Cheers Gadget,
I would love an easy fix, but they rarely seem to come along. I looked for any marks on the top of the pistons potentially indicative of valve - piston contact, but there is nothing. Even with the exhaust valve decompressed there is nothing to indicate contact.
I don't have a stethoscope and my best efforts with a finger in one ear and my deaf ear up to the engine indicate the sound is coming from the upper barrel area - just where you would expect a diesel knock to come from. I have heard enough lower end; small end/gudgeon; and valve noises to discount these. Also, I have replaced the big end bearings, small end bushes, gudgeon pin, valve guides, valves, valve springs etc. As I mentioned earlier, these engines do not have any rings below the gudgeon pin, however, there does not appear to be significant piston wear. In addition, the bore still has some of its factory cross-hatch hone marks, even at the top of the bore (the bore is not chrome plated i.e. post 'Listard').
I don't know if backlash is likely to be an issue and I assume Lister made allowances for backlash in their gear design and placement. I suspect the level of engineering precision in these engines was perhaps a whisker or two less than that in Swiss watches. The timing gears and idler are in good condition with no appreciable wear.
The knocking noise occurs just before the exhaust valve opens so overlap can't be a contributing issue.
I also wondered if the push rods / tappets were perhaps hanging up and getting slapped by the cam, and this contributed to why I replaced the valve springs, however, both tappets and rods move freely and are well lubricated. The rollers in the tappets similarly work perfectly. I did remove the inspection door and look at the cam - tappet contact while turning the flywheels with the injector out and they work as they should without any hanging or slap.
As I suggested on yesterdays post, I'm now wondering about a reduced bump volume due to insufficient valve seat recess; or replaced valves that have a slightly different head shape / thickness. I imagine this could have a similar effect to carbon build in the upper cylinder or on top of the piston.