Topic: Why does a cam chain sprocket break ?

[reinax]

Hello,

I have a CB125J (cb125S3) that's been difficult to get going.
Finally I seemed to have gotten it working until all of a sudden it died on me again.
Strangely, I couldn't turn over the kickstarter any more nor get it to move in 1st gear, the engine seized !

I found out today that this was the result of a broken cam chain sprocket. The valves and piston seemed to have remained intact. It could have been worse. I ordered a new sprocket and will replace it shortly... But i'm wondering, What caused the sprocket to break ? Has anyone seen this before ? What did i do wrong ?
maybe I should mention that i was having doubts about my cam chain tensioner, which seemed to be hardly of any use ...

Any ideas ?


Thanks,

[ekpent]

Did you check the cam to see if it may have seized. Sure a valve did not smack a piston. Is it a sprocket with holes around in it ?
   I have broke an ear off a cam sprocket mount on a 750 once, maybe things just break sometimes.

[reinax]

I opened everything. As far as I could see none of the valves seemed hit. It is indeed a sprocket with holes in it and that is exactly were it broke. But Why ?

[ekpent]

See it broke at the mount holes. Some of the 750 sprockets are swiss cheesed with additional holes around the sprocket to make them lighter.
   Wonder if maybe those mount screws were loose and it started to get s good wobble going and maybe egging the holes a bit and putting some undo stress on the holes causing it to break ??.  Are the mount holes perfectly round when you put the halves back together ?

[Tugboat]

Just throwing this out there: maybe they used the wrong bolts in each hole? It does matter... One shouldered, one not.

[754]

Wondering sprocket was cast..cost savings these days

[reinax]

Thanks for all the suggestions so far!

Acrtually, there was no 'they'. Everything has been disassembled and reassembled several times by myself, so if things were done wrong, I did it. btw i'm not a mechanic and I don't have any special tools. So for ex. tightening the bolts on this sprocket is somewhat difficult as the cam starts to turn when you put force on the bolts.

The sprocket was what I found on the bike, which wasn't really fiddled with all that much. So i presume it's original.

@tugboat: I checked both the exploded view on cmsnl and in the haynes manual and both sources show twice the same bolt. anyhow I didn't notice any difference in bolts in the past...

You won't be able to see it now anymore since one of the bolts was completely damaged as wel as the thread of a hole in in the cam. I wonder if this means i'll have to buy a new camshaft ....

[reinax]

Thank you,

That's soon good information.
I see why it says expert under your name

guess i'm going to get myself a torque wrench.