Further, given this (non-regulatory) requirement has been out for 20 years, why do you only borescope if compressions are low?
If compressions have been good, I have NEVER seen anything of concern in the cylinder. Others may have, maybe, but if the doctor uses his stethoscope on a young person and hears nothing of concern, and there are no symptoms, would he then do an ECG anyway?
Continental's borescope recommendation is a butt-covering thing. The manufacturer's have a lot of that, and you can't blame them. They're being sued all the time for stuff that was never their fault. I don't recall Lycoming demanding borescoping.
Moreover, borescopes haven't been around that long. What in the world did the old guys do without them?
Nothing wrong with a borescope, but to suggest that it is superior and replaces the compression test ignores the fact that leakages are often detected long before anything shows up visually. It also finds any cracks in the head casting, and Transport Canada demands that check for on-condition engines in commercially-registered aircraft.
In a previous career many years ago I was the foreman in an air brake component remanufacturing plant. We tested every overhauled component, and used soap and water to find leaks. Nothing that showed a leak was released until it was fixed. We also, on the air compressors, pressure-tested the coolant jackets, and sometimes we'd see the soap bubbling for no apparent good reason. It was usually due to microscopic cracking of the castings, often from some mechanic overtightening the coolant hose fittings. We could not see those cracks. Only the air leaking and making bubbles found them. Visual inspection was useless. Air has 1/800th the density of the coolant, and it finds leaks fast. We didn't have one, but there were ultrasonic leak detectors that needed no soapy mess. They detected the high-frequency sounds made by air escaping under pressure.
In Canada we have an AD that applies to all aircraft using the exhaust system as a source of cabin heat. There have been fatal accidents when the aircraft occupants were disabled by carbon monoxide leaking into the cabin. The AD calls for removal of the heat muff around the pipe or muffler, and a visual inspection using a 10X magnifier to look for cracks or small holes, and a pressure test using low-pressure air to further test suspect areas. I found that doing the pressure/soapy water test found leaking cracks that were undetectable with the magnifier, so I did it on every AD check. Still, there are mechanics who either just take a little peek under the muff, or pencil-whip the AD inspection. A shop across the field from us went to do the check on a Cherokee, and when they took the muff off they found a hole in the muffler you could nearly put your fist into. "No wonder I was getting headaches," the owner said, and that was in the summer. He'd only recently bought the airplane and hadn't even used the heat yet, but there was enough CO getting past the cabin heat valve. When they took the muffler off it fell apart in two pieces. The previous shop got a visit from the TC inspectors.
Incompetence is everywhere, and aircraft maintenance is no different. Mechanics sometimes take shortcuts, and owners sometimes insist that the time not be taken (it's expensive) to do some "stupid" inspection. That's owner incompetence. Sure, there are cheat mechanics out there, but there are way more cheap owners.
