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For sale is a Stewart-Warner Tachometer Guage 3500 RPM RPM x 100 551BABJ5. See pictures. I don't see any sign that this was ever installed. The chrome ring appears to have no pitting.  I think it sat on a shelf inside the shop I bought it from from it birthday. The diameter of the insert portion is 3 1/4" and the diameter of the chrome portion is 3 7/16".