I was driving my matching #s '70 Corvette L46 (350/350) the other day & noticed an intermittent miss at idle through the exhaust on the passenger side. Decided to look into it today. I cranked it up with hood open & could hear a slight tick sound coming from under the distributor cap. I pulled the cap & noticed the #8 contact was sooty. I removed the #8 spark plug to find that it had no center electrode. Otherwise the plug looked good ... ceramic & ground electrode were intact, no damage, color was good. I dissected the plug & found the upper portion of the center electrode with spring on top was there but the entire bottom portion was gone!

My dilemma is this. That nickel electrode had to go somewhere. I do remember when I was driving the other day a single sharp sound that I thought at the time was a pebble striking the windshield. Could that have been the electrode being expelled out the exhaust? ... or even worse being impaled into the head chamber (Trickflow aluminum) or piston (forged TRW)? ... or even more bizarre .... it being coughed out my filterless carb connected to a fresh air chambered L88 hood thats open to the windshield?

The car ran pretty well even with the messed up plug so I don't think any catastrophic events occurred. I just don't know if I want to take any chances with a matching # block.

I see only two choices because I don't have a borescope & don't want to buy one ... #1: replace the plug & see what happens. #2: pull the top end & check the head & cylinder.

Any feedback is welcome. Thanks Mark

 

That's awful, the plug manufacturer should actually be paying for any damage that it could have done. I can understand I personally would probably want to rip it apart and see what happened, but that is a lot of work, you could chance it, if you have the time i mean it might be worth it to take a peak.

 

Over the decades I have had many similar happenings, so if the engine runs decently, and has good enough compression on that cylinder, leave well enuff alone, and obviously replace the plugs, you MAY have a bad production run....

 

I went through this with a Mitsubishi 3000GT. I pulled uip to a stop light and all of the sudden I could tell I had lost a cylindar and I could hear a faint tapping. I took it in and the mechanic called me and let me know that it was the electrode that had come off and was in the cylindar bouncing about. Ended up having to pull the head on that one. If I were you, I would go option 1, put a new plug in and fire it up, chances are the electrode went out the exhaust and is no longer in there if you don't hear it.

 

I have torn apart an engine just to find a bad gauge was the culprit. Heres a crazy idea...if you do hear any faint tapping in there, use one of those cheap auto-zone type retractable magnetic pick-up tools through the sparkplug hole.(I guess itll pick it up?) Should'nt be a problem with your pistons and heads.

 

Turn the engine over with the spark plug removed. It will shoot out of the hole with the compresssion !

 

It either went out the exhaust if your lucky or it is stuck or mashed in the top of your piston.I had a guy drive one for a year before it finally broke the top land off the piston right where the electrode went into the piston-it was clear to see afterwards because it broke off on cranking one morning and stopped the piston dead-he got lucky-bent the rod was all -replaced piston and rod and back on the road--be safe go in and look for it-if yours would fall apart at 6000 rpm you wont be so lucky---just a few hours of labor and you can go back to putting your foot in it and not waiting for the explosion--Good Luck

 

I saw a tool that would be great for a job like this. It was a camera with about a foot long bendable head and a light in the end(I think Xenon). It was about 1/4 or 3/8 inch around. I thought it was a neat tool at the time, but didn't know of a direct application for it. Bingo. I think I saw it in the Skymall catalog on an airplane. Anyone else seen it? I will search for it later tonight. At the time, I didn't think it was too expensive (other than normal, inflated Skymall prices). Hope someone else has seen it and has a better memory than I.

 

Injested a valve cover screw once. Don't ask how. The screw became imbedded into the top of the piston after bouncing around for a while. Chiseled and ground off the high spots then drove on happy till I sold it a few years later.

It is worth doing a tear down IMHO if you do not have access to a bore scope and you do hear a tapping noise.

-Mark.

 

Thanks for all the responses. I'm going to run a compression & leakdown test tomorrow. If everything looks good I'm leaning toward replacing the plugs & firing it up. If I could find someone local that would loan me a borescope for an hour that would be the best.

My main fear, as dwncchs and stingr69 spoke about, is that the electrode or most likely even a small piece of it, is embedded in the piston which won't show on the tests. If that comes loose somewhere down the line, especially at WOT, it's all over.

stingr69 ... the tapping I mentioned was a very faint one coming from the misfire at the #8 distributor contact. I hear no tapping or anything that would indicate a major mechanical issue.

DoodahVette ... the tool you saw was a borescope. The basic ones are $200 to $300 dollars ... not cheap. If I can't find a loaner, I'd rather spend a few hours tearing down & assembling the top end.

Sam74c3 .... I tried a telescoping magnet through the plug hole & didn't come up with anything metallic at all, just a little carbon.

Thanks again ALL! Mark

 

I agree with the fella who suggested a small magnet. Fix it permanently to a 10 or 12 ga. solid copper wire (unless you can find one on a bendable cable). Fish it out through the spark plug hole. Just keep moving it around the perimeter of the piston until you're sure you've swept everywhere it could be. It could be that it imbedded in the piston, but mainly you just want to be sure that it is still not LOOSE in the combustion chamber. If it doesn't show up on the magnet and it still runs OK....leave well enough alone. But you should really try to find it if you can. Good luck!

7T1Vette

 

edit. Didnt read whole problem

 

If I may ask?
How did you relate a ticking in the Distributor with a bad plug?
My dist clicks a bit (Accel pointless module) so I am just curious
Thanks

 

I didn't really. The noise just caught my attention when I opened the hood. The noise had never occurred before so I pulled the cap. The #8 contact was all black & burned looking which obviously led me to the plug. I'm assuming since the distributor noise was new, the cap contact looked trashed AND the plug was bad that the noise was related to a misfire/arc/rotor noise/etc. Once I get this sorted out we'll see if the two are actually related or I'm just out of my mind.

 

I am still wondering how this could represent a bad plug?
This is something I would relly like to learn about...any more input would be great
Thanks

 

Matching # !!! I'd pull the top end..no question. I say this, as I'm # matching as well (72 LT-1) and theres no way I could start that engine, let alone, WOT wth the possibility of ...well you know the scenario Just my 02...Muddy

 

First of all, running the engine again when you know that something is possibly in the cylinder, and hoping for the best is a high risk proposition. Comparing the cost of a teardown after a catatrophic failure, then the rebuild to PAYING someone to boroscope should be obvious. Secondly, the piece that you are trying to retrieve may not be magnetic.
Good luck, but hoping that nothing will happen in the future could lead to a high $$$ end.

 

With new( not stock) heads, pistons, and plugs, you MAY need to check clearances, different plugs have different depths into the cylinder, it is POSSIBLE to have the piston hit the plug electrode, I would have it scoped and recheck plug clearances.........good luck