Apparently the original MGA tachometer drive seal was just a fat rubber o-ring, as it only needs to keep oil from crawling up hill out the top of the spindle. The speedometer drive seal is a proper garter spring seal, because it resides below oil level in the gearbox. As so happens, the speedometer drive seal will also work for the tachometer drive. For replacement procedure see GT-106.

Many years ago (mid 80's I suppose) Moss Motors had a cross reference note in the Engine section of their catalog saying the tach drive seal was not available, so use the speedo drive seal in its place. No problem. Now they seem to have improvised some sort of substitute seal for the tach drive with a different part number that is $11.85 as opposed to the nice speedo drive seal that is $4.35. I will continue to use the speedo drive seal in the tach drive.