br4inl3ss
Scoot Enthusiast
Posts: 493
Location: Québec
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Post by br4inl3ss on Apr 6, 2023 12:54:18 GMT -5
hi.
how do i know if my 139qmb chain is worn or not ?
received camshaft and rocker, waiting 2-3 weeks for the chain and guide ( logistic problems i guess ).
cannot wait to see if it bent a valve and see if its better with brand new cam.
but i wont if its too risky... but to know if its too risky i have to know if chain needs immediate repalcement or not.
ps i dont care reopening the engine again when i receive chain and guides.
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Post by aeroxbud on Apr 7, 2023 3:24:39 GMT -5
Same as a chain on a motorcycle. Any tight spots or if you can pull links away from the middle of the cam sprocket.
If you have an old and new one together. You can compare the side bend and total lengths to see how much the old one has stretched.
If you have one coming I would replace it anyway.
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Post by 190mech on Apr 7, 2023 5:56:59 GMT -5
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Post by GrumpyUnk on Apr 7, 2023 9:15:33 GMT -5
The timing chain has a tensioner that should take up slack due to wear. It is bathed in oil. It doesn't have a lot of load moving the cam & valves, so should last a long time. I'd be surprised if one was worn before 10,000 miles of use.
Given a worn chain, you can check by setting the crankshaft at TDC, and moving the cam so the two small holes are even with the top surface of the cylinder head. Any wear will keep the two holes from aligning exactly with the surface, generally allowing the two holes to be CW of the edge of the cylinder head when the chain is tight. A good unworn chain SHOULD allow aligning the two holes exactly with the crank at TDC. Worn links/pivots will allow a slight elongation, allowing the cam to be slightly 'behind' with a taught chain. Just a tiny bit, which should not hurt unless you are racing for $$$. The tension is on the 'front' side of the chain, and that will allow the cam to lag slightly.
tom
ADDED: Generally, there should be no 'loud noises' from the cam & followers. If things are assembled properly, and the correct rocker setup is used for the installed valve stem length, it should possible to rotate the crankshaft through 2 complete turns without having any binding beyone the slight resistance from compressing the valve springs. No parts of the valve assembly should clank or bang into any other parts. With the valve clearance adjusted to .003-005 there should be no clatter and very little noise. Essentially, you should be able to go smaller as long as there is any clearance, as the cylinder will not expand as much as the cylinder head, increasing the valve clearance as the engine reaches full temperature. Should is the operative word.
tom
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Post by snaker on Apr 7, 2023 10:19:29 GMT -5
Same as a chain on a motorcycle. Any tight spots or if you can pull links away from the middle of the cam sprocket. If you have an old and new one together. You can compare the side bend and total lengths to see how much the old one has stretched. If you have one coming I would replace it anyway. On point. Grumpy makes a good point as well with the visual check. I've done much more work with motorcycle engines than GY6 Bikes usually had the multilink chains, I forget the name. But it was pretty normal to see the chain guides wear as much as the chain if not more. I would consider it standard to replace the guide/guides as well as the chain if that was the case. The sprockets held up well Does anyone here go with the notion that guides are consumables or are the guides generally good for life on the GY6? |
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Kaos
Scoot Enthusiast
Posts: 185
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Post by Kaos on Apr 8, 2023 20:18:55 GMT -5
Same as a chain on a motorcycle. Any tight spots or if you can pull links away from the middle of the cam sprocket. If you have an old and new one together. You can compare the side bend and total lengths to see how much the old one has stretched. If you have one coming I would replace it anyway. On point. Grumpy makes a good point as well with the visual check. I've done much more work with motorcycle engines than GY6 Bikes usually had the multilink chains, I forget the name. But it was pretty normal to see the chain guides wear as much as the chain if not more. I would consider it standard to replace the guide/guides as well as the chain if that was the case. The sprockets held up well Does anyone here go with the notion that guides are consumables or are the guides generally good for life on the GY6? I've never had enough wear on a GY6 to need to replace the chain guides. I put 24k miles on my last one before other stuff broke in the engine. I've also put 30k on a bigger bike before I finally had to replace a chain guide, though it was pretty badly worn at that point. |
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