I tried re-adjusting with the engine running and the valve covers off but the whole engine started running rough so I stopped, worrying that I could be hitting my valves with pistons.
Since I know every single valve is a half turn past zero lash,
Having messed with some but not all of the rocker adjuster nuts, DO you know that they're all 1/2 turn past zero lash? Or are most of them 1/2 turn, and others are some random amount where you quit messing with them?
Are you SURE you did the job properly "a couple years back"?
As said--
if you're adjusting a running engine, y' gotta turn the adjusting nut slowly, or in small steps, and allow the engine to recover before turning it more. There was a time when I "liked" adjusting lifter preload with the engine running. Now I do it with the engine off. Everyone has their favorite method. There's probably a dozen-plus ways to do that job, and they all work. Some take longer, or require more precision/attention. As usual, there's
two hundred ways to screw it up, though.
If you showed up in my driveway, I'd pop the covers and start from scratch. But that's me, and I've done this before. If I was motivated, I'd pop the intake manifold and inspect one or more of the lifters and cam lobes. Or
maybe sneak a videoscope probe down some pushrod holes in the head and inspect the cam lobe(s).
would it be safe to just take the covers off and add 1/4 turn to each one and then let the engine run for a bit so the lifters figure themselves out? (maybe twice if they're still clacking after the first try?)
Probably.
Thousands of years ago, GM spec for preload on small-blocks was one full turn. You'd be at 3/4 turn, 1 full turn if you do it twice. But that assumes that the plunger travel on the lifters is as large now as it used to be; and I'm not sure it is. I got into trouble once due to a cam company selling me restricted-travel lifters with no notice in the box. Would be nice to verify plunger travel on a lifter or three once the top of the engine--manifold, pushrods, rocker arms--is apart.
My opinion on preload is that when it comes to noise, ANY preload sufficient at cold and hot engine temperature, is as quiet as
any other preload that doesn't allow the plunger to top-out, or bottom-out. Within the adjustment range of the lifter plunger--close enough is good enough to
settle the noise. I'm not saying "any preload" is good enough to get max valve lift, or minimum guide wear, or to last the life of the engine.
For those who prefer to just-barely preload the lifter, normal wear might remove the preload making the valvetrain noisy. So to compensate for normal but not excessive wear, you'd have to add more preload during the service life of the engine. For those who use "factory spec" or more for preload, preload should last the life of the engine--because when the valvetrain wears enough to remove the preload, the rings and valve seal are probably toast, too.
(Don't mind taking the intake off, but I don't wanna drain coolant again)
Well, you're not getting the intake off without draining coolant.
Have you considered that with that many miles on an engine, that the rocker arms and valve guides may be worn-out? The bores could be worn? Valve springs weak? The timing chain may be loose? 300,000 Km isn't that much, but prior maintenance and useage, and "luck of the draw" does affect what condition the engine is in.
