Cam runout

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Make sure its the Merc big pin Iwis, Andy did not push it and first offered the std Iwis but the big pin version is the best, its pre-stretched so no need to pull the timing cover after first 1000 miles to check tension and the big pins mean less need if any for subsequent re-tensioning.
 
kommando said:
Make sure its the Merc big pin Iwis, Andy did not push it and first offered the std Iwis but the big pin version is the best, its pre-stretched so no need to pull the timing cover after first 1000 miles to check tension and the big pins mean less need if any for subsequent re-tensioning.
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Pre stretched you say... hmmm...

I musta got the wrong ‘un and maybe that’s why the new IWIS chain I tried to fit when building the 920 motor is still on the shelf? It was too tight, so I fitted the used IWIS from the previous motor...
 
gortnipper said:
Get an IWIS cam chain. I think Jim Comstock stocks them in the states. If not, The Chain Man.

https://the-chain-man.co.uk/
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pics showing chain differences

Cam runout
Cam runout
Cam runout
 
Is there a problem with the original chain or the substitute? Other than if the original has proved to be not strong enough the extra weight of the IWIS must be a disadvantage.
 
All you really need is a chain that will stretch evenly (and not break) so it will not pulse. ?
 
If you check your cam timing with a new stock replacement chain and then again in 10,000 miles you will be amazed at how many degrees the timing has changed.
The last stock replacement chain I used lost 6 crankshaft degrees in a little less than 10,000 miles.
 
comnoz said:
If you check your cam timing with a new stock replacement chain and then again in 10,000 miles you will be amazed at how many degrees the timing has changed.
The last stock replacement chain I used lost 6 crankshaft degrees in a little less than 10,000 miles.
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That chain must have, in 10,000 miles stretched like elastic! A few of those together would be good for holding my Levi's up!
 
comnoz said:
If you check your cam timing with a new stock replacement chain and then again in 10,000 miles you will be amazed at how many degrees the timing has changed.
The last stock replacement chain I used lost 6 crankshaft degrees in a little less than 10,000 miles.
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Jim, I've been trying to figure out my problem with variable chain slack and having difficulty identifying and measuring all those areas where variability might be introduced. I tried to calculate what effect 0.010" variation in center distance (due to runout) should have on the amount of slack. I calculated 1/4" (up-down movement) on a single run of the chain, so that would combine to 1/2" for the top and bottom runs (although modified somewhat by the deflection of the chain by the fixed tensioner). Chain wear that allowed 3 degrees of camshaft rotation would have a similar effect to almost 0.060" of runout introducing well over 1" of additional up-down movement. Did you see anything like that much chain slack?
 
ggryder said:
Jim, I've been trying to figure out my problem with variable chain slack and having difficulty identifying and measuring all those areas where variability might be introduced. I tried to calculate what effect 0.010" variation in center distance (due to runout) should have on the amount of slack. I calculated 1/4" (up-down movement) on a single run of the chain, so that would combine to 1/2" for the top and bottom runs (although modified somewhat by the deflection of the chain by the fixed tensioner). Chain wear that allowed 3 degrees of camshaft rotation would have a similar effect to almost 0.060" of runout introducing well over 1" of additional up-down movement. Did you see anything like that much chain slack?
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There was a lot of slack and I needed to adjust the tensioner several times over the summer, but I don't think there would have been an inch of up/down play. -Maybe, but it's been several years ago.

When it was new there was plenty of clearance between the lower run and the case -without the tensioner.
After a summer's use I could pull the chain down with my finger and it would lay against the case.

I got the degrees change by checking the cam centerlines with the used chain and then again after I installed a heavy duty chain.
I don't have an ignition pickup on the cam to make it easy.
 
My post here is somewhat academic. Not wanting to muddy the waters here but when we use the term cam chain stretch "I think" the phenomena is cumulative wear and not a permanent deformation (stretch) of the individual metal components.

This apparent stretch (cumulative wear) probably has more to do with the chain components rubbing surface (barrels and pins) metallurgy, surface finish, and lubrication.

Any chain or metallurgist here care to chime in and enlighten.
 
Stretch in a chain is like rings bedding in, it's just the initial rough machined finish being reduced to a much finer running surface giving a higher rate of wear at the start and then plateauing out. A chain is just a series of shafts and bushes and each one wears increasing the distance between the pins by a small amount but over 50 links its 100 pins and so a thou wear in each pin gives 100 thou of wear in the chain.
 
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