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Discussion Starter #1
Hey all,

So today when commuting home on my 1983 CM450A I got about a block from the office when there was a nasty crunch sound, I lost all power, and I felt something grinding somewhere along the bike's drivetrain.
I pulled over immediately and found that the chain looked like it was off the front sprocket. I thought the chain had broken but when I pulled the side cover off I found that the chain was intact but the front sprocket was totally gone.

I searched all over and around the road where it occurred but couldn't find the sprocket or any other parts anywhere so I'm not exactly sure what failed or why.

Inspecting the end of the countershaft and the rest of the area where the sprocket lives doesn't show a whole lot of damage.... There are two big gouges that were scraped along the side of the case where it looks like the chain dug in when the sprocket failed but neither of them touch the countershaft or the oil seal surrounding it. The bottom edge of the case housing was cracked off where the sprocket undoubtedly smacked into it when it let go and the grease nipple for the swing arm bushing got snapped off. Here's a picture I snapped of the damage:
CM450A left side damage.jpg


There's a bunch of grease in and around the swingarm bushing and a fair amount of chain lube that has gunked up the area but I don't see any evidence of oil leaking from the countershaft seal.

Both the chain and the sprocket were essentially brand new, having been replaced less than 500 miles ago and until today had no problems at all. I went on a ~150mile ride yesterday with no problems at all (really happy it waited until today to let go, not yesterday 100 miles from home!) though I've got to think that that ride definitely contributed to the failure since my daily commute is less than 10 miles and I rarely get the bike over 40 along the way.

As to why it failed, I'm guessing that either the retaining bolts for the sprocket plate were way out of spec and torqued themselves loose enough to come right out... or the chain tension or alignment was way off which wore down the same bolts to the point of failure.

So, going forward I'd love some input on what else I should check out for damage. I have a replacement sprocket on hand and ordered a new retaining plate and bolts. Will replacing those be enough to let me ride safely? I'm going to remove and thoroughly inspect the chain but barring finding any obvious problems is it crazy to reuse it? From the point of failure to being pulled over at a dead stop I traveled only 30 yards or so so I'm hoping the chain didn't get much abuse in that short of a distance, especially since its practically brand new...

Am I missing anything here?

Thanks for any advice, it is appreciated as always!
-Joe
 

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That's gnarly, glad it just kinda stopped and didn't throw you off or anything.

How tight is your rear sprocket on the wheel? I went to replace mine and realized I needed a new spacer (could also need new circlip or washer and if you're really unlucky cush rubbers) because I could tilt the sprocket towards/away from the wheel a few degrees each way. I could see that putting pressure on the front sprocket.

I'll let someone with more experience answer the chain and bolt part, I wouldn't know myself
 

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Discussion Starter #3
Yeah, I consider myself very lucky that it was more or less a nonevent considering how bad that could have been if the chain got in the wheel or what have you... Since the chain didn't break and because of the way its routed on the frame it stayed more or less in position just super super loose.

I'll check the rear sprocket torque and see what sort of shape the spacers and such are in. Last time I was back there everything was A-ok and has remained so to a visual inspection but I'll definitely give it a thorough going over.
Thanks for the tip!

-Joe
 

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A dab of blue Loctite on those new sprocket retainer bolts would be a good idea.
 
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Discussion Starter #5
Definitely! Already have the Loctite tube at the ready :)

I'm hoping with that plus proper torque and quadruple-checking chain tension and alignment I ought to be good to go for the reinstall... IF all that fails it just isn't meant to be.
 

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Discussion Starter #6
UPDATE:

I went for a walk while on lunch at work today and low and behold there was my missing sprocket right on the sidewalk about half a block up from where I was looking for it after the breakdown:

found sprocket.jpg

It looks to be in near perfect condition... all the teeth are intact and basically brand new, the inner splines are perfect, the threads on the retaining bolt holes are good as new... the only thing that makes it look in any way like I didn't just pull it right off the bike myself are the minimal surface scratches on the back side from the wild ride it took across the pavement. Damn glad that thing didn't end up going through someone's windshield or into a pedestrian along the way.

From everything I can see it just looks like the retaining bolts just worked themselves free and the clip holding it on the countershaft then torqued over to the point where the thing just slid off the shaft and went flying.
Unless anyone has a good reason not to I'm just going to reuse this sprocket and install it with a ton of thread locker to call it a day.

-Joe
 

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I would reuse it, be sure to ease any new burrs with a file first.
 

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I have replaced the retainer on several bikes (mostly dirt bikes) with snap rings and never had a failer. Not sure what size I used but I got them from the hardware store.
TOOLS
 

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Maybe it's the picture angle but it appears that sprocket is done. The teeth look like points instead of flat topped and the valley's appear elongated. Have you been lubing the chain every 300 miles with chain lube? not WD or oil.
 

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Discussion Starter #10
I think it is just a bad angle. The teeth tops are all flat and the valleys look stock to my eye... I did order a new replacement though so when it arrives I'll measure both with some calipers to compare before reusing this one. Might just end up using the replacement regardless since I already bought the damn thing...

This may be a better picture:
sprocket zoom.jpg

Here is the exact same sprocket and aside from the difference in photo quality and the fact that one clearly unused while the other has some mileage they look pretty much the same to me, geometry-wise:
https://4into1.com/parts-unlimited-...l350-cb-cj-cl360-cb-cm400-cb-cl-cm450-cb500t/

-JOe
 

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Discussion Starter #12
Thanks for the advice! The stock replacement sprocket came, was installed, and gave me no trouble whatsoever throughout the whole 350 miles I rode to Jersey and back this weekend.
When I installed it I was careful to apply a generous amount of loctite and dialed the torque in just right on the retaining bolts. I inspected both when I got back home this evening and confirmed they didn't budge at all throughout the weekend.

I threw the sprocket that came off into the "extra parts and bits that I think will be useful for some project some day but in reality will never touch again" drawer in my shop. ;)

-Joe
 
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