The squish band thickness is not something that should always be .030" or .040" on every engine. It is something that is a major engine design parameter and is often different from engine to engine for many good reasons - even different between different builds of the same engine. Typical engine builds show the squish thickness to vary between less than .008" to more than .068".
The squish band doesn't really have anything to do with, as you say (edited for spelling only) "piston/head clearance too far apart and the piston won't be able to transfer its heat to the head correctly." Its job is to concentrate more of the fresh mixture so it can become part of the power making process more quickly. Even this comes at some loss because the mixture that's in the squish area does not become part of the power making process. Engines tuned for maximum power will take advantage of design parameters to reduce this trapped loss and turn it into free power whenever possible by making it either narrower or thinner or any number of other combinations that'll achieve this goal. You can notice this in the combustion chamber shape of modern road racing engines - very narrow parallel squish band width with a large blend radius leading into a large radius, shallow bowl area.
Modern squish designs consider the maximum squish velocity as one of the most important design parameters - one which ultimately controls all other squish related variables. The software programs from Blair, TSR and Bimotion all include this feature as part of the two stroke engine design data crunching - it's probably the most important head design program to own. This software makes it easy to design a combustion chamber to the correct maximum squish velocity, squish band thickness and width, based upon the engine's maximum operating RPM's, octane requirement, engine displacement and other fixed mechanical parameters. It makes it possible to run as tight a squish thickness as possible without the onset of detonation. The ideal squish area is the one that loses none of the fresh charge to thickness and doesn't allow the piston to come into contact with the head. Four stroke drag racing builders (and a two stroke tuner or two...heh) have been using the "zero" quench design (squish as we call it) to extract the most power from a given amount of fuel for quite some time. That is, the perfect quench is one that allows the piston to just "kiss" the head at maximum RPM under full load and not detonate or damage any parts in the process.
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