Yes I have experienced it on an MGB GT in the 70's which I did a lot of tuning work on to stage 3. The plugs were Champion N9Y's and used to split the insulators lengthways.

(That's the insulator inside the plug that shrouds the centre electrode, not the insulator you can see when the plug is in the engine)

Caused by distributor problems over advancing the ignition timing, detonation etc - rebuilt the distributor - different advance curve etc.

Edited by focussed on 11/01/2019 at 21:58

Plugs breaking up is very rare but it can happen - as said above, the ceramic around the centre electrode can crack, or the ground electrode can fall off if the weld fails. Either problem can damage pistons, gouge bores, etc.

The problem is made worse by the increasing use of 10mm thread plugs in modern cars (very common with smaller-displacement, 4-valve-per-cylinder engines): the tightening torque for these is much lower than larger plugs, which means:

1) it's easier to strip the threads in the cylinder head if the person fitting the plug is ham-fisted

2) it's also easier to put excessive stress on the metal body of the spark plug, which in turn can cause the ceramic to crack

The tightening torque with a brand-new 10mm plug (like those in Vx Corsa petrol, or many modern motorbikes) is just 10 to 12nm, which is really not very much at all. Or if you don't have a torque wrench, 180 degrees extra after finger-tight when the plug is brand-new, and just 30 degrees extra after finger-tight when re-using.