Sorry, I'm too late in joining this thread
Just an idea, why don't you use the M47N (M47TU) crankshaft?
It has 90mm stroke compare to M47 which has 88mm stroke, both crank has similar design
By using existing piston and 135mm rod (M42, M43 & M44 has 140mm), you'll get 1995cc and 1.5 conrod ratio
You can also use 86mm diameter S50 B30 Euro M3 piston and you'll end up with 2091cc
You're right, this would be possible.
This configuration:
* M47N 90mm crank
* 135mm Rods
* Standard pistons.
Would have the piston sit 0.36mm below the cylinder deck.
This is not good for your squish effect. The stock M42 has
the piston protrude 0.15mm above the cylinder deck
Also you have 9mm more stroke with the standard piston,
and combustion chamber, which compresion ratio (CR) do you
think this would yield ? Well you could probberly machine
the pistons to lower the CR. But this would weaken the piston
crown, and i would think that as theres a lot of material removal
thermal problems might arise. As the "heat sink" capabilities would
not be as good with less material. I would not be comfortable with
this during sustainéd high load. Well these prblems with the piston
might just be in my head. But then again they might not.
About the crank, sure it could be made to fit. As the M47 it
needs some machining. But the shank is not the length it needs
to be so this would have to be lenghtend. So the shank would
end up being a two piece design. I dont like that. Try and look
at the images in Metric Mechanics M42 booklet. You'll see what
i mean there.
All in all i think the M47 crank, 140mm rods, custom pistons are
the best way to go. Simple crank modification, together with
good rod ratio. And with custom pistons you will have the luxury
of being able to select your CR.
Sorry if i come off as being defensive, i'm just explaining the pro's and con's.
as I see them..