I was changing my seized on spark plugs when some of the ceramic part shattered off and fell down the hole, I tried to hoover out what I could but just could not tell if I got it all, so I risked starting the car and for 4 days now it's been running fine with no noises, would the damage of happened instantly or could it still happen, I had a mobile mechanic tell me it will be fine now.?

Ceramic is brittle, it would've shattered within the first few strokes, I wouldn't worry about it now.

That's what I thought but I've had a few different opinions on it, I've had a mobile mechanic say that the damage would of happend instantly and should now be fine if the car is running well, but I've also been told it could be sitting in there and at any point cause damage, just about anxious as I've got kids and do long journeys on the M25.

34 minutes ago, TomsFocus said:

Ceramic is brittle, it would've shattered within the first few strokes, I wouldn't worry about it now.

Posted just now

That's what I thought but I've had a few different opinions on it, I've had a mobile mechanic say that the damage would of happend instantly and should now be fine if the car is running well, but I've also been told it could be sitting in there and at any point cause damage, just about anxious as I've got kids and do long journeys on the M25.

If you want complete peace of mind, the only option is to pull the head, or maybe get a small endoscope to plug into your phone.

I really can't see a small piece of ceramic causing sudden catastrophic engine failure though.  If there are large lumps they could be denting the cylinder or valve which could lead to a slight loss of compression on that one cylinder.  Smaller lumps should just get battered into dust and blow out through the exhaust ports.  If it's a turbo engine, the turbine fins may get a bit of damage as it passes, but again won't be a massive issue.

Obviously I can't categorically state that the engine won't blow up tomorrow, but that can happen to any car at any time anyway.