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Mike Muir
Mike Muir
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Kevin E Offline OP
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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

Sprocket Original.JPG Sprocket New.JPG Sprockets 01.JPG Sprockets 02.JPG
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Yes, exactly right. But don't throw out the disc, somebody might need it. You could have re-used the same sprocket, just reverse the disc, if it had been in good condition.

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Q
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back in the day there were 2 different sprockets supplied
Your older sprocket , without the shoulder is a 68-0544 ... fitted originally to ET ignitions
.the new sprocket , with the shoulder ... is a ... 68-0205
I believe the sprocket with the shoulder just mimics
the width of the thinner ET sprocket and ET dowel spacer

there are also two different distance spacers used behind /or before the sprocket
to achieve proper Chain alignment

68-0686.. thinner ... 7.9 ~ 8 mm
68-0689 ... thicker by about 0.48mm

using the rotor broach and key just transfers the crankshaft timing slot
to a timing mark on to the outside of the rotor
it does have a secondary function of helping generally locate the rotor
But i think most of the rotor location is being supplied by the clamping force of the rotor nut

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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.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
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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 klempfsbritishparts.com]
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 .

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Kevin E Offline OP
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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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I would try to reassemble it the way it came apart, in other words, use the timing disc and rotor and ensure the rotor is aligned so that the timing marks are in the right place as you have already suggested.

The original sprocket looks fine to me and they generally last well, but obviously, I can't see any wear from where I'm sitting.

It's important to ensure the primary chain sprockets are aligned to avoid chain wear. Usually, a straight edge is used across both sprockets, it's tricky to get right because the chain wheel wobbles and other factors but needs checking. Any misalignment is adjusted by using shims under the engine sprocket or removing material from the sprocket with a lathe.


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Kevin E Offline OP
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Originally Posted by gunner
I would try to reassemble it the way it came apart, in other words, use the timing disc and rotor and ensure the rotor is aligned so that the timing marks are in the right place as you have already suggested.

The original sprocket looks fine to me and they generally last well, but obviously, I can't see any wear from where I'm sitting.

It's important to ensure the primary chain sprockets are aligned to avoid chain wear. Usually, a straight edge is used across both sprockets, it's tricky to get right because the chain wheel wobbles and other factors but needs checking. Any misalignment is adjusted by using shims under the engine sprocket or removing material from the sprocket with a lathe.

Hi Gunner,

I have decided to use the original sprocket and save the new one for another project. After having a good look at it I agree that it is in perfectly usable condition and it saves me having any worry about the worn keyway.

I have made up a jig to hold the clutch drum in place and used my depth mic to check the distance from the crankcase/chaincase face to both the clutch drum sprocket and the crankshaft sprocket. It's a pretty easy jig to make and is just basically a tube of the correct OD and a thick washer to temporarily hold the drum firmly in place using the mainshaft nut. If you don't have a depth mic you can use a straight edge and feeler gauges to check for any misalignment. I found on mine that the crankshaft sprocket needs to come out, away from the crank assembly, so I will make a shim to go between the spacer and sprocket.

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The 'tit' on the spacer is not for driving the rotor, it's only for lining up the timing.
A yank on this site had a right go at a couple of us who had done similar things
by driving rotors with stuffed keyways and loose centres by putting pegs in them
and driving them from engine sprockets. (although they had lasted for years like it)
Just thought i'd mention it.
The crap design of the alternator is such that if the rotor rotates on it's centre, the
magnets push out and contact the stator. Lucas sixpenny alternators, great eh?
Lining up the primary better than a mm is what is needed. The clutch will wear if not.
Certainly if you cane the old dog.

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Kevin E Offline OP
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Originally Posted by NickL
The 'tit' on the spacer is not for driving the rotor, it's only for lining up the timing.
A yank on this site had a right go at a couple of us who had done similar things
by driving rotors with stuffed keyways and loose centres by putting pegs in them
and driving them from engine sprockets. (although they had lasted for years like it)
Just thought i'd mention it.
The crap design of the alternator is such that if the rotor rotates on it's centre, the
magnets push out and contact the stator. Lucas sixpenny alternators, great eh?
Lining up the primary better than a mm is what is needed. The clutch will wear if not.
Certainly if you cane the old dog.

Hi Nick,

I'm not an expert on this matter but the way I see it is this: -

If the keyway in the crankshaft is in line with the rotor when it is in the correct 'timed' position for the ET ignition system then what is the point of the timing disc and its peg? There are two holes in the rotor. One is marked up for 'early crankshafts' and the other is marked up for the 'late crankshafts'. If the rotor is in the correct position when on the timing disc peg but not in line with the crankshaft keyway then what do you drive it with, if the timing disc peg is not up to the job?

When I had my engine at SRM back in the early 80's for the end feed conversion I supplied them with the complete bottom end and I had also asked for a new alternator stator and rotor to be supplied. SRM called me and asked me if I wanted them to drill the new rotor to use with the original timing disc, as the keyway in the crankshaft was in poor condition. They also suggested that I could have the keyway repaired but that if I wanted to stick with the original timing disc that would do the job and the keyway would not be needed.

I went with the timing disc option and it has been driving the rotor ever since until my recent re-build. It is still in great condition and will be getting put back in the engine.

The timing disc peg seems to me to be a perfectly acceptable method of driving the rotor. Magnets coming loose in the rotor can happen with a normal crankshaft key drive, so I don't see the relevance of that particular subject in regard to the timing disc drive?

Cheers,

Kev E

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Kevin E Offline OP
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Originally Posted by NickL
The 'tit' on the spacer is not for driving the rotor, it's only for lining up the timing.
A yank on this site had a right go at a couple of us who had done similar things
by driving rotors with stuffed keyways and loose centres by putting pegs in them
and driving them from engine sprockets. (although they had lasted for years like it)
Just thought i'd mention it.
The crap design of the alternator is such that if the rotor rotates on it's centre, the
magnets push out and contact the stator. Lucas sixpenny alternators, great eh?
Lining up the primary better than a mm is what is needed. The clutch will wear if not.
Certainly if you cane the old dog.

Hi again Nick,

I also found this in the BSA Factory Workshop Manual, which I think vindicates what I said in the earlier post.

Maybe you could show it to the 'yank' you mention?

Rotor Key_LI.jpg
Last edited by Kevin E; 06/02/21 10:50 am.

Moderated by  Allan G, Jon W. Whitley 

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