u/BluejayBeautiful7247

1991 mazda miata. Yes the third one down is missing its bridge. car started feeling like it wasnt running on all four cylinders and then started making a knocking sound which i originally thought was rod knock, so I parked the car overnight and left it and checked on it this morning, decided to gamble and drive it home and the knocking completely went away. Pulled my wires and spark plugs just to check only to find this. Cant remember exactly how long ago I replaced these plugs but it couldnt have been more than 10k miles ago. How the hell does a spark plug even lose the bridge like that???