Mikey:
the 2004 rm125 has a longer rod, but it still has the same 54.5mm stroke and 54mm piston, i am yet to see a 04 but i am not sure how the cylinders will interchange easily with the older bikes. as you know, the 58mm big more makes 144cc. what year is the bike??? Last time i spoke with eric, he was building himself a 2002 rm155- 58mm piston with an extra 2mm stroke- its easy to do since the older rm125 (97-99 i think) rod is 2mm shorter so no spacer place is needed. I beleive Eric is getting into doing the stroker kits since the wiseco cranks are available and another company is doing the stroked versions on a kind of off the shelf deal, so the kits are going to be much cheaper than the elaborate work done by a certain shop located not far from glen helen raceway.
from what i understand, if you go any further than 2mm stroke, you run into serious issues with port and ignition timing and your pipe and ignition wont match up with that much increased stroke, and you will also have to wonder how the bike will rev since i am a bit sceptical of some of the 167 dyno curves that i have seen on the net. and then you have to put all of that power through the RM gearbox, asking for trouble IMO.
if Eric reads this, sorry for taking the liberty of answering on your behalf and please make any corrections. there has been a few threads on this before, so do a search on stroking since Eric did an article on this very topic and the rm case study. it is somehting i will consider when by engine gets its overhaul next season if the stroker kit isnt much more than a replacement crank.
and if Rich reads this, come on mate, spill the beans, i am sure you have had your hands on this 155 somewhere down the line:) give us the full scoop.
cheers, Bruce