Ever gotten money back from a machine shop?
time is money, and money is time for those in the business of service. More service means more $$$$$. I went to a engine shop that built drag motors, had them install hardened seats and new valve guides in some 906 big block heads. When I got home, I checked what they did. One head was perfect, the other head all 4 exhaust valves let alcohol run right threw. In a dark room, I shined a flashlight through the exhaust port and all 4 valves had a 1/8 moon of light coming through.
It took me 4 hours of lapping at home to get those 4 valves to seal. I didn't say a word, but they've never done anything else for me as far as machining goes.
- I do as much of my own shade tree work that is possible. Never lost a cam or engine, ever. But I don't double check, I triple check because I know how many mistakes I can make from sunrise to sunset :D