I said "about" 42mm cause it is "about" 1 5/8". The op, I believe, just wanted a measurement that would let him know how the mod might look with the tank on it. .725mm would not mean much. Plus it measured slightly below the 1 5/8" mark on my tape measure.(so what does that calculate to?) As far as 13mm to replace 1/2", it makes little difference with wrenches that 1/2" is actually 12.7mm. Typical engineer thinkin, like you said.
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