too far into the combustion chamber which allowed the top of the piston to hit the end of the electrode. (which explained your hammer impact observation)

And the impact force may have damaged the thread or that the installer overtightened the plug (which may have been too long) if it was the incorrect plug which damaged the end of the thread.

Then the owner seeing the previous plug may have purchased a replacement plug based on what he pulled out of the car - which would have repeated the error.

Just a theory to consider

Regards

Rep

yeah, I thought of that too. I didn't pull another plug to compare, but I asked him pointedly whether he'd replaced just that plug, or all of them, and he insisted that he'd replaced all of them. I was skeptical, but didn't want to irritate the guy too much by verifying his work... Until you mentioned it, I'd forgotten all about that part of the conversation.

You would think that if someboby had replaced a normal plug with a long nosed one and the piston had hit it a few times that the driver would realise his engine is only running on three........surely!!!!

Well maybe not.

I beleive there are people out there who just get in and turn the key and thats as technical as they want to get.

Looks like this could be another of lifes little unexplained mysteries.:what: