Why would you want to put a 4 stroke motor into an old frame??? Why not just buy a YZ450F if that is what you want? The YZ250 with a bigbore kit like the 285 is a huge improvement in the powerband.
Welcome to E2S.
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My bikes
Foster Bike
00' Banshee (Shit my buddy don't ride it!)
84' FXE Shovelhead
72' Norton Chopper
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