While normally used for finding and diagnosing issues within human beings, stethoscopes are also excellent tools for seeking out engine troubles. It’s easy to tell when an engine is making weird noises, but discovering exactly where they come from can shave time from the diagnosis and repair process. You can also avoid silly mistakes like replacing your alternator when it’s an idler pulley making that infernal squeal (nice move on my buddy’s part). Examination with a stethoscope can reveal valve train noises or spun bearing locations in toasted motors, and an old engine builder I worked with refused to sign off on an engine assembly until he’d gotten it running on the stand and listened to a laundry list of critical areas with his ‘scope.
Even if the uses are limited, stethoscopes are cheap, and while specialized mechanics’ stethoscopes are available, the standard kind works fine. If you have a community college or university with a medical school in the area, their bookstore will usually have good offerings, but as usual, Amazon has a wide selection and good prices (from $4 to over $100) that almost make a good stethoscope a cheap-ass tool.