decided since i bought truck last year with 162000 miles on her and now currently have 166000 on her, i would replace the cap and rotor before winter.

i removed the cap and rotor and the whole tip on the rotor was gone.

what would this have caused?..the truck always started up fine.
what is the purpose of the cap and rotor?
i replaced it yesterday.

 

Don't know what would have caused it but does the truck run better?

 

Im not a genious or master mechanic by ANY means but if i didnt know any better id say its for sending the "spark" through your spark plugs....someone can come along any time now and correct me.....