A10 Super Rocket knocking

[SSWIFT]

Whilst out today the engine was not developing full revs and when over about 50 mph in top gear there was a knocking from the engine. Back at home I determined the following:
1. There is a knocking when swinging over with the plugs out that sounds  as if it comes from the timing case. Took off outer and inner cases and nothing found.
2. The knocking happens when the exhaust valves open and close.  Took the rocker box off and knocking stops.
3. Put the rocker box back on and the noise came back, then took the idler gear out of the timing case and the noise stopped (not the crankshaft big ends etc).
4. Took the head off and nothing obviously wrong with the exhaust valves.
Does anyone have any ideas?

Also with the head off I can see there is a lot of slack in the STD pistons.  Does anyone have any suggestions about getting a re-bore; who could do it, what size pistons and where to get them?

Many thanks

Simon

[JulianS]

Check idler and cam bushes for wear.

The mainshaft pinion can work loose and fret on the key causing a knock - I know. It happened to my A10.

[SSWIFT]

Thanks Julian, but would what you describe account for the knocking stopping when the exhaust valves were not opening and closing (rocker box removed)?  Also I did one other test which was to put the rocker box back on with only one push-rod in place - RH exhaust.  This resulted in the knocking occurring only when that valve opened and closed.

[Jackson]

Something sloppy around the exhaust push rods/cams?
Just guessing, but if it sounds when the exhaust valve opertae and dispappears when the rocker box is off...?

[JulianS]

Noise likely from the valve gear anywhere from the cam upwards. No pressure on the valve train/gear with rocker box off so no noise.