Spark Plug Insulators Loose

[skinny z]

I've been wrenching on this same engine for the last 10 years or so. And other similarly built SBCs for decades before that.
The car has been on the shelf for the last couple of years for various reasons although it's primarily because of the weak short block, excessive blow-by and lost compression. It was also built to be a little more tightly wound with freshly rebuilt and ported heads and a bigger cam. That in itself probably accelerated the end of the short as there were simply too many miles accumulated. It developed a high RPM misfire that eventually worked it's way lower in the rev range to the point where I parked it. I've yet to determine the root cause although there are a few contributing factors I'm aware of. One of which is this newly discovered plug issue.

Anyway, that's a bit of a backstory.

I'm prepping to yard the engine out before the snow flies here and when removing the plugs, I found number one would spin between my fingers before I put a wrench to it. Turns out the plug wasn't loose, but the porcelain had come free from the nut. I found two others damaged this way as well. It's plain to see that combustion gases were leaking past as well.

I've changed hundreds of spark plugs in my time. This engine has decent access to all plugs and installation or removal is never compromised with respect to stressing the plug.
What gives? Anyone seen this before? This is genuinely a first for me.

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If I hold that plug by the metal body, I can spin the insulator and tip easily. A closer look shows that the entire insulator will spin where my thinking before was that it cracked radially behind the nut.

[Racer71]

I’ve seen combustion leaks past body on a semi regular basis on production vehicles typically seems that they’re overtorqued upon loosening. Also certain brands seem to have a higher rate of this type of failure. Being that the ones I’ve seen are daily drivers I’ve never looked into it further

[Roundybout]

I got one right out of the box that I noticed right away before installation (NGK). Never had that after removal from a running though.

[skinny z]

I've never come across it period. I've seen insulators snapped off or cracked from impact or tool misalignment but this is new to me.

Somewhat interestingly, my local (Canadian) parts store dropped this line of AC Delco plugs. I ended up ordering a set from Summit. Perhaps there's an issue with the authenticity? I bought them about three years back.

[MadBill]

Lots of counterfeit parts out there and AC has definitely been targeted by the R & D (Rip off and Duplicate) artists...

[Schurkey]

I bought AC-Delco spark plugs for my Trailblazer via an "Amazon Partner" based in Texas.

Got suspicious, sent this photo and some others to GM. Confirmed counterfeit.

Dirtbag in Texas wouldn't respond in writing. Wanted me to telephone him--so there'd be no record of our conversation, I suppose. Eventually Amazon refunded my money (Not the schmuck in Texas.)

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