Week 21: 14th May

Back to the wretched Atlas sprag which worked once on Monday morning but required a bump start to get home…

By happy coincidence I was able to pick up a complete Zane replacement on Wednesday. I was initially disappointed to find that it was not a simple case of off with the old and on with the new.

The sprag’s are broadly the same but operate on two very different systems. The Atlas is derived from the 120 triple roller/plunger/spring setup whereas the Zane has the modern one way sprag bearing arrangement.

When I came to fit I found that the ring gear pushed too far back on the crankshaft leaving the top edge of the crankshaft splines on display – the consequence of this is that the ring gear just ‘kisses’ the engine balance gear and also makes it not possible to lock up the sprag against the back of the rotor. To solve this I had a spacer washer made up with some additional shims. I was pleased with my solution which not only shimmed the ring gear correctly but the stepped washer also prevents lateral movement in the needle roller that sits in the sprag.

I thought I had a Zane alternator case but it turns out not so. (it’s my gearbox cover) This then caused my next problem which is that the Zane idle gear is countersunk by 14 mm whereas the Atlas original just 9 mm. To get the Zane part to fit I therefore need to either get the gear or the casing altered. Over next week I am going to try and get the gear altered but apparently cutting through the case hardening will be the main obstical.

On a compare and contrast the gear ratio’s are different::

Atlas idle gear 48/21 teeth whereas the Zane runs 50/19
Starter gear on the Atlas is 84 teeth and the Zane 86

I got round the problem of not being able to fit the Zane idler gear but putting in the Atlas original. I took advice from the micapeak list and Hande responded confirming my worse fears: ‘In your mixed system you are running a gearing ratio of 10/48=4.0 and 86/21=4.1, which works out to a total ratio of 4.0 x 4.1=16.4. This is the tallest ratio you can get with the parts – and the worst in my opinion. This loads the starter motor and battery the most. In percentages compared to the Zane setup it is 27.6% higher/taller gearing, and compared to the Atlas setup 14.6% higher. So it spins the crank about a third faster than the Zane setup or a sixth faster than the Atlas setup – if your battery can take it! And I think it can’t.’

Anyways the good news is that the Atlas now starts pretty well on account that the new sprag grips the crank far better.

Other things on the horizon are the roadworthiness test for the Atlas on the 31st May. I have to get the indicators, horn fixed. Possibly fit a new front tyre and disc pads + check out the headstock bearings which I think are loose…If I can get this done and finally bring the sprag up to Zane standards then maybe I will have the time to get a proper run at the other bikes in the garage!

Nick 🙂

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