Mk3 Sprags, a new design

A little while ago i bought at auction, a box of spares that contained the parts removed from an 850 commando that had had the Alton electric start conversion fitted. In the box of bits was also the earlier Alton parts that the PO had replaced as Alton updated them. One of the parts was the alternator rotor attached to most of the sprag clutch. (this was the small hub version, later replaced by alton with a larger hub design) Looking at the sprag, it is of, a far more robust design (i call it a roller cam sprag) and bigger in diameter, than the Mk3 dog bone sprag. Never hearing of an Alton sprag failing, i started to search for a similar design of sprag to fit the Mk3 sprocket and drive gear.
Unlike the dog bone sprags (cross and morse, ringspann, steiner and all the chinese ones) i couldn't find any size specification charts to see if there are any roller cam sprags made of the size required, so i took the empirical way of buying one and adapting it to fit.
It was my intention to alter the size of the drive gear snout and the engine sprocket bore to suit the purchased sprag, but after due deliberation i dismantled the sprag and reassembled it, in a way that no machining of drive gear or sprocket was necessary.
Assembling the roller cam sprag into engine sprockets and drive gears that i have as spares it seems to work perfectly (these six engine sprockets vary in sprag recess size from 2.283" 2.284" x2 2.286" 2.287" and 2.288") drive gear snouts are mostly 1.625" with a couple i have sleeved to 1.632" All sizes and combinations were tried with the new roller cam sprag and all worked perfectly.
A road test is now the next step as i know what works on the bench might not work in practice.

Keep us posted on your results, looks really good

trident sam said:

Keep us posted on your results, looks really good

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Great work, Peter!

trident sam said:

Keep us posted on your results, looks really good

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Morning Sam
After getting the starter motor reliable the only thing left that lets the Mk3 down is the fragile nature of the dog bone sprag. Ive had a few failures of the sprag in the past. If your lucky, the sprag starts to slip or not engage at all. If your unlucky, the sprags collapses and jams the starter train which leads to starter motor destruction as it cannot withstand the high revolutions that engine will drive it to. The rotor windings are thrown outwards till they grind their way into the field coils (and this is only at 3k engine revs or so)
If you look at a new dog bone sprag, there are three little plate 'T' bars spot welded to the outer cage. These are prone to breaking off. There is a thin wriggly
ribbon spring between the cages that energies-es the individual dog bones, this is prone to bending/breaking.
All of this makes the dog bone type of sprag (in this application) a bit fragile. I have had all the above failures occur with my Mk3, and with Mitzi not have a kickstart she has to start on the button.

Roller cam sprags are used in automatic transmissions. Are you saying you found one that fits with no modification? I hope the road test is positive.

My MKIII has been working perfectly but should it ever quit.........

Yep, been there with an exploding starter armature. It was a fitting end to the two pole Prestolite. Dyno Dave replacement is light years ahead.

dobba99 said:

Morning Sam
After getting the starter motor reliable the only thing left that lets the Mk3 down is the fragile nature of the dog bone sprag. Ive had a few failures of the sprag in the past. If your lucky, the sprag starts to slip or not engage at all. If your unlucky, the sprags collapses and jams the starter train which leads to starter motor destruction as it cannot withstand the high revolutions that engine will drive it to. The rotor windings are thrown outwards till they grind their way into the field coils (and this is only at 3k engine revs or so)
If you look at a new dog bone sprag, there are three little plate 'T' bars spot welded to the outer cage. These are prone to breaking off. There is a thin wriggly
ribbon spring between the cages that energies-es the individual dog bones, this is prone to bending/breaking.
All of this makes the dog bone type of sprag (in this application) a bit fragile. I have had all the above failures occur with my Mk3, and with Mitzi not have a kickstart she has to start on the button.

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Thanks Peter, the starter that you modded for me is still going strong, must be 10/12 years ago now. Good luck with this latest venture.

Looks good, going through the drive sprocket, but looks more difficult to emulate (as a one-off) than Yves' version - unless I'm seeing it wrong

Good luck with the new design, would be great if it's successful. Suspect there are many MK3 owners who would be interested in doing this, please keep us posted.

This is very encouraging for us with EStart machines. Mine has never worked. I replaced the old broken sprag once and the new one failed also. Luckily the bike is a 1-2 kicker. Now all the gears and bits are in a shopping bag somewhere in my garage but I plan to get back to it over the winter next.

JimNH said:

Roller cam sprags are used in automatic transmissions. Are you saying you found one that fits with no modification? I hope the road test is positive.

My MKIII has been working perfectly but should it ever quit.........

Yep, been there with an exploding starter armature. It was a fitting end to the two pole Prestolite. Dyno Dave replacement is light years ahead.

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If the sprag jams it will kiss goodbye to the starter motor. Dyno Dave starters are not immune to this failure either. It will destroy them just as easily as a prestolite.

dobba99 said:

If the sprag jams it will kiss goodbye to the starter motor. Dyno Dave starters are not immune to this failure either. It will destroy them just as easily as a prestolite.

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I didn't mean to imply that a Dyno Dave starter was immune, just that the Prestolite was a POS.

Years ago sprag jammed, kicked it and it freed up. Happened another time and it didn't free up. It was late at night and far from home. I changed everything including a Dyno Dave starter and it's worked fine since.
If it were to lock up now I'd call a tow.

In addition to the DD I upgraded the charging system which has brought its own issues through the years which hopefully is solved with a Shendegen open type regulator. So far, so good.