what causes the white part of spark plugs to crack ??

they've been in for 2k miles and where the right ones for the job according to the local motor factors some sort of champion plug :S

second time this week its borken down first time went through a ford and got water in the ecu after i found the cover was off (waited 2 hours just for the rac to turn up )

just put a really good end to an even better week that