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Allan G, Kev Ev
Total Likes: 2
Original Post (Thread Starter)
#849986 05/25/2021 7:05 PM
by Kev Ev
Kev Ev
Hi all,

I am hoping you can help me clear up why I have a difference in my primary drive crankshaft sprockets.

I know that my bike is a bit of a mix and ever since I have owned it it has had the rotor drive via the 68-0722 timing disc with the peg that fits into the hole in the rotor.

I wasn't aware that the sprockets were different but this has become apparent (and seemingly obvious) after buying a new 68-0205 sprocket from BBB.

This is different to the original sprocket fitted to my bike in that the centre hub is extended at one side by 0.268" from the sides of the sprocket teeth.

The original sprocket is flush with the sprocket teeth on both sides. I believe this is the 68-0544 sprocket and is missing the hub extension to accommodate the rotor drive timing disc.

As I am using a standard battery and digital ignition system I do not need the rotor to be 'timed' and as such I believe it will be ok for me to use the new sprocket without the timing disc and drive the rotor with the standard crankshaft keyway and woodruff key?

Am I right in my assumptions, or am I missing something?

The first of the attached photos show my original sprocket (68-0544?), the new sprocket (68-0205) and the last two are of them both together, showing both sides.

Cheers, Kev E
Attached Images
Liked Replies
#850024 May 26th a 05:52 AM
by quinten
you can pull the dowel from the
disk , ( or maybe flip the dowel to face the sprocket )
and the timing disk becomes a simple rotor-spacer .
... and then the rotor timed with a key in the regular slot .
[Linked Image from]
the various pieces may have started life on different models
but they all had to stack up inside the same space in the standard width primary case .

whats a little surprising is that they make 2 different sprockets ( the ET sprocket can play both ways )
except maybe that the wider sprocket was the "normal one" on hand and generally used
and the fewer thinner ET sprockets were the bespoke exceptions .
1 member likes this
#850285 May 30th a 04:43 PM
by Kev Ev
Kev Ev
Originally Posted by Mark Z
If the timing disc is of the same thickness as the protrusion on the new sprocket, and the rotor with the timing disc was original equipment for that engine, then the sprocket alignment should be correct on substituting the new sprocket. What is the year and model of the A65 in question? With ET ignition, I'd expect it to be a Hornet or a Spitfire.

Hi Mark, sorry about the delayed response. I've been a bit tied up with work stuff of late.

The bike is a 1966 Lightning but when I bought it decades ago I was told it was a Thunderbolt. It had a single carb head on it and there were many things on it that weren't original. It certainly has all the frame, wheels and brakes for a 1966 Lightning and the original engine bottom end was matching with the frame as a 1966 Lightning.

I have since fitted later crankshaft casings, because the chain had gone through the top of the originals at some point. I bought a decent second hand set and sent them off to SRM back in the 80's, along with the original crankshaft assembly etc for the end feed roller bearing conversion.

Ever since it came back from them it's had the crankshaft sprocket and timing disc driving the rotor, although it's never actually needed it.

The rotor keyway in the crankshaft is not in great condition, so I will probably machine the new sprocket to suit the timing disc and utilise it for driving the rotor, rather than take a chance with the dodgy keyway.

I will make sure the ignition timing marks on the rotor are correct with a DTI and timing wheel before I put the head on. This will be to accomodate strobe timing, as I now have a later chaincase on with the inspection cover and timing pointer.

Cheers, Kev E
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