My gen 4 at 4,000km needed all new valves. Thereís no warning, the top end will all of a sudden sound like shit.
My rebuild schedule is bottom end every two years and top end every two years alternating so itís not one big bill at once. Stretching it out until failure will result in a single very large repair bill. Good luck.
I plan to do a full rebuild on my zx10rr at about 4000km, the Thai Kawasaki Sbk team changes valves every 2000km.
If you keep the engine completely stock then they seem to last pretty good but once you add a little extra (race cams) then they donít hold up so well. My teammates just had to have their BSB spec engines completely rebuilt, one lasted just over 4000km and the other bent a valve just under 4000km.
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