Used Cam Chain for Rebuild?

[lakeview]

Can o' worms time. Most of you say just buy a new one. But there must be some test once they are on the bench to see if there is any service life left in them.
By analogy, if I have a drive chain and when you hold it out sideways, if it just curves down to the ground, then it's done.
Look at the chains in the pic. The top and bottom ones are loose and sloppy, but the centre ones still resist bending the wrong way. I think they could be reused.
Let's have some opinions, please.

 

[gggGary]

https://www.diamondchain.com/understanding-wear-life/
Diamond sez 3% stretch over the spec length in a straight line.
EDIT! This is about power transmission limits. The cam chain is more about alignment and timing. See toomanys post below Used Cam Chain for Rebuild?

 

[gggGary]

If the engine is together,with a good front guide no more than 1/2 way on the adjuster? I have never seen a cam chain fail or worn out cam sprockets...........

 

[Jim]

Just measured the chain that came out of my SG. Stretched, it measures 32-5/8". When I put the new one in, I compared it to the old.... stretch was about half a link. So if I subtract half a link, we get 32-15/32" length for a new chain. I'm guessin' that's accurate to within a 32nd". Try applying the 3% rule Gary found and see if they fall within it.

Jus' asked google what's 3% of 32.5 and she told me .975. So just roughly..... if you're under 33.5, you pass the 3% rule.

 

[TwoManyXS1Bs]

Another take on this.

The 3% stretch limit would apply uniformly to any section of camchain. And, all camchain slack is taken up at the rear tensioner.

Looking at the front straight section of the camchain, along the front guide, from centerline of the camshaft to centerline of the crank sprocket, is approximately 12".

3% of that would be 0.36".
How much camshaft retardation would occur with that 0.36" of stretch?

From post #22.
http://www.xs650.com/threads/excessive-timing-chain-slack.42215/page-2#post-422476

A 36T cam sprocket, a chain pitch of 0.3061".
Gives a cam sprocket circumference of 36 x 0.3060 = 11.02".
The cam sprocket retardation factor is 0.36 / 11.02 = 0.032668 of a turn.
0.032668 x 360° = 11.76° of rotation, at the camshaft.
That's 23.52° of crankshaft rotation.

My camchain is pretty stretched, at least 10° retarded at the crank timing mark, and I've exceeded the limits of points plate adjustment, and have to use larger points gaps. And, I run my ignition a few degrees retarded anyway.

I think the worst I've seen on here is about 15° retarded, well beyond normal ignition adjustments .

These are only about halfway into the 3% stretch value.

I'd say that the 3% stretch limit is appropriate to power transmission applications.

But, for critical timing applications, my call would be for a 1% limit.
Maybe even a 0.5% limit.
(Which is about where Jim's camchain worked out, 1/2 link over 106 links)

Feedback guys?

 

[lakeview]

Thank you 2M, that is the perspective I needed and one that should have been obvious to me.
Equating drive chain and cam chain wear is wrong as the drive chain has the limited function of transferring motion from one sprocket to another, while the cam chain not only turns the cam, it also has to do it with a precision that results in the bike running properly.

 

[MaxPete]

I agree with all of the above, especially with the notion of a fairly tight spec on permissible timing chain stretch (and my gut tells me that 1-3% of new length is reasonable).

The other thing is that a chain has MANY parts, each of which will have its own tolerance and all of those variations will combine (the technical term is stack-up) - so there could be a fairly significant variation in the overall length of even a set of new timing chains. Thus, a used chain that, when new, was originally on the “small” end of the tolerance spectrum could potentially be shorter than a brand new chain that is on the “long” end of the tolerance band.

From what I can see, the chains second and third from the bottom in Lakeview’s original photo above look much better than the top and bottom chains.

Pete