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 ....

[calj737]

Those bolts should be torqued to a proper spec. To avoid the cam turning while tightening, put the bike in a high gear, have someone depress the rear brake, and then tighten. This added "restriction" should allow you to tighten properly without damage to anything. Torque is likely in the 14-18# range. But seek the manual for verification.

You may be able to repair those threads. Verify the correct thread size AND pitch (8MM is 1.25, 10MM is also 1.25, 6MM is 1.0 on these bikes) and use a tap and some cutting fluid. 1/2 turn in, 1/4 turn out with the tap. Progress slowly, and keep squirting fluid in there as you go to aide in clearing the cuttings and assisting in the cut. Understand, that cam thread will be pretty hard metal, so use quality taps and plenty of fluid. A good set of taps and dies are essential for maintaining these bikes. Make sure you get METRIC! Far too many holes, threads, etc get buggered up over the years.

And replace both bolts once cleaned up.

A good idea is to use some LocTite (not on the sprocket bolts) in certain applications. Blue is for strong, non-permanent installation. Red is "gorilla" strength. Anti-sieze is also useful (think side cover screws, etc that you frequently need to remove). These are all thread "lubricants" and provide better hold, less likely cross-threading, and added torque (LocTite) when strong holds are needed.

You don't a lot of special tools, but you do need a few "essential" ones to deal with engines and drive train/safety systems. Or maintenance and repair of these older bikes.

If you have had the sprocket and cam off the bike previously, and weren't successful at getting it properly fastened, then probably the cam "wobbled" itself and the sprocket into a situation where the stress of the chain snapped the sprocket. Consider yourself lucky that other damage didn't occur.

[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.

[calj737]

I didn't put that there, and would like to remove it. I'm not an expert at anything but smoking cigarettes and drinking espresso. And even then, I'm sure there's someone out there more knowledgable about espresso than I. 

Lots of these things come from experiences (good and bad) and some from pure common sense.

In addition to a torque wrench, grab a good set of metric taps and dies. There's lots of times that you may need to re-tap a hole where the threads get buggered. Especially in fine thread applications. These kits usually come with a thread pitch gauge also (looks like a set of feeler gauges, but the edge is serrated and marked with .5, .75, 1.0 and so on) and this can be really important with the larger bolts. Some 10mm and 12mm bolts on these bikes are fine pitch and if you stuff a M10x1.5 bolt in a factory M10x1.25 hole, you're going to learn some new four lettered words the next time you remove it.

Best of luck-