Timing/idler gear/adjuster follow up

I have posted this topic in recent days but now I have new insight into my problem, which is that after I reinstalled my Series 1.5 4.2 l head after it was rebuilt, the upper timing chain is too loose and will not rotate when I rotate the crank and lower timing chain.

I believe I have now identified the source of my problem. This is the eccentric adjuster is not moving the idler gear up and down as much as it should. When I rotate the eccentric adjuster shaft 360 degres, the maximum up and down movement of the idler gear is only about 1 cm. By contrast, I estimate this vertical distance should be about 3 cm. As a result, the adjuster cannot pull the chain tight enough to engage the crank driven intermediate timing gear.

But why the too small an adjustment? My only guess, as I have not disassembled the timing system yet, is that the eccentric lobe is slipping on its shaft.

This is bizarre because the picture in the parts diagram suggest that the shaft and the eccentric lobe is all one piece. But I doubt that as it seems that it would be difficult to manufacture this as only one piece. But that is another guess on my part.

I would like to buy this as a new part and inspect if it is one of two pieces but can’t find a supplier that sells it.

Apologies for very long story but if you have made it this far, my requests for help/ advice are:

  • Can you tell me if this part is one piece of two?
  • Or can you suggest another reason why my idler gear does no move up and down as much as I believe it should?

I think a video of the eccentric, moving through its range, would be helpful.

Even with a severe head plane, I never found there to be too little adjustment.

Does this help? Eccentric on the left.

Unless I’m missing something, the photo of the eccentric shaft provided by Mike suggests that the sprocket will only move about 1cm from one extreme to the other; the same as you measured. Where did you get the 3cm figure from?

Can you pull the chain one link shorter on one of the sprockets and see if it’s within adjustment range. If it is, you can shorten the chain.

Standard engineering to produce that part, a lot less involved than a crankshaft. No way would there be a 3cm adjustment.

Agreed, plus 20 chain links.

You have the wrong upper timing chain?

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im hoping the attached will help. as you should be able to see the top chain runs around the main sprocket on the underside as viewed from the top of the engine. as such if you manually lift the camshaft sprockets and rotate them a little the top chain must engage with the main sprocket AND rotate. you might need and assistant or three but you should understand my point. if as others have suggested you have the incorrect length top chain you can replace it in situ by using a chain with a split link. there is much in the archives on this method which is quite simple.

Mike, are you the one who posted the picture of the two timing gears, the eccentric piece and another shaft? If you have the actual pieces rather than just a picture, could you make a measurement for me please
Measure the distance from the center line of the shaft to the outer edge of the long side of the eccentric lobe. And then from the center to the short side of the lobe.

Then I believe the up and down movement of the idler gear should be equal to the difference between these two numbers. After think more about, I guess you guys are right that this answer will be closer to 1 cm than to 3.

And assuming the answer is only 1cm, I want to go back to my basic problem and ask for your advice.
This was a salvage car and engine I bought s so I had never seen the engine run. However, before taking the head off, I cranked the engine with starter motor and measured cyl. compression Ok on most cylinders but bad on two so I took off the head and had it rebuilt.
This suggests to me that the top chain was rotating before. Otherwise, I wouldn’t find any compression and also some valves would be damaged by Collison with piston but they were not.

So chain and eccentric adjustment ok before removal. So why would the chain be too long now? Have I done head reinstallation wrong or is it something else?

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You COULD always count the number of links…yes, a tedious job, but it would eliminate your concern there. IIRC, the lower chain has 82 links, while that upper chain has an even 100.

Yes, but unfortunately, I took that photo in 2012, so those parts are long ago installed. Looking to see if I have another set lurking somewhere.

I can’t find any reference to it in the factory service manual, but my recollection (from years ago) was that I found it was possible to fit the eccentric 180 degrees out, and when it was 180 out, it was not possible to properly tension the chain. This just all sounds familiar.

Is one of the outer runs of the top chain slack?

Hi Mike your reference to the cam being 180deg out …as you turn the cam it moves the sprocket up and down…so tightens and loosens the chain if you keep turning it (on the bench)… So depending if you install the cam with the threaded section at the bottom as in your last photo then as you turn anticlockwise as per the service manual you lower the sprocket and tighten the chain…But if you install the cam with the threader section at the top and then turn anti clockwise you raise the sprocket which loosens the chain…Steve

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Here’s the easy way Phillip
Max up and Max down, near as dammit

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Thanks for picture of max up and down and I think we can now all agree that distance is between 0.5 inches and 1 cm ( 0.4 inch).

But that still does not explain why my chain slips with eccentric max down when it presumably did not before I removed the head!

I think someone has asked this of you before, but I don’t recall seeing any answer. When the head was being worked on after you removed it, was the surface skimmed? Secondly, how thick was the head gasket that was originally on the engine, and how does it compare to the gasket that you fitted when you re-installed the head? Any reduction in the head gasket thickness and head skimming are amplified by a factor of four by the chain’s route between the bock and the head. Apologies if I missed the answer to this question earlier.

Remember that the eccentric take twice as much slack out of the chain as the distance it moves full stroke. It affects two runs of the chain, not one run only.

There is no way your chain can slip over sprockets IMO, even with a worn chain or a skimmed block and head. Any slipping would be noisy and obvious as you turned the crank. Something has been assembled wrong. Study the parts book drawings and photos here. I suspect your problem has an extremely easy answer as it is such a bizarre’ issue, but your unfamiliarity with the engine is letting you miss it. I bet if one of us was in your garage we could solve it quickly. Send more pics? . .