As much as I hate answers that start with it depends it really does depend on the quality of your spark plug socket and what is essential is that it has a rubber insert. If that’s the type that you have then there is no problem using a knuckle bar but the damage occurs when the extension between the socket and the knuckle bar is not held absolutely square to the spark plug. Any force that puts any sideload will shatter the ceramic without fail in fact it’s not a bad idea to have a spare spark plug handy as the threads may well be stretched if it’s been overtightened and I would recommend fitting a new plug afterwards.
You might be tempted to put your breaker bar/knuckle bar directly onto the socket but don’t do this it’s much easier to keep everything square and in line if you use an extension on the socket and put your knuckle bar onto that.
Under no circumstances should you ever use any kind of impact and I hope that part was tongue in cheek as that’s a guaranteed way of damaging the ceramic insulator of the spark plug.
No point at this stage going into why it’s been overtightened in the head as unless it’s been filled with water and left for a long time that’s almost certainly what’s happened many people have no idea how little these plugs should be tightened up. They really are finger tight and a slight nip afterwards they don’t come loose, they don’t blow out, that’s just an Internet myth and old wives tales,
Good luck!
BR
Colin
