The anti seize on the spark plug threads cause a ground path issue, hope that clarifies it for you.

 

You got off easier than I did. The first and only time I stripped a spa rk plug hole I sent the heads out to Big boys for time sert and a Street Port. I Also put a Big bore kit in. It was expensive but a fun repair. My spark plug blew out of the cylinder head while I was riding down the road stayed on the plug wire and was sparking . Drove home 10 miles on one cylinder. After I took the spark plug off the plug wire of course. I'll never install another spark plug without anti-seize and a torque wrench.

 

Does it make it better or worse?

 

Worse. The spark plug grounds through the plug shell to the cylinder head. Anti seize can insulate the plug interrupting the ground path. Saw this at the GM Training Center on a car brought there by another dealer tech. Erratic misfire, moving from cylinder to cylinder, scope pattern was crazy. Pulled the plugs,cleaned off the anti-seize, reinstalled, no more misfire, perfect scope pattern. Tech spent a week chasing that issue,which he caused with anti seize.

 

Sounds like the guy put on too much. Were the plugs black oxide coated? Tapered seat or crush washer? None on the porcelain? If using a old sunnen scope, it sounds like a 16KV coil and distributor, not a 40KV like what is used now..

Antisleeze is commonly made out of powdered copper or aluminum and high temp grease. It is typically conductive but molybdenum disulfide is not. Too much and a tapered plug hole may not ground a black oxide coated plug. Crush sealing washers help make contact.

The good thing is that most plugs now days have a conductive zinc or other coating.

 

I've always used a dab of Nickel Neversleaze on my plugs. Just a bit, not frosted like a cake. Has always worked out fine for me on a variety of equipment.

 

As stated erlier I am pro anti-seize for spark plugs. The more the better, just dip as much of the plug in the jar and spin two or three times. Make sure your fingers are completly silver or copper colored. Loctite Stick, they have thread lockers and anti seize, silver or high temp copper, I use the copper. It is like chapstick, very neat to use and you can just put a little bit only on the threads. Just don't over tighten, I don't use a torque wrench for spark plug. (Yes, I was raised by a pack of wolves.) I am not a complete jack wagon, I only drive in the left lane to pass and no katchup on hot dogs.

 

i have experienced this, not with my HD but with my cb72 honda which has a ***** of a sparkplug hole to get at. I worked out what was happening fairly quick and washed it off the plug and it was all good to go....maybe i used too much , who knows?

 

Well, I learned something today. I've never seen or heard of a expandable tap.
Also, I like your idea of checking your threads on new plugs before committing to installing them. Shouldn't have to, but mass produced stuff can be pretty cheesy these days. Actually, not even mass produced. Exhaust systems come to mind.