It's a long time since I hung up my licences but in those days every aircraft was required to have two-year or three-year C of A overhaul and inspection, depending on schedule. Once put together the aircraft was test flown fully loaded and had to attain its scheduled climb performance, from memory Tiger Moth 580 ft/min, C150 620 ft/min, PA28R-180 970 ft/min. If they failed on climb it was usually due to poor compression due engine wear.
In support of Ancient Geek we had several Lycoming 'fours' running perfectly after 3,000 hours 'on condition' and one IO-360 with over 4000 SMOH. Of course we changed oil regularly, with spectro-analysis every 100 hours and results recorded on graphs (no computers then!) We also differential compression tested every 100 hours, again keeping graphs to build a long-term picture.