cylinder 4 middle back is wedged by the o2 sensor, if the socket cocks on the plug, it snaps it.
ive done it before myself on cars with headers that were a tight fit, good old cyl #8 with a/c on a big block chevy= 3 universals and extensions lol and a snapped plug every now and then.
since then i always use spark plug sockets. the rubber inside keeps the plug tight to the socket, and it cant fall out of the socket either.
if you dropped a plug, that could have cracked it too, but then you would have also needed to drop one from the other set of plugs as well maybe?
spark plug socket.
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