I thought unobtanium would be shinier

Been chasing my tail for a couple years trying to keep the electric starter on my '79 KZ750 working. After replacing everything but the starter clutch gear, I determined it had to be the culprit. This particular starter clutch gear is unique to the 750 twins, they haven't been made in donkeys years and NOS is pretty much unobtainable. So I tried a couple other used ones, tried having a used one machined (that was a disaster BTW). Finally gave up and decided the bike would just be kick start only.

Then I finally found one NOS on ebay. It was ridiculously expensive. I tried to talk him down, and still ended up paying entirely too much for the steel cog. But I have it. It's mine. And eventually my right leg will go back to being the same size as my left.





One thing you might notice is that the surface of the boss is sort of a matte finish. It doesn't have grooves cut in it like some of the others do (like for the KZ650). It looks like it's been media blasted or something.

So if you're going to attempt to salvage an old one (like I did), don't bother trying to cut grooves into it (like I did). Just have it trued up so it's nice and round with no flat spots. And then have the hardened steel blasted with something that'll produce a matte texture.

I'm excited to get this on the bike and confirm it solves my starter clutch woes. It was an expensive piece and I feel like I've achieved some right of passage, paying way too much for what seemed to be an unobtainable part. I just thought unobtainium would be shinier.

They like all manufacturers want to sell bikes, not parts.
I'm sorry, I don't like crotch rockets or cruisers. I always said, If Harley would put out a transverse inline four cylinder, roller bearing lower end, 5 speed transmission, Babbitt bearing duel overhead cam with direct lifter motorcycle that looked like mine and handled like mine, I buy one. They didn't so I won't.
Steve
I'm not in to plastic. :woohoo:

I bought a very good used starter clutch assembly (including everything) for my 79 KZ750 for about $80 from one of the online bike dismantlers a couple of years back. Didn't need it and sold it to somebody here. I think you can find them online.

One thing you might notice is that the surface of the boss is sort of a matte finish. It doesn't have grooves cut in it like some of the others do (like for the KZ650). It looks like it's been media blasted or something.

So if you're going to attempt to salvage an old one (like I did), don't bother trying to cut grooves into it (like I did).

Mine has some fine grooves in the hub surface but I don't think it needs them as long as the clutch is good it will grab.

To little to late, but this guy makes sprockets. I bought my new ignition system from him, he is very cool and ridiculously smart. He whould be the guy to call if you had to have one made. He also does off set sprockets.
www.7thgeardesigns.com/products/sprockets.html

Interesting surface finish on the starter gear boss for the 750 twin. The fours have concentric machining marks on the boss - exactly the same finish as the exterior of the sprag clutch housing. This maybe means the surface finish is unimportant as long as it has correct OD and is not too smooth?

When they slip, is it wear in the springs, rollers or boss -or a combination of all three?

Making these or reconditioning them is batch work to make it economic, sooner or later someone will have to start making these again I guess.

Tyrell Corp wrote: When they slip, is it wear in the springs, rollers or boss -or a combination of all three?

Certainly any or all of those could cause it to slip. I replaced springs and rollers several times and it kept slipping. The OD on the boss doesn't appear to be the issue either. The difference between the NOS one and my used ones is barely measurable. . I think it's slight irregularities in the boss and the "polishing" of that matte surface over time.

The problem I had with having a gear reconditioned (or made) is that I didn't have any NOS to use as a guide. Didn't know whether the OD on the boss should be much bigger (it shouldn't) or what kind of surface the boss should have.

if I were reconditioning one now id just have it trued and blasted.

It may be a couple weeks before I can install this bit of unobtanium and see if my problem goes away.

I know the feeling. With the cost and it being scarce, it's a shame to bury in in the engine. I don't think they expected these bikes to last like they have. Planned obsolescence became the industry standard and still is. Unfortunately aftermarket died with their improvements leaving us nowhere to go.
I've been watching a NOS alternator rotor this clown had for $300 plus outrageous shipping. It now is $500. Yeah, I've been bidding and buying used ones but they don't last. The one I just got was starting to get loose, I now have three. I told the seller about it, sent him pics and he offered to sell me another one. I thought about filing a not as described but he could block me from buying from him, so I let it go.
At this rate, this one little part will be worth as much if not more than the bike is.
Steve

swest wrote: At this rate, this one little part will be worth as much if not more than the bike is.
Steve

Yeah, I might have gotten off easy. The NOS starter clutch gear was $180. Considering I bought the bike for about $650 it seemed like a ridiculous amount for a little steel cog. But it's like they say, it's worth whatever someone is willing to pay for it.

Ever since the bike became kick only I find I'm choosing to ride it less and less, opting for my Versys more and more. I decided it was worth $180 to change that.

I wish I could have kept the little 550. It would have taken some of the burden of my Z1. Fun too.
Steve

swest wrote: They like all manufacturers want to sell bikes, not parts.
I'm sorry, I don't like crotch rockets or cruisers. I always said, If Harley would put out a transverse inline four cylinder, roller bearing lower end, 5 speed transmission, Babbitt bearing duel overhead cam with direct lifter motorcycle that looked like mine and handled like mine, I buy one. They didn't so I won't.
Steve
I'm not in to plastic. :woohoo:

I got to concur on this.

The difference between the NOS one and my used ones is barely measurable. . I think it's slight irregularities in the boss and the "polishing" of that matte surface over time.


Yes, I have three on the bench now. The one that looks best, really smooth all around was slipping. The slightly notchy feeling ones weren't slipping.

Could modern oil be part of the cause? This was a problem back in the day, my local bike shop used to do through sump starter clutch installs quite regularly.

I wonder if there is a way of bench testing these? As in applying a lot of tourque by hand to see if /when it will slip. Maybe repeat the test with the parts hot out of the oven too? Are these worst cold or hot?

Tyrell Corp wrote: Could modern oil be part of the cause? ...
I wonder if there is a way of bench testing these? ...Are these worst cold or hot?

I've wondered about the oil too. I'm using J-SON rated oil, but I imagine modern oil formulas could be slightly more slippery.

I'm too young to know whether these starter clutches were problems back in the day. But I've heard they're a well known weak point in the design.

Mine was worse cold. But I assumed that was because the bike was just harder to start cold. So that put more strain on the starter clutch.

I'd try roughing that smooth one up and see what happens. I would have done that with mine but it's hardened steel and I didn't have any way to really scratch it up.