I've read a lot bout minimum piston to head clearance like .040 general rule of thumb for steel rods, etc. But how much piston to head clearance can you run before it starts becoming an issue with being to much?

Somewhat dependent on bore size and chamber shape, but after .060" the knock suppression is weak, at .100" it's about gone.

The effect still works in the roller 318s which have something like 0.080 piston to head clearance. But it'd work even better tighter. 0.040 is the most common recommendation, tighter than that is workable if one could stop before the piston runs into the head at redline. Piston rock, rod stretch and crank flexibility all play a part.

Running the piston into the head repeatedly can lead to all sorts of bad things which will cause one to bash one's head repeatedtly into the wall.

R.

No it doesn't. Working over the years, in a Chrysler dealership, there have been numerous software updates and half a$$ed fixes for 5.2's and 5.9's with spark knock issues. Maybe a thinner head gasket would have been a better fix.

P-H is 2 different issues: quench (knock suppression) has a limited range of effectiveness, with a small bore and plug near the exhaust valve you may not need to be really tight - Jenkins reported minor pumping gains in a non-critical SBC by going up to .060". This may also be true of alcohol engines where high comp is safer.
Squish is the turbulence and mixture homogenization caused by mixture being expelled from the tighter space and (if you're lucky) aimed at the plug - this is generally helpful even if very loose.

My take: measured carefully (piston rock) I want .040"

Generally what Polyspheric said
I run under 4" bore tight pistons .025
440 6 pack type piston .030- .040
loose pistons .040
aluminum rods and loose pistons.050 but you loose some effect which is made up by race gas or alky
two things
the squish- turbulence mentioned above
and the quench two cold surfaces close together squeezing out the fuel molecules and quenching detonation
low compression motors can use bigger area than High
on a turbo alky build we had to cut the quench area down as we were blowing the fire out- burn pattern on the pistons- took a while to figure out
in addition tight quench helps with timing variations and good/ bad fuel
just gives you more room to tune without detonating your motor
or when you get a load of bad gas
more important in a highly loaded application- say motorhome or boat or towing and less important in a motor that is past the detonation range and running high rpm- and race gas
which is why you could race an open chamber low compression MOPAR but they suck on the street almost the same with high compression and open chamber