The old guy at the shop in Dayton told me my torque wrench technique was backwards, and he was right
I was torquing down some intake bolts last spring and this retired mechanic named Ray who hangs around our shop in Dayton watched me do it and said "son, you're pulling the wrench like you're starting a lawnmower, hold it steady and let it click." I got defensive at first because I've been doing this 11 years, but later that day I looked it up and realized I was gripping way too far down the handle, which throws off the reading on a click type wrench. Now I choke up to the middle of the handle and pull slow, and my comeback on head gaskets has been dead on ever since. Ray still gives me grief about it every time he stops by for coffee, but honestly he earned that right. Did anybody else get corrected on something basic way late in the game, and what was it?
Learned I was setting my spark plug gap with the wrong feeler size after like 8 years of guessing by eye. My dad finally asked me why my gaps looked wide and I just stood there like an idiot. Now I check every single one with the gauge and my old truck idles smooth as glass. Basic stuff, but nobody ever taught me right the first time.