Paranoia And Urban Legend.

Timing Chain paranoia and urban legend.

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Against my better judgment, and since this will be my final post here as I have now passed on my last remaining 300D to my grandson, I will throw in my two cents worth on this subject anyway.

Timing chains don't Ð''just break', and there is always an underlying reason other than that they have done over *** miles or the chain elongation exceeds * degrees if they do. Their lifespan is heavily dependent on oil change intervals and your driving habits. These chains are massively over-designed to minimize wear and the working tension (typically > 30 kg) is barely 1% of the ultimate breaking strain. You could pick up your car on it with a handsome safety margin. Chains can be thoroughly inspected and wear measured in situ using a good quality digital caliper (0.001mm), gauge blocks and pin micrometer. In addition the cam sprocket must be closely examined for wear (magnifying glass) and valve timing measured at #1 intake valve.

A chain worn in excess of 6Ð'Ñ" should be replaced not because it is in imminent danger of breaking, it isn't, but because the difference between the pitch of the chain and the pitch of the sprocket teeth is now excessive. Instead of the load being spread evenly between all the engaged sprocket teeth (25 at the cam.) the entire load is now on the leading 2 or 3 teeth causing sprocket and chain wear to increase rapidly and exponentially. If you have never had a quote on a new set of sprockets, guides AND chain make sure you are sitting down when you do!

After over 50 years in this business, 19 of them as a development engineer in Perkins Diesel R & D test shop deliberately testing engines to destruction, I have never seen an OEM timing chain fail through fair wear and tear. Collaterally yes, over 90% followed by bad riveting on replacement, rarely these days manufacturing defects in OEM chains and, last but not least, fitting sub-standard aftermarket timing chains using snap-on master links. It follows that a timing chain is most likely to break after a new one has been fitted.

Valve timing and offset woodruff keys

The TDC marks on the camshaft thrust washer and tower are not, and were never intended to be, a precise method of checking valve timing. They are there purely as an assembly aid to position timing chain on the cam sprocket correctly and must never be used for key selection. Misalignment of them is a cue to take a precise reading with a DTI on the #1 intake valve and the result can then be used to select an offset key to correct if error does not exceed 6Ð'Ñ".