What are some lesser known items or tricks in your profession that most people don’t know about?
For me one thing would be Thrift drain cleaner. I was having issues with my drains being clogged and nothing really worked, but looking around on forums and advice from plumbers lead me to Thrift and now my drains work great.
I don’t think you understand what everyone else is talking about so I’ll tell you instead of piling on.
They’re talking about socket sizes for nuts/bolts. In the US we have fractional inch socket sizes (1/4", 1/2", etc) whereas metric sockets are sized by full millimeters.
So while you are correct in saying you can convert all inch values to millimeter values, you can’t always use a metric wrench on an imperial bolt, or vice versa. For example, a 6mm wrench will be slightly too small for a 1/4" bolt, and a 7mm will be slightly too large.
3/4" and 19mm just happen to be close enough to exactly the same that you can use both wrenches on either size bolt.