increasing m42 bore size

[-318is-]

ive searched about but there's not alot of information, or at least i cant find much, so im going to come right out and ask

im drawing up a plan to increase displacement by means of an over sized bore. and maybe while im there fit lower cr pistons if i go turbo mode! or slightly raise the cr as ill be having a bigger bore. now question is
 
how far out can you bore a m42? or whats the biggest piston that will fit the conrods of the m42 and still be safe as a over bore

is there a usual oem part used when increasing bore/displacement?

and lastly will it run ok if i use a high lift cam and bosch 204cc injectors to provide the extra air and fuel it needs with the motronic tweaked, i understand increasing bore will not affect timing so shouldn't need the standalone, but rather just a little more air and fuel as above.

is there any objection to my theory on fuel/air?

[dude8383]

I think the safest you can go is 86mm... but some guys have gone a bit further. Anything past 86 though, and don't have much cylinder wall left between them.

[M5wanaB]

Use some pistons out of a e36 M3 with the S52B32 engine, basically any M3 engine from 96-99. You will increase the bore to 86.4mm and you will lower your compression to 8.5to1 or so. I would do a little more math to get the CR just right but If you deck the block and head a little to straighten them out yous should end up around 8.5-9.0 to 1. This would be perfect for a turbo application, you would also retain the M42 crank which is forged, I think you may want to swap rods too thought, I know the M50NV rods are pretty hefty and should handle some good boost.

[-318is-]

cheers, i was under the impression the m42 had forged crank and rods, and the pistons being the only weak point along with the headbolts. which would need arp bolts to stop the head lifting. that being over 15psi, even seen some youtube vids and thread stating there running 7-12 psi on stock internals, and another saying he rund 7psi stock internals, stock ecu but chipped, then running 19lb Bosch injectors

so these s52 pistons, do they have the same measurement as the m42 ones from wrist pin to crown?

if i decide not to turbo. as i can easily do this later (may have to replace head if i skim to raise cr on non turbo setup) so what cr would be best.

looking around i may even start with a m44 engine as it has a slightly longer stroke and if im right 86.4 bore would make it shy of 2.0L altho i have seen some thread saying 2.2l maybe that was some thing else

you say the s52 pistons give a low cr, are we talking skimming the head loads to get the cr back?

[Warsteiner]

US S52 has a 31mm CH. M42 has a 31.65mm CH.

Not sure what kind of compression that will give you.

You really have to plan everything from your intake to your exhaust and anything between:-)

Cheers,
~Ralph