<VV> Spark Plugs Flying

[Brian]

Yesterday, two spark plugs blew out of one of my engines.  A PO had put inserts in the heads but they didn't hold.  Number 5 went first follolwed by number 2.  Got real distance on number 2.  At least ten feet in the air. Looks like my only option is to pull the heads and have them welded up, drilled and taped.  A local Corvair friend said he had the same issue on his Rampside.  Says he replaced with an 18mm plug!?  Said he had it laying around.  No idea of the original use.  I never heard 18mm plugs, at least in the past 50 years.  Anybody?
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