New Dvelopement MK3 Sprag not engaging

GRM 450 said:

Hello all,

My '77 destroyed the sprag assembly last weekend.
Who sells a new reliable kit at present?
How far away is the cNw kit?
I need the sprag 06-4733, crank gear 06-4731, and engine sprocket 06-4681.
The sprag is smashed/cracked, both the crank gear and the engine sprocket are dented (as Matt says) so both need replacing.
Should I replace the needle roller bearing and the inner race also? they look ok. (60-4711 and 06-5820)

Regards
Graeme

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cNw said:

Hey all,

While developing the cNw electric start conversion, it was our intention to also see what we could do to possibly better the MkIII starter set up to make it more reliable and better performing. We wanted to see if we could somehow incorporate what we learned from the new starter kit and make it a relatively easy retro fit. In other words, replacing as few parts as possible and not alter any of the major components

The first thing we determined was that the sprag clutch design is in fact pretty good and well built compared to others available. Interestingly enough, this sprag is being used in a number of demanding applications without any issues. It appears that the problems are associated when used in the 1975 electric start system

So the next step was to look closer at the other parts associated with the starter system. Having taken a lot of the MkIII primaries down, one thing that I have certainly noticed is the damage caused by the sprag engagement pins on the front sprocket surface and the crank gear surface. The surfaces become wavy and this will certainly be a hard surface for the sprag to grab onto since it may 'skip' as it's trying to get a bite and fully engage

When I went down to Special Tool Solutions (that are developing the cNw electric start conversion) John told me to bring some parts to do some testing. While there, we used a Rockwell tester and the case hardening on both the front sprocket and the crank gear was at 30-40 HRC (Rockwell C). It was determined that this case hardening was 0.030-0.040 deep. We then cut the part in half and tested them for hardness below the case hardening and found an extremely soft HRC of 10....or less. Almost down to the Rockwell B scale.

What this means is that even though there is a case hardening (and not very hard at that) the surface below it is extremely soft. In other words, it appears to give away when pressure is applied and literally 'dent' the surface. This uneven surface is then really hard for the sprag to get a positive engagement on

It seems that most of those that replace all of the parts associated with the starter system end up with a fully functioning and reliable set up. This for as long as the parts don't get compromised such as the 'denting' described above

When STS started the development of the new starter kit, material quality was on top of the list. It was decided to use an S7 tool steel for both the front sprocket and crank gear. This tool steel is extremely durable and strong measuring a HRC 60-62. While testing the starter system with 1000 plus starts to date, these surfaces are not showing any distortion or wear whatsoever. Is it overkill as far as material ?....maybe, but the bottom line is that it gets us piece of mind knowing that it will hold up over time

So what John at STS and I are thinking about at this time is to finish up the testing on the e-start and when we are confident that the complete kit is just right, move forward with developing an upgrade kit for the original MkIII system

This kit will tentatively consist of a front crank sprocket and crank gear both made out of S7 tool steel. It will also include a new sprag bearing, same design as the original but with 18 engagement pins instead of the 14 that was on the original sprag. So a very straight forward install

Stay tuned

Matt / Colorado Norton Works

http://www.coloradonortonworks.com

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