I've seen these used in motorsport, where you can inspect the piston crowns/combustion chamber etc through the spark plug hole..

Just wondered if anyone has used one and how good a diagnostic tool they are..?

 

Endoscope. They are great if you want to see inside something without taking it apart. great diagnostic tool for some things

Depends if you can get a flexible one, as the ones I've seen may still need you to pull the engine in a subaru as you know, the plugs aren't the most accessible

The other thing is that you need to know what your looking at

 

These things cost a fortune, got a few at work, they are the muts but it would be cheaper to buy a new engine!!!!

Not heard of many car garages who have em' either.