Rotax Sparkplugs

I recently experienced the same thing.  The crack covered the entire circumference of the ceramic part of the plug.  It caused arcing and a rough running engine during the run-up.  

Tried the Rotax plugs back in 2020. Hate the Rotax plugs. The engine started running crappy after about 25 hours and this flying my motorglider in airplane mode only. (Gliding, lots of low power cold operation with in-flight restarts.) Replaced them with the NGK's which survive 100% leaded fuel and glider mode torture testing for ~150 hours without problems. Bought from a Rotax dealer, they weren't fake, but built like the cheapest possible imitations. Pull the caps off any way but straight (good luck) and you get this: