Another cam timing question

[Randys01]

Compensation for projected chain stretch I would argue is not a lineal event...once the chain stretches from intitial use, the rate of stretch slows way down.

I stripped my cam boxes years ago to make it look like the older 6.3 variant. I kinda like that look over the "de rigueur" matt black look of the 70's. {Astons..I think Bentley etc all went thru that phase ]

That Tech bulletin on the valve timing etc reaffirms my own held belief that you need to compensate for both cams as they will be technically slightly different values. When I did mine, [near new chain ] one key worked for both as the difference was minute. I reckon by the time it's worth  the difference, you should be looking at a new chain.

Daniel: Rumb has answered your query spot on.

[s class]

In our shop we do a lot of chain/sprocket/guide/tensioner replacement jobs on 350/450/500/560 engines.  On average a couple of them per month. 

My observations are that generally, a well maintained engine of approx 200 000km will have around 10 degrees of stretch.  By that I mean that with the right hand cam lined up, the crank damper shows 10 ATDC.  ON the other hand, I also see a lot of poorly maintained 107's that have had long intervals between oil changes, and it is not unusual for them to show 15 to 18 degrees stretch with only 120 000km on the odometer. 

IN general, with these 117 engines, we find that fitting a new chain will result in remaining cam retardation of approx 2 degrees on the left, and 6 to 7 degrees on the right.  If camshaft sprockets are then also replaced, the situation improves to approx 0 to 1 degrees late on the left, and approx 4 degrees late on the right.  In almost all cases, we install 4 degree offset keys on the right cam, and usually then it is bang on 0, or possibly 1 degree early.  Given that chains stretch, our policy is to start off with the cam a degree or two early, rather than late. 

The situation gets more complex if the heads have been removed and skimmed excessively, because then the chain path is shortened. 

I might add that we never rely on the stamped markings on the camshafts, we always measure cam timing with the dial gauge method, as per that table rumb presented - it gives crank timing for 2mm lift on the cams. 

Raueda, in your case where there is some doubt, and the possibility that offset keys may already be installed, I would start by measuring the valve timing using a dial gauge, as you will then have accurate readings that are not subject to unknowns.  I have seen cases like this where cams have been advanced to cater for highly stretched chains.  I had such a case on one of my 6.9's. 


I have done only 3 x 6.9 chain jobs in my time (well thats quite a few given that there are only about 10 6.9 cars in south Africa...).  I found that the above general principles applied equally, with the only difference being that I did not replace the cam sprockets in the 6.9 cars because they are prohibitively expensive. 

[marku]

What about when the heads have been excessively skimmed as I had to do on my 450. It had been done very badly previously (it wasn't level front to back) although it was running OK when I got it although rattling a bit. Fitted a new chain as grooves had actually been worn by the old chain inside the heads. All this put everything out even making tappet adjustment just about impossible. So fitted shims under the cam towers equal to that taken of the heads putting both cams back in perfect alignment. Bit unorthodox but seems to work. 

[raueda1]

2 years later I can now close the book on this topic.  As others advised I replaced the chain, the rails and most of the sprockets.  The cam sprockets were substantially worn, as were the sprockets for the hydraulic pump and distributor.  The sprockets were out of a parts car, but in much much better shape than the old ones.

As it turns out somebody DID advance the cams for whatever reason.  I got zero-offset keys which should put everything exactly in place.  Mystery solved.  It will be interesting to see what effect all of this has on performance.

Thanks to all.  What a long strange road it is.  Cheers,