I have used a 91 top end on a 1998 bottom end. you just have to do a little coolant line work since the 91 head has 2 coolant spouts instead of one. I would still double check your piston to head clearance, port timing, and so on. the 91 motor had a shorter stroke, my 91 and 98 cylinders were the same length and I was raising the ports anyways, but that all should be checked.
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2004 CR250
1984 YT60 Tri-Zinger  1991 KX500 Hillclimber 2004 custom chassis.  1998 KX310 Hillclimber  1998 CR250 Hillclimber  |