My Gramps had a unit of measurement when shimming rod bearings. I would say for example, "how much on the Bentley, fifteen hundred or two thousandth?"
He would reply, "Finer than an RCH". the RCH being a red c**t hair, since red is the finest hair and the hair down below is thin........well, you get the picture.

Funny, all my wives and most of my lady friends have been and are redheads....

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