Cylinder head wear...

[dhirocz]

I experimented with running zero lash with hydraulic roller cams in small block Chevy's vs preload vs running with extra clearance. I did notice that it always ran best with a little extra clearance, but too much made it extra noisy, hammered the valvetrain and set off the knock sensors so I desensitized them after getting it set where it was reliable and efficient.

I began to notice if I set my lash cold (prior to initial startup) then checked it hot, the clearances changed, understandably so. I started noticing the changes correlate to valve stem length and material (oem vs stainless vs inconel) vs temperature. So I try to use that experience as a guide for setting them to run as close to my desired spec as possible. On an engine like that I would check valves at least twice a year, depending on usage. Even OEM, at most 200k before you are into it for an overhaul or repair that requires you touch the valvetrain. Keep in mind this is a sbc, not an isx.

...which brings me to my point. Which is...why the hell 500k is a good overhead interval is beyond me! An engine is still an engine. Way too much going on and too many possibilities for those dimensions to change. Must be 'corporate and financial engineering' at play.