Ok , share some secret  
 
how glow plug life last  depends how lean you run and what Fuel ( of course Nitro X la). usually will last at least 60 mins plus. I used my up to 5 times play at the track in racing condition. 
 
how to determine a glow plug is in good condition, Use your glow starter, ignite it to see how bright it glow. If it still very bright keep using it.  
For racing, I suggest look at the wire, if it is still shiny, use it. If it get grey colour, use a new one. 
 
good luck |