How much can you shorten them by before getting into bother? I'd like to raise the compression of my 8.5:1 pistons to around 10:1. Presumably I'll need to have the carb rejetted as well, and advance the ignition. All advice welcomed!

Why 10:1, as that would probably mean using octane booster as well as 97 or 98 grade fuel?
Also, raising the compression usually requires that the ignition advance is reduced, not advanced, because the rate of flame spread will be faster.

ken
p.s. I doubt that 10.0:1 is possible with 8.5:1 pistons + shortened barrels, unless you choose to ignore the sound of pistons hitting the heads...

Fair enough. I don't know why I've got 10:1 in my head, I'm sure I read it somewhere. But let's say I had my barrels shortened by 0.5mm each. Is that feasible? Would it give a useful increase in power? Or is none of it worth the bother and should I just build myself a good spare engine with no mods?

just build a good engine

decent barrels new rings, if you need to buy a replacement set get the 9:1s

dress the oil pump cover if its not too badly scored, get a new one if it is

get everything set up well sort the mating faces of the manifold make sure its got a good insulater under the carb,
do the valves and guide oil seals

take the primary jet out to 107

set the timing to 32* max advance and it will be tipitytopity

Sean