Half White/Half Black Spark Plugs

Evening everyone,

I've encountered something strange across all my spark plugs. For context I have a 1973 Gran Torino with the 351C 2V and is completely stock. About 2 months ago I did the first tune up on the car since I got it, and changed the spark plugs. When I took the old ones out I saw half of the electrode and insulator was white and the other half was black. This was present on all 8 plugs.

Today, I decided to do a compression test on the engine (I got 125 on all 8 cylinders with a cold engine). When I took out the relatively new plugs, I saw the same pattern I saw in the old ones, which makes me think there's something wrong, potentially in the heads?

Has anyone has everyone seen this before or have any idea what could be causing this? Thanks everyone!

Here is a picture I took of the plugs:

US 2V's are open chamber so not much shrouding as there would be in a closed chamber, and they don't pattern the plugs like that anyway. completely stock as in never been cracked open ever? i'm leaning toward valve guides and the seals are crumbled out. a little shade on the plugs i'm not worried about but the implications are disastrous. worn guides and stock valves lead to a dropped valve or valve head separation. the worn guide allows the valve to land on the seat out of position exerting awkward pressure where the valve stem and valve head are joined together by friction welding. enough awkward pressure and the friction weld fails and the valve head comes loose in the chamber. all sorts of damage, it can even punch a window through the cylinder wall. when my 1st Cleveland pulled this stunt, the valve stem got loose from the keepers and slid down the guide acting like a punch press making little slugs of piston material. real cool



Edited by Rockatansky - 21-December-2024 at 5:24AM

Since the black/oil stain is only on half or just less than half the plug's insulator, it could point to an intake manifold gasket leak, pulling oil/oil vapor from the valley. This dark side of the insulator facing the intake valve, but there are lots of variables to consider and/or look at.  

That's good to know the oil consumption isn't very bad. With that I'd try to work with it as long as you think the oil pickup might not be clogged. Watch the oil pressure closely, often it will be low or varying to the low side if the pickup is restricted. If you think the oil pressure might be low, try hard to stop running the engine then. It's a big job to R&R the engine to do minor things like the pan/pickup/pump, or the valve seals or springs, but fixing any oiling damage is a full scale rebuild.

I'm late to the party, but in my experience half white/half black plugs is caused by oil burning and the incoming fuel charge "washing" the half of the plug clean that faces the intake valve.

Yeah, I should have elaborated more.