The magic is in the piston. You can use the piston from the 450 (actually, ’04-’05 YZ450F & ’04-‘06 WR450F piston marked 5TJ) with the 400 connecting rod (5BE). They both have the same pin diameter (18mm). Since the 426 piston pin bore is 19mm, a 426 con rod is needed in order to use the 426 piston and jug with a 400 bottom end.
Parts required for the conversion:
1x 5TJ-11631-00-00 PISTON (STD)
1x 5JG-11603-00-00 PISTON RING SET (STD)
1x 5TA-11633-00-00 PIN, PISTON
2x 93420-18082-00 CIRCLIP
1x 5JG-11181-00-00 GASKET, CYLINDER HEAD 1
1x 5BE-11351-00-00 GASKET, CYLINDER
1x 5JG-11311-00-00 CYLINDER 1
BTW, I've had this setup for a few years with no problems. Today it’s time for a new 5TJ piston and rings.
Edited by umu24013, January 16, 2012 - 01:19 PM.