I spent some time on this issue with my older metro 6/1. Excessive lash on the cam can be a contributor, but it is NOT the only source. The other big source is that the tappet and cam profile cause the tappets to not follow the cam, and instead to lift off them after the lobe peak and then slap down on the following cam slope.
You can confirm that this is your issue by lightly pressing the rocker arms down on the push rods with your fingers... the audible clack should disappear. Ronmar suggests using very soft springs under the between the rocker and push rod to take up this (valve lash) slack. Hotater put fine lead shot in his pushrod cups to prove that this was the issue...it made it go away until the lead was pulverized. I even tried making lightweight pushrods out of carbon fiber tubing (with steel ends) to see if it would help but it did not.
The valve tappet clacking is very pronounced on my Metro 6/1, even with a bronze idler and 0.055" offset idler to get an appropriate valve lash. It does not seem to adversely affect cam lobe or tappet wear, and my engine has about 2600 hours of running time. I have meant to implement Ronmar's spring method to stop the clacking but always seem to have a project of higher priority.
I think the bronze idler was a good stopgap measure but the solution to early idler gear failures has clearly been solved by XYZer's research and development of offset idler bolts. I don't see much point in a bronze idler for a properly set up engine, though they certainly won't hurt. They don't solve the tappet clacking.
Best Wishes,
Bruce
