After the second year I got smart & bought a cheap borescope, just to see how it would work.
I now have 3.
Saves a lot of time pulling stff apart that is not economical to repair and the pretty pictures of ugly engines goes down well with the customers.
It also means that I can get away with not charging for quotes ( well guestimates really ).
No one likes paying $ 30 to $ 70 to be told their equipment is not worth fixing.
OTOH a repair shop can not afford to spend all day working out they will not get paid.