Tach Drive Tech Tip
Richard Finch
Cookeville, TN
Reader Larry Dussack's technical tip about the tach drive reduction box shorting out his TD's electrical system (a problem I too have encountered several times during the 25 years I have driven my TD), reminded me of an even more obscure tach drive problem.
My car used to tear up tach drive reduction boxes with distressing regularity. It took several boxes (all bought from Moss!) to find out what was happening. Any longtime owner of a T-series MG eventually has to have the generator rebuilt, or perhaps replaced the old generator with a rebuilt unit. This is what I had done, and unbeknownst to me, the generator rebuild job included cutting off a few of the threads on the generator end plate where the tach drive collar screws on. Why were the threads cut off? Who knows! Perhaps they were damaged, and it seemed harmless to someone to cut them away. But the result was not harmless...
Instead, with the tach drive box collar screwed down tight, the head of the tach drive was pressed too tightly against the generator shaft. Shaft pressure ultimately tore up the gears in the drive box... time and again.
Solution? Simple, I filed off part of the end of the tach drive head, leaving plenty for the head to engage the slot in the generator shaft, but removing enough so that the shaft did not exert a back pressure on the drive box gears.
An obscure problem, but one that has probably occurred to T-owners other than myself.
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