[quote= LaceyDucati ...
" The squish should be ideally parallel or slightly tapered the other way, ie. tighter at the edge and getting larger towards the centre of the chamber. "
____ I-myself am not an expert on squish-band angles, and I'd certainly expect the likes of Nigel to know and/or understand such tech.application more-so than myself. _ So it would be of fairly reasonable interest if he (or someone-else [such-as like Eldert]) would offer an explanation as to
why it would matter any at all for the mating squish-surfaces to rather be tapered with an expanding-gap facing
TOWORDS the combustion-chamber, (if not kept exactly parallel).
__ I-myself don't see how being other than perfectly parallel (with a
slight-taper in
either direction) could make any
notable-difference whatsoever.
I can understand how it could possibly make life a slight-bit easier on the compression-rings if a tapered squish-gap had it's widest gap-opening faced away from them (and towards the comb.chamber), however it seems obvious to
me that
that oppositional taper-orientation would naturally tend to rather diminish the forced
injection-effect which the squish-band is intended to provide.
And in fact, just as a nozzle-attachment connected onto the end of a water-hose vastly increases the intensity of the velocity-strength of it's expelling water-stream,, I'd of-course likewise naturally expect a squish-taper oriented with it's
narrowest-gap/opening faced toward the c.chamber, to
assist and accordingly increase & intensify the desired*effect of the squish-band !
(* It's my understanding that the intended effect of a squish-band is to provide an extra intensified
RUSH -('charge') of combustion-chamber
turbulence [in the air/fuel-mix], so as to cause a faster & more complete combustion-burn [thus providing improved burn-efficiency and resulting increased power-production].)
So with the properly tapered-squish providing a "
nozzle-effect" to
further intensify the 'squish-effect', thusly makes good natural/logical sense
to me !
__ Therefore I, for one,, don't think it's any mistake that there's a taper of the squish-gap and that it's orientated in the particular direction which it has been found to be
pointed towards ! _ And I'm thinking that the Mark-III's
high-performance piston may quite-likely
intentionally exploit the functional feature which I postulate -(referring-to the "nozzle-effect"). _ And-also,, I can see where as the squish-gap is getting squeezed-down towards it's minimum space-gap (as the piston rises near-against the head), that the ordinary squish-effect alone then merely squeezes-out it's fuel-mix.volume with
gradually greater & greater
linear-intensity
until the nozzle-effect can next (rather '
superlinearly') finally fully-initiate it's
FINAL (even
further intensified)
BURST-
injection producing additional/
bonus turbulence-charge into the
THEN extra-turbulent/swirling burning-gases within the c.chamber,, (as a simple parallel squish-band/gap would-
not produce such an
extra-
kick at that last point [right-at TDC]) ! _ (
It's sort-of kinda like giving the squish-effect an intensifying magnifier-lens, isn't it !? _ And after-all, which would ya expect to most disturb the air within a bag,, blowing your breath into it with your mouth simply opened, or rather with your lips puckered [to create the "nozzle-effect"] ? 
)
So I'm quite inclined to believe that the angles of the squish-bands of the piston & head have been rather
purposely designed so that the present squish-
taper provides a rather
useful function (as Ducati must've intended).
(And besides, should we really think that the likes of ourselves are possibly near same as educated engineers who are well qualified to second-guess any intended reasoning established by the designers of the piston & cyl.head ?)
" 33 thou is fine but 12 thou is way too tight. "
____ Why is it that much less than .030" is "too tight" ?
I'd expect that a tighter squish-gap would increase the effectiveness of the purpose of the squish-effect.
What negative result could there possibly be with a squish-gap that's supposedly too tight ?
__ In a related case,, I once had a 76.4mm w-c.350-piston installed within a n-c.Mark-3 engine, and it just-happened to have a squish-clearance of near-about .009" (which I
thought was good). _ And it ran just fine for several thousand miles like that !
However after experiencing much hard running, it later began to develop a top-end
tacking-noise which eventually evolved into a considerably louder clanking-sound that finally warranted a rather extensive investigation. _ And it was then found that the con.rod-bearing had developed near-about ten-thousandths of slop, which had resulted with piston & cyl.head squish-band surfaces that had become as barren as possible with absolutely no carbon build-up whatsoever (even-though the piston-crown was found still well covered with a fairly deep layer of carbon-deposit !).
So aside from the disturbing noise of the piston making overly-positive contact with the cyl.head, (thus
no squish-gap/clearance),, the engine
still performed it's usual power-production, (as far as I could readily discern).
And I certainly can't blame the original overly narrow squish-gap for making the rod-bearing go bad. _ So exactly
what consequence supposedly ought-to be expected with a squish-gap that's left well-under the recommended 30-thou.minimum squish-gap ?
The only thing I can think of,, is that during engine warm-up, the involved eng.parts may expand at differing rates which may tend to at-least temporarily eat-up a fair portion of the preset squish-gap clearance. _ And-so possibly, a wider squish-gap is thus-
then required to avoid any possibility of piston-to-head contact (whenever the engine is no-longer cold).
Dukaddy-DUKEs,
D.Bob