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Honda Cg125 noise/vibration from output shaft?

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moto42
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Joined: 15 Dec 2025
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PostPosted: 14:43 - 22 Dec 2025    Post subject: Reply with quote
stinkwheel wrote:
No harm swapping them out for all they cost.

Do you know anyone with a lathe? Be interesting to get the mainshaft between two centres with a dial guage on it to check for runout. Given your reported tight spotting of the chain.


Very true, I'll get them popped out whilst I polish the casings up a bit and replace when re-assembling.

I don't know anyone with a lathe unfortunately no. I don't suppose it'll be too hard to find one in the industrious city of Manchester though. Is there another way I could check for run-out at home possibly with my amateur toolbox?
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moto42
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PostPosted: 14:51 - 22 Dec 2025    Post subject: Reply with quote
jeffyjeff wrote:
stinkwheel wrote:
....Be interesting to get the mainshaft between two centres with a dial guage on it to check for runout. Given your reported tight spotting of the chain.

I think a mainshaft with excessive runout would cause multiple tight spots in the chain, every 16 links or so, depending on number of teeth in the front sprocket.


As far as I could remember the tight spots were a bit more spread out than that. I'm presuming an excessively tight chain could cause a bent mainshaft? However, I don't recall ever having excessive tension on my chain whilst I've ridden the bike and with the bike only having 3000miles on the clock, it feels unlikely.. Then again, only an instinctual feeling.
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moto42
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PostPosted: 14:56 - 22 Dec 2025    Post subject: Reply with quote
A100man wrote:
Im a bit out of my depth here, however I'd suggest leaving the bearing to start with and part reassembling the gearbox only - a bit at a time -until you can identify the true source of the grumble.


I'm also the same, was only 6 months ago I first passed my test and touched a motorcycle for the first time lol.

Anyway, I was tempted to do the same whilst waiting for some parts to come; Re-assemble gear cluster, selector fork/drum etc. and see if everything spins smoothly, gear selection shouldn't be an issue but wouldn't hurt to test also whilst there. Then, I'm sort of hoping it's a meshing issue as it would allow me to narrow it down to the gearbox and renew as necessary. If not, I'm not sure what to test next apart from main-shaft run-out as the bearings do look, feel and sound in good knick.
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jeffyjeff
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PostPosted: 19:16 - 22 Dec 2025    Post subject: Reply with quote
moto42 wrote:
... I'm presuming an excessively tight chain could cause a bent mainshaft?

On the surface, you'd think that a tight chain would not cause a bent shaft, as the radial force caused by the chain would move with the rotation of the front sprocket. Regarding maladjusted chain slack, I would rather err toward a loose chain than tight. An overtight chain might not tweak the shaft by itself; but when you are riding the motorcycle, (especially with a heavy load or a pillion passenger) any road condition that causes the rear suspension to rapidly compress could spike the axial load on the shaft and damage the chain, sprockets, or shaft. Remember that as the rear suspension compresses, chain slack tightens. The more the front sprocket and swingarm pivot are offset, the greater the effect.
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jeffyjeff
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PostPosted: 16:59 - 23 Dec 2025    Post subject: Reply with quote
If the shaft was bent due to a load placed on it from a tight chain, I would expect it to be tweaked between the outer bearing and the butt of the shaft. Attempting to measure runout at that location using V blocks and a dial indicator will be difficult due to the (sprocket) splines on the end of the shaft. If you support the shaft on the bearing journals, you could try to ascertain runout with a fixed pointer, but actually measuring the total runout might not be possible. Runout at the sprocket mount could confirm your theory that a tight chain contributed to the cause. Runout between the V blocks could confirm the cause of your vibration, but not that it was caused by a tight chain.
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jeffyjeff
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PostPosted: 17:13 - 23 Dec 2025    Post subject: Reply with quote
By the way, according to the CMSNL diagram, the sprocket mounts to the transmission countershaft, not the mainshaft. Any damage to the mainshaft could not possibly be caused by a tight chain.
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