The difference can be seen in the photo but the biggest difference is the centre seal arrangement. The centre seal on the R2/R4 is a separate seal that sits beside the bearing (available from RGV spares here http://www.****.com/index.php?page=shop.product_details&flypage=shop.flypage&product_id=482&category_id=41&manufacturer_id=0&option=com_virtuemart&Itemid=26&vmcchk=1&Itemid=26
The R5/6 centre seal is an intergral part of the bearing and seals up against the web. Honda also put a thin Oring seal which sits in the thin groove you can see around the centre bearing.
NSR250 MC18 R2/R4 1988 Crank
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NSR250 MC18 R5/6 1989 Crank
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All the bearings are available from mixalisen who has already posted here but the R2/4 bearings come with a pin now to stop the bearing spinning inside the case so the cases will require a small amount of machining to accommodate the pins (R5/6 already has the pinned bearings). The Japanese rebuild here http://88nsr250rsp.web.fc2.com/crank.html
shows the look if not a complete explanation.
The outer crank seals at both ends (and the middle seal for both the R2 and R5 MC18) are also available from ****** at RGV spares.
Comparison MC18R5/6 and MC21 Crank Webs
This shows why an MC21 Crank is too big for an MC18R5/6. As dunkenb has said you need to machine down the webs by 2mm as otherwise its a direct drop in with the same bearings, seals, and rod dimensions. Given the spacing I would assume that the later R5/6 crank would not fit into the R2/4 cases but have not had a close look at this
Last edited by geezagillard on Wed Feb 10, 2010 7:29 am; edited 3 times in total