Tach Reduction Box

3 Nov

Recently my tach gearbox failed. The unit was a riveted type (Fig 1) so it is not easy to determine the fault. I drilled the rivets out and discovered major wear in the minor gear shaft housing resulting in the minor gear having a hit and miss engagement with the major gear. I sourced a replacement from Moss USA; this unit was screwed together (Fig 2).

Above: Fig 1 – riveted type. Below: Fig 2 – held together with screws.

Fig 3 – grease not where it should be!

I decided to inspect the gears before fitting the unit to the dynamo. I was very surprised to see the factory greasing had come nowhere near touching the minor gear, as demonstrated in Figure 3.

Moreover, it was apparent that it had not touched the major gear. This is evident in Figure 3 as the base plate was completely clear of grease.

Clearly you cannot rely on factory greasing (or some after-market parts! Ed). I added extra grease directly to the gears plus a little 140 grade gear oil. My aim in adding the oil was to minimize waxing overtime. After about 100 miles this unit failed. Being a screwed unit it was easy to inspect. The inspection revealed that the small gear was loose on its shaft. Moss forwarded a replacement but unfortunately it’s a riveted one. I suggest that purchasers of these unit request screwed units and they inspect the gears for greasing prior to fitting, and the security of the minor gear on its shaft.

John Langley
Lake Hawea, New Zealand.

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2 Responses to “Tach Reduction Box”

  1. Dirk W Dondorp 22. Nov, 2016 at 5:43 pm #

    Not that hard to change the rivetted ones to screws, or blind rivets ! Never seen a srewed one from the sellers.

  2. Peter Jones 29. Dec, 2016 at 12:45 pm #

    ‘Tis always a problem with new bits–you’d probably have done better to re-bush the old one !

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