YZ250f Big Bore

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Oct 15, 2003
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#1
I want to build a big bore 250f to run in the 250 class at the pro level. Having researched the various forums, there seems to be a question of how reliable the various combinations are. Also, both AMA and CMRC (Canadian) rules say that you cannot change the stroke. This seems to rule out the 302 conversion. Is the 280 up to the job, or does it cause rod bearing failures. We already had a 450f rod bearing fail!.
 

Rich Rohrich

Moderator / BioHazard
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#2
2mm over is the farthest you should bore & replate if you expect reliability similar to the OEM parts. Anything beyond that will dramatically increase the frequency of crank/rod maintenance.