hee hee ,,,Kev now you`re really showing your age
Ok,,,pistons down the bore isn`t really an issue if you`re not chasing every single pinch of HP like as in a full house race donk, where we run comp ratios as high as 14:1 & piston sticking out the top of the block etc
Just """One""" of the reasons that it`s good to have the piston flush with the head gasket is so that the piston can shroud the gasket somewhat & creat less of a disturbance for the inlet charge & also protect the head gasket somewhat as well,,,minor worry in a road donk & basically un-noticeable on an engine dyno.
Yeah,,,there`s allways some carbon build up on every single piston engine around the top edge of each cyl,,,combustion does get down around the top of the piston (above the top ring) anyways , whether the piston is flush or low,,,& as things go, you just can`t stop that & there`s no "real" issue with that unless you`re running stupicly rich & then you`d have stuffed valve guides pretty soon & a very short engne (rings & bearings) life anyways...obviously it helps to have your engine running correct mixtures
the slight bit of """Dryness""" that occurs at the upper end of the cyl from pistons sitting so low in the block has not created any premature engine wear that i can measure on my 1510 or any other engine that i`ve set up that way.
My stroker 1510 engine has pistons down about 1/4 of an inch, it`s about to have a re-ring & bearing job & be shoved into ,,,well,,,one of my minis somewhere allong the line
not quite sure which mini i`ll shove it into at this stage really,,,most of my bricks allready have engines allocated,,,i was actually thinking of modding the 1510 donk to take another Bimmer Motorbike twin cam 16 valve cyl head , but in Naturally asspirated form with injection. Won`t be for a while tho as i`m going strong on the Monaco fastback with Bimmer twin cam turbo injection mini at the moment & also have some headway on the Turbo charged alloy V8 power plant , & am also still creating the subframe for the Yamah R1 powered thing, so i`m a little busy to bother about the 1510 roady donk at the moment
But still,,,don`t be too worried about pistons being low in the bores for a roady, just don`t run too high a comp ratio if you do
Most """decent""" race donks (as Doc said) run upwards of 12.5:1,,,,,,, & as i said earlier i commonly run 13.5 & 14:1 ...BUT!!! there`s no drama with pinging when you run lovely clean race fuel like Av-Gas & have it tuned to perfection,,,MMMmmmmmm,,,yummy tell your mummy
Combustion `chamber' was the fact the pistons stopped about 3/4" from the top of block.. 
