Timing gear knock

[SAJ]

When I got my roadster I imported a touring engine from Snyders, reconditioned by Schwalms. It ran nicely until I burned No. 3 exhaust valve.
I removed the head, replaced the valve and since then I have had a camshaft knock that I diagnosed as end thrust "walk" by screwing in a polythene bolt in place of the timing pin, wherupon I could remove or return the knock at will by pressing it onto the fibre cam gear.
First I checked everything was free in the oil pump and distributor drives, mindful of what James Rodgers has said about this. Then I replaced the spring in the plunger and made a packer of about 0.3inch to put inside the plunger. This made no difference.
So I then tapped the casing and put in an adjustable end thrust bolt.
Again, the noise is still there, unabated.
I just removed the side plate to observe the gear running and a video is attached here https://www.youtube.com/watch?v=iPLZXvFUf9E (There is a bit of gasket hanging into the housing at the top, but doing no harm)
I then put the poly bolt in the timing pin hole and observed it eliminating the knock as seen in this video https://www.youtube.com/watch?v=MmKJyDLO1cM
Lastly I measured the backlash with a DTI at 0.008 inch as in the photo attached (measured in 4 places around the gears).
I know it should be .003 to.005, but I did not think a fibre gear would be so noisy at 8 thou backlash.
Plus, if I replace it, would I get an oversize gear?
The last video shows the gears meshing and there are no teeth missing etc as you can see. It all looks good to me. http://youtu.be/L0qaBGVlhgs

I know metal gears are said to be better in a matched set, but I don't want to remove my engine to get the C/S gear off, because I already have two other members cars in bits as favours and I am short of time before rallies coming up.
Diagnosis and advice please. I don't understand why the noise only started when I replaced the head gasket and valve. Nothing else was done except a valve grind all round.
Merry Christmas to all
SAJ in NZ