How to identify it
By crossing three pieces of data: the supplier’s lead time, the criticality of the equipment it is fitted to and how often it is used.
For example, a $4 gasket with an eight-week lead time, fitted to the machine that stops the line, is a critical spare part. A $3,000 motor you can get in 24 hours is not.
The metric that gives it away
Break down your MTTR. If waiting for materials takes up more than half the time, you have a spare parts problem, not a technician problem.
The usual mistake
Sizing stock by value instead of by risk. You end up with a storeroom full of cheap, fast-moving materials and without the part that really stops production.