Originally Posted by
bjr_93tz
Being a quiet day, I decided to run the dial gauge on the valve rockers at various positions.
Both rockers/tappets had dimples from the valves so I ground them flat again, only needing to take off about 0.04mm.
I'm pretty sure I put new cam bearings 6 months before the problem started, making them ~ 2 years old now but not having done much work. The dial gauge tossed up some interesting results. When the cam was loaded ie during a valve lifting part of the cycle, the clearance on the other valve dropped quite a bit.
0.06mm of clearance (at TDC) on the exhaust valve reduced to 0.01mm when the intake valve was being opened.
0.07mm of clearance (at TDC) on the intake valve reduced to 0.02mm when the exhaust valve was being opened.
Not sure why this is happening but I'm guessing this isn't "normal" for a new engine otherwise YS wouldn't specify the clearance to be set between 0.0mm-0.1mm. It might be something that creeps into an older engine like mine. This is something I'll review when I change the cam gear bearings. I'm guessing the bearings have been letting the cam shift when under load, unseating the other valve and allowing it to leak.
Anyone else out there notice their valve clearances change like this?
I would think that when the engine is running, the valve would seat in the dimples as that is where they want to naturally sit, so I wouldn't think it would cause any issues.
I've never replaced cam bearings in any of my engines. I check them when I replace bearings, but never felt one that seemed loose or rough. Being so small, they would have to be extremely worn to cause any problems. Even then, I suspect they would fail before affecting engine running. As for the clearance variation, with everything being solid, not sure how that would happen. I've never felt the need to check with a dial guage - Just set and forget. Never had any problems doing it this way. Might have to check mine during the next rebuild if I remember.