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As SaltyDog suggests, but in more detail -
1) Grab the connecting rod, and see if it wiggles at all in the "in and out" direction. Note that the "bottom" end of the con rod, riding the crankshaft, is attached with two bolts. There are also two bearing shells between the con rod and the crank. Anything more than one RCH is too much.
2) Grab the connecting rod, hold the piston absolutely still, and wiggle in and out again, this time feeling for play at the top (outer) end of the con rod, at the rod-to-wristpin-to-piston interface. Ditto one RCH.
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I am stumped!!!
Any chance something is contacting the valve cover itself? Maybe some tell tale sign on the back side? You mentioned not hearing it with them removed, just grasping at straws I guess.
Rocker arm end play, perhaps?
When parked for a week or more, my R75/5 makes a ticking noise for a few seconds until the oil reaches the rocker. Maybe you could consider an oil pressure deprivation on one of the noisy side rockers. In the same region, could it be a rocker arm roller bearing going bad?
When you’ve got everything back together and running, if it still makes noise try advancing the timing a degree or two. When your bike was new, gas was not a problem for its high compression pistons. You have restored compression but now gas is a problem for high compression engines. Under light throttle ignition knock is a soft tapping sound. Hard throttle can produce a noise like a loud rattle. Scary sound. For several reasons, the knock can be on one side only.
When you’ve got everything back together and running, if it still makes noise try advancing the timing a degree or two. When your bike was new, gas was not a problem for its high compression pistons. You have restored compression but now gas is a problem for high compression engines. Under light throttle ignition knock is a soft tapping sound. Hard throttle can produce a noise like a loud rattle. Scary sound. For several reasons, the knock can be on one side only.
Last night we watched James Bond in Goldfinger and this thread came to mind.
Maybe someone has placed a bomb in there ?
Could happen
Nick