Bought a $180 borescope last month and it saved me a whole engine pull
Customer brought in a Cherokee with a rough running issue and I was about to start yanking cylinders, but the borescope showed a small exhaust valve burn on cylinder 3 in about 15 minutes. That $180 camera saved me a full day of labor and the customer a lot of money, which felt pretty good for once. What's the one tool you bought that paid for itself faster than you expected?