What causes a spark plug to crack?

[farmerbrake]

Title says it all. Had a rough mag, turned out to be a spark plug was cracked.
So what causes it? And how long do plugs usually last in planes?

 

[Henning]

Typically plug insulators crack for two reasons, someone drops them or breaks them with a pick while trying to dislodge a ball of lead, or the engine is experiencing detonation or preignition. I would have to see it to be able to determine which.Typically massive electrodes go between 500 & 1000 hrs, and fine wires go TBO.

 

[hotprops]

some ******* dropped it and did not man up .common now a days

 

[farmerbrake]

some ******* dropped it and did not man up .common now a days

I don't think this is how it happened this time around, but I wasn't there all the time when the mechanic was so I guess it is possible.

Typically plug insulators crack for two reasons, someone drops them or breaks them with a pick while trying to dislodge a ball of lead, or the engine is experiencing detonation or preignition. I would have to see it to be able to determine which.Typically massive electrodes go between 500 & 1000 hrs, and fine wires go TBO.

Refresh my memory. Preignition is when there gets to be a buildup in the cylinder and it becomes a hot spot and ignites the mixture before the spark plug ignites it
Detonation is...?
Either way neither is caused by a too lean mixture right? That's what one of the guys tried to tell me but I didn't quite buy it. Not looking to get into the 'lean debate'

Sent from my SAMSUNG-SM-N910A using Tapatalk

 

[Henning]

I don't think this is how it happened this time around, but I wasn't there all the time when the mechanic was so I guess it is possible.

Refresh my memory. Preignition is when there gets to be a buildup in the cylinder and it becomes a hot spot and ignites the mixture before the spark plug ignites it
Detonation is...?
Either way neither is caused by a too lean mixture right? That's what one of the guys tried to tell me but I didn't quite buy it. Not looking to get into the 'lean debate'

Sent from my SAMSUNG-SM-N910A using Tapatalk

Correct on Preignition, detonation is the same basic thing in the cylinder, however the spark plug is the source of the ignition. Either way what happens is that you see the big max pressure hit in the combustion chamber befor top dead center and basically it's trying to drive the piston backwards and it creates incredible shock waves in the combustion chamber from the over pressure that can literally break chunks out of the heads of your piston.