Just my personal view Jamie, but rather than have pistons protrude from the block I'd have their crowns skimmed, there's easily scope to remove 10 or 20 thou from a "Seven Workshop" piston. On a 3-bearing engine I'd run them flush with the block, on a 2-bearing I'd allow another 10 or 20 thou for crank whip (e.g. if you miss a gear at speed).
An alloy shim plate is a neat solution, but if unavailable I have in the past doubled-up paper gaskets to get a bit more thickness. It didn't leak any worse than any other arrangement I've tried. Typical paper gaskets compress to around 15 thou thickness in use.
On my 3-bearing engines (with Reliant con rods) I use rather more substantial spacers i.e. 1/8" or so. These are hand-made from sheet aluminium, I think it would be tricky in anything less than 1mm thickness. Keep in mind when inserting spacers you need to watch out for pistons hitting the deck at bottom of stroke too.
An alloy shim plate is a neat solution, but if unavailable I have in the past doubled-up paper gaskets to get a bit more thickness. It didn't leak any worse than any other arrangement I've tried. Typical paper gaskets compress to around 15 thou thickness in use.
On my 3-bearing engines (with Reliant con rods) I use rather more substantial spacers i.e. 1/8" or so. These are hand-made from sheet aluminium, I think it would be tricky in anything less than 1mm thickness. Keep in mind when inserting spacers you need to watch out for pistons hitting the deck at bottom of stroke too.