If the goal is to mimic a 300 mm International size, 11 13/16 inches is almost perfect (300.04 mm), again with the same 1/64 tolerance. Obviously a conscious decision was made NOT to match 300 mm, although the reason is not apparent to me.
--- On Sun, 4/5/09, Jeremiah MacGregor <[email protected]> wrote: From: Jeremiah MacGregor <[email protected]> Subject: [USMA:44375] RE: Records To: "U.S. Metric Association" <[email protected]> Date: Sunday, April 5, 2009, 9:59 AM The RIAA spec is very interesting as it butts up against an interesting problem in converting millimeter dimensions to fractional inches. I would say that in 1963 when the spec was made (possible from an older spec) that decimal inches were rare and not popular and it was common to express all (or most) inches in fractions. So, how do you convert a rounded metric number to a fractional inch and then come up with a usable fractional size that is in either 16-ths or 32-nds? Anything smaller is not practical. 302 mm converts to 11.89 inches. The nearest fraction is 11.875 mm which is 11-7/8 inches. This however is only 301.625 mm. The reason for the asymmetrical tolerances is to accommodate rounded numbers in both units, the 302 mm in metric and the 11-7/8 in inches. If we add the 1/32 tolerance to 301.625 mm we get 302.42 mm. The average of the two is the 302.02 mm you noted, which for all practical purposes is the 302 mm intended. However, in inches there is no common fraction to equate to 302.02 mm. So the closest common fraction was chosen and the tolerance was made asymmetrical. If the RIAA spec were ever to be updated it could simply drop the 0.02 mm extra you noted and simply make it 302 mm +/- 0.4 mm. Would you agree?
