Damaged Spark Plug

[4sfed]

This plug was removed from a good running vintage sports car after 3 race weekends. The other three are normal. Any idea what could cause this type of damage?

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[superpursuit]

Check your thread length. Is the thread length the same as the cylinder head thread length? It appears in the photo that it may be too long.

[JoePorting]

Plug is probably too long for application and hit the Piston.

[MadBill]

It looks for all the world like an under/no-torque issue. I have a couple of plugs with missing center electrodes and the insulators softened and leaning off to one side. They are from a stock 1988 Firebird run in a test at Road Atlanta that year. The instrumentation showed knock retard even on 100 octane unleaded. When the mechanic went to check the plugs, these two were only finger-tight. (from the factory)

The heat tulliped the intake valves so we backed off the adjusters and carried on.

I've got pix; will post if I find them...

EDIT: On second glance, I think super is right and Joe mostly so. The last half inch of thread is sooty, suggesting excess length, but it looks melted, not battered. Maybe it was both too long and too loose

Must have been operating under extreme conditions, say a turbocharged LSR Alfa Romeo..

[4sfed]

It's the correct plug for the engine ... I've built dozens of these. I haven't seen the engine myself, only photos from the client. I'm guessing it was loose, possibly arcing to the head? He said it unscrewed easily, that he didn't notice the damage until they were all out.

The engine is in a vintage road race sports car ... 1500 cc, 12.5:1 CR, 8500 RPM, tuned on a dyno. Three weekends would be about 6 hours running time.

[MadBill]

I just now pulled the aforementioned plugs off the "Offerings to the Gods of Speed' shelf and the threads are indeed a bit sooty...

[BigBlockMopar]

The last thread looks a bit deformed and doesn’t seem to line up across the damaged area.
I wanted to say arcing at first, but after seing the threads more closely I tend to think that maybe something went through the cylinder and smacked the plug right next to the groundstrap.

This doesn’t explain the side damage still, if they are indeed the correct length.
Maybe it started arcing after the hit?

[4sfed]

Looking through a borescope, there's no damage to the piston to indicate mechanical damage. I can't see the valve seat, but the damage is right next to the exhaust valve, which shows no signs of distress.

We've installed a new plug and will run it tomorrow ... checking after each session.

[MadBill]

How do the threads in the head look?

[4sfed]

The bottom 3 threads are burned ... 1/2 normal height. At the location of the damage, it's burned to the root.

[Kevin Johnson]

Somewhere I have a nice 50s vintage rock catcher dual trumpet set. They would stop a very large stone but not smaller ones. Your customer would (should) know immediately if he was using one, of course. Maybe the item was metallic and fostered arcing if caught by the strap of the plug.

[cardo0]

Try and duplicate that damage with a oxyacetylene torch. My guess an air leak there acting like a blow torch.

[In-Tech]

My first thought when looking at the picture was the plug was loose.

[John Wallace]

Do a compression test and/or leakdown?

[Newold1]

Could be a pre-ignition result, although its in a strange place as it usually shows up on the valve edges or pistons. Do a quick compression test and see if that cylinder is close to the other cylinders.