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ATD fibre gear


Well Known and Active Website User
VOC Member
On the Comet, the fibre gear of the ATD does not line up with the rest of the gear train, it requires to be brought forward over 1/8". I am considering maching a recess into the face of the gear before riveting it to the ATD metal flange. This would bring it nearer into line leaving a rivet length of 1/2". Has anyone considered this, or any other remedy.:confused:


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VOC Member

Hello, not to sure as I have not come across this before. It looks like a good fix, my only thought is that is it the correct ATD.
Regards John.

Hugo Myatt

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VOC Member
It is terrifying adding to any thread with all you real engineers about but at least it stimulates conversation. Anyway nothing ventured nothing gained so here I go again. John, I have a beautiful brand new ATD languishing on the shelf. I tried to fit it first to my Comet and later I tried it on two other Vincents. Even with the nut fully tightened the tapers would not grip. Either it was the wrong taper or the taper was machined too deep. It seems that not all ATDs on offer come up to original specification.

Comet Rider

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VOC Member
ATD fit

Hi Hugo,

Just to add my twopennies worth, another common problem is that the nut which retains the ATD onto your mag, can become threadbound before pulling the 2 tapers together. If you run a countersink inside the nut to releive the first coupole of threads it will overcome this.


Prosper Keating

Well Known and Active Website User
Non-VOC Member
This can be due to a number of reasons. IMO, the most common reason would be years of various owners lapping ATDs or drive pinions to magnetos, wearing everything away. Shortening the retaining bolt as suggested is certainly one solution but before you do so, remove it and check that the tapers fit one another. If you feel any rock with the ATD pushed firmly home, one of the tapers is probably deformed. It is most likely to be the female taper.

In this case, if the disparity is not excessive, you might be able to lap the two tapers to one another using fine valve grinding paste. This should, of course, be done off the machine! It would also be a good idea to disassemble the ATD and magneto so that you are just working with two parts and can obtain a much more accurate result. There is the added advantage of avoiding any grinding paste entering the magneto or remaining within the ATD, as even a tiny amount of paste is not something you need in your engine.

Disassembly of the ATD is quite easy. From the 'inside', the side opposite the bob weights, drift the centre with the female taper through the retaining collar, which is usually punch-locked to the centre. Ensure that the ATD is well-supported and that you use a soft drift, brass being an ideal material. It should be very slightly smaller than the diameter of the centre. It doesn't require much force. If the female taper is beyond easy reclamation for whatever reason, you can always retrieve the centre from the old ATD, which presumably fitted your magneto well. Reassembly is simply a matter of flipping the ATD over and supporting the retaining collar as you drift it all back together again. Afterwards, re-lock the retaining collar by centre-punching it at 120° intervals.

I had to do this with the Comet recently. In my case, the female taper had been damaged because someone had tried to run a rev counter off the ATD, having cut a slot in the head of the retaining bolt. It must have come loose at speed. In fact, the centre was s**** but the ATD was otherwise as-new and now the motor settles smartly down to regular, metronomic tickover.

Prosper Keating
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