Starter Clutch Failures

Both of my KZ750 twin starter clutches have failed.  One dramatically, one silently.  They sure do seem to be prone to it…

750 #1:

The starter clutch exploded and seized my engine on my “first” 750 twin first.  I was riding along at about 30mph in town when the engine just rapidly seized and the back wheel locked up.  I got a handful of clutch and rolled to the side of the road.  I pushed the bike home, pissed off.  A week later I got around to checking out why…

It had automatically backed 1 of the 3 mounting allen screws/bolts out (the other two were already loose too), dropped a roller out, jammed against the starter sprocket or something and cracked in half.  I was lucky it wasn’t worse.

Backed Out Screw

This is immediately after removing the assembly from the seized engine. Note the melted screw head at top right. All 3 screws were loose and backing out. The pin and roller had freed themselves and wedged somewhere between the assembly, the starter sprocket and the engine case to cause the engine seizure.

Cracks in Starter Clutch Housing

Now from the other side, I hope you notice the housing cracked in half, the missing metal chip (hole in housing in foreground) I recovered (in background).

I just pulled the rotor, removed all the debris, got a replacement starter clutch from a spare KZ400 engine, removed all the pins, springs and rollers (only kickstart now), and reinstalled the *housing* only with RED Locktite on the 3 screws.  So the final difference from stock is that it’s just missing pins, springs, and rollers.  When you push the starter button, it just goes “Wheeee…”

Since I did this bike’s repair first, I naively thought I needed the housing as a spacer (of course, if I had it to do over, I would have left the housing out).  More on that below…

750 #2:

So, shortly after, I was working on my *other* 750 twin;  because I swore I would pull the rotor to examine the state of the starter clutch before that thing saw the road that season.

This time, after I pulled the rotor off, I was looking at what looked like the backside of a starter clutch, still on the starter sprocket.  Lo and behold, it *was* the starter clutch.  The three bolts that mount the starter clutch *to* the rotor had ALL sheared cleanly, separating it from the rotor.  No shattering, no other damage, just a completely free-floating starter clutch. It looks like one of the three bolts was a little worn like it had been grinding like my other bike, but only a little.  I’m sure this would have caused damage in the future…  So I had to get the little bolt bits out of my rotor…

Three Sheared Starter Clutch Screws

This is where all 3 screws were sheared, and the starter clutch was effectively free-floating in there. How long before one of the screw pieces got free and made it's way toward the crank? Fortunately, we will not find out.

For this bike, I left out the *complete* starter clutch assembly.  This is because (as I mentioned above) I later realized that I do not need the housing as a spacer, but I *do* need the starter sprockets and chain to hold the sprocket steady while running.

IMPORTANT: I got some great advice over on the KTOF about an oil journal that exists on the crankshaft right under the starter sprocket.  Without the starter sprocket in place on the crank, oil pressure might be reduced; by how much, I don’t know.  Since I don’t want *any* reduced oil pressure, I put it in.  And if the sprocket is on the crank, I may as well have the starter chain to hold it steady, and the starter to hold the chain, and the ankle bone’s connected to the….

Conclusion:

Now, all is well.  And I can kick start them forever now.  But do the pics make you cringe?

So is everyone going to go get a gasket (or make your own), a rotor puller (or rear axle), and check them out?

On a related note… If you decide to repair rather than remove, it appears as though Parts N More offers a starter clutch rebuild kit (though I’ve never bought anything from them myself).

Sorry if I sound like an evangelist, folks.  But my engine seizing – even at relatively low speed – might have been my scariest moment in my brief motorcycle history…
😉

UPDATE, 20120602:  In the recent days I have removed the starter sprocket and chain from 750 #1.  I thought I was hearing a noise in there, and decided to take the cover off and inspect the start clutch bolts and such, just-in-case.  Given my past experience, I didn’t want to risk it.  I found that everything was as I left it, even after thousands of miles.  So I re-assembled, leaving the sprocket on the crank in place (to keep pressure against the oil journal in the crank), along with the starter clutch housing and 3 bolts (without pins, springs, and rollers of course).  I would have taken the start clutch housing off too, if I had a working torch on hand; I used red Loctite and the 3 bolts wouldn’t come out… Ironic, don’t you think?  😉

 

64 Comments

  1. Carlos

    I have the 82 CSR. Curiously enough the cam chain tensioner bolt is exactly the right thread size, but I’m hesitant to hit that with a hammer. Probably will just buy an M18 bolt. Last question: how did you torque it back on? That might be where I’ll have to make that tool with the c-clamp

  2. Biquetoast

    @Carlos –
    Ah, gotcha.
    I never did anything fancy to torque it on. Just a torque wrench on the left-hand thread bolt, to spec (whatever that is, I don’t recall).
    The lefty thread never gets loose as far as I know, and the press-fit is all you need on that angled shaft fit for the flywheel (just make sure it’s absolutely clean).
    -‘Toast

  3. Carlos

    Hey Bique,

    I posted what I did to get that flywheel off over at the KTOF forum if you’re interested.

  4. Dan

    Because starter clutch sprockets for a 1980 kz750g, part #21170-003 are impossible to find new I just took mine to a brake shop and had them turn the surface on it with the machine they do rotors with. We got all the imperfections out of it removing very little material. I don’t know if this is going to work but I’m going to find out

  5. Biquetoast

    Dan –
    I actually think that’s a great idea, and have heard in the past pf people fixing it this way. I personally don’t know, since I disabled my starter clutches, but there ya go…
    -‘Toast

  6. Allan j

    My bolts have broken on my twin twice now this time I am going to get longer grade 8 bolts and put locking nuts on the outside of the stater unlike u guys I am too lazy to sit there and kick my bike do u think that this idea will keep the bolts from backing out ?????

  7. Biquetoast

    @Allen –
    Sorry, but I think that’s a very bad idea, and I don’t want you to get hurt. I don’t think that you should be introducing new tiny metal bits that can come off and fling around the engine. Especially if they broke again.
    Anyway, I don’t have a great answer, but if I were tyring to make the thing work (rather than disabling it like I have), I’d probably turn the boss on a lathe, get all new parts, use blue Loctite on the bolts, and check them annually.
    …by the way, my bikes start in one or two kicks. 😉
    Be safe man!

  8. charlie

    do I need a gear puller to pull stator housing on a 77 kz750?

  9. Biquetoast

    @Charlie – Believe it or not, you only need a bolt. The 78/79 can actually use the rear axle as a puller, but the 76/77 must use a narrower bolt if I remember. You just screw it in, tap with a hammer, and “POP” the stator housing comes off. If you can’t find the correct size bolt, Z1Enterprises.com sells the “puller”, which again is just a fancy bolt.

  10. Bruster

    Probably “beating a dead horse” but both of my ’77Bs have had starter clutch failures, both caused by the socket head bolts backing out. Both caused the clutch to lock up, causing the starter to be rotated by the engine continually. You won’t make it very far that way. My solution was to replace the bolts and blue locktite them in place and that has been successful for me so far. This is certainly a troublesome aspect of this bike.

    I have a few other suggestions about these bikes to increase their reliability based on experience, one being that if you are using the original ignition coil you are asking for a breakdown…

  11. Jake

    Hi, thanks for tips and info.

    I got me a situation and need some help, been trying to get the stator off with the axle. In the process I managed to crack two of the magnets in the housing. Is the part damaged beyond use? Maybe some crazy glue?

    Thanks again for your help.

  12. Jake

    Sorry, I got the terms mixed up. I meant trying to take off the Dynamo flywheel and the magnets that are in it.

  13. Biquetoast (Post author)

    @ Jake –
    I have to be honest here… using that part again would be dangerous, maybe even life-threatening. If those ever came apart during operation, it could be catastrophic damage with those chunks o’ metal moving around in there. Do yourself a favor and toss that in the bin and get a new one.
    Just my 2 cents…
    -‘Toast

  14. Petr

    Hello, I read your topic and I have a question.

    The starter clutch of my Kz750 1978 is damaged. The only way how to fix the problem is to make a new starter clutch gear. In order to do so I need to find out the accurate diameter of the part which runs on 3 rollers in the clutch assy. According to my information, the diameter must be very precise.

    Can anyone help? Thank you

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