info on topend

[desertfox89]

hay i was wondering about what the average hours are on valve ajustments,or replaced.also replacing pistion and rings. i have 2002 426 and have about 40 hours on the bike just checked valve clearance and everything was in spec. i was just wondering iv heard storys of bikes top ends going out with little hours on them and also heard of them lasting a lot longer with out a top end rebuild.might be a stupid question but just wondering what kind of hours some of you guys are getting.

[grayracer513]

YZF's in general hold up better than the rest because Yamaha stepped up and used the stuff their engineers told them to.

My 450 is still in spec, and has never been reshimmed. It certainly has more than 40 hours on it. My 250F was just rebuilt (top only) at something like 400 hours.

The life cycle of the valve will go something like this: Sometimes maybe a little tightening in the first hours of operation, followed by an extended period during which the valve clearance will not change at all. This may be multiple years. Eventually, it will need a reshim of some of the valves, and the clearance will again remain stable for perhaps 50 hours. Then one day, it will suddenly get to be hard to start, seem not to have its normal cranking compression, or you'll find during a clearance check that it's lost .10-.15mm clearance. You'll reshim it, and it will need to be redone again in less than 20 hours. It will then be time to replace them, as that will be an indication that the coating on the valves is worn through.

Something you can do as a preventative measure to avoid other damage, is to replace the timing chain every 1-2 years. It can save you replacing a crankshaft or possible catastrophic valve damage.

[MichiganMXer]

I've got an 01 426, with a lot of hours on it. Starts on 2 kicks, no black smoke..and I mean none. Idles great, doesnt bog and I've never cracked the head on this thing.

I seriously considered opening it up and doing it myself but I might do more harm than good. I'm gonna wait for some symptom to come, but i'm still waiting.

Knock on wood.. ??

[642MX]

426's are bullet proof. You may never need to do a top end job unless you have extremely high hours on the bike or you are very very hard on it. Keep an eye on the valve clearence and change your oil every 5-10 hous and it will run a long time.

I got scared and sucked into all this top end replacing stuff too. So I opened mine up to find very little wear on the piston and rings. So I went ahead and put a 13.5:1 piston in and the engine has more rattles in it now that it did with the old piston.

[desertfox89]

thanks for the info its nice to no i got a bullet proof bike as long as i take care of it. it kinda gives me a good idea on time to check valve ex.reason i asked is my buddys bike had about 40 to 50 hrs on his 450 and had to replace the hole top end 1000 dollars later not going to mention what brand it was.