After many years of trying to get the starter working I gave up. New sprockets, sprags , 4 brush motor and heavy duty leads. Stiil slipped after 2 or 3 starts! I've looked at several fixes ken merrick etc and wonder if anyone can recommend a possible fix? Also recommend an engineer who would tackle it. Have all the relevant bearing shells etc. Thanks Roy. Doc says kick it over again and hernias applenty gauranteed!!!
The sprag was never installed correctly in the first instance - no secret, just look up DC sprag clutches, problem now is that there is no real and easy way to remedy it. It seems the Norton engineers of the day went to the pub and forgot to take the sprag instructions with them, or the administrators ditched them.
I found this when someone down under said the same sprag is fitted in automatic gearboxes and never fails, that discounts the sprag itself.
Frustratingly, some have more luck than others, but when the inner chain case changed 17 times during the short production run of the MK3 it would be impossible to find out why they do, there are so many variables. Some are still using the 13 sprag with success.
You should not be struggling to kick a standard MK3 to get it started, unless you have increased the CR with a standard cam, though not sure why anyone would use this route. Out of kicking it and putting it on the centre stand, I would have thought the later would more hernia inducing.
You also have the timing plug on the RH crankcase, use this to set the ignition - the workshop manual is wrong, it is not TDC, but 30 Degrees BTDC, and thus far more safer to kick over.
From what I can gather, some of the mods works well, but seeing how tight a modern sprag is in the correct working envelope, making it any tighter would scare me.
There is a sprag on Aliexpress getting good reviews on Access Norton, working on bikes where the Uk/US version fails. It's Chinese made and cheap but works better, odd but seemingly true.
If all the components including the battery and leads are in good condition, and you refit the starter assembly as per the excellent OldBrits instructions in the link below, all should be ok. https://www.oldbritts.com/e_start_gears.html
I,ve given up on measuring the sprocket and gearwheel, having renewed everything in the past a couple of times but with sprag failure still occurring after only moderate miles. A used gearwheel and engine sprocket will be ok if smooth and in good condition, but replace these if badly scored or notched. Use the recommended 200cc 20w 50 mineral oil in the primary.
You may also want to try one of the Aliexpress Chinese sprags mentioned in John Holmes post, I,m using a smilar one bought from Alibaba about 3 years ago, along with a good used gearwheel and engine sprocket, 4600 miles since fitting in March and still going strong on an e start only Mk3.