Rebuild Question

I have a 01' WR426 that I bought used and it runs good but id like to rebuild the top end this upcomming winter. for two reasons, so I know its new, and just in case it was maintained poorly by the previous owner(s). I want to go with a larger bore/piston, cams, valves, springs, etc. My question is will I need to get a forged rod if I run a bigger piston? with the engine work as mentioned will the stock carb. with a jet kit be sufficient? Will a new timing chain be nececcesry? Iv rebuilt motors before but not on the newer bikes..I like the vintage stuff its all so SIMPLE! Just trying to get an idea of everything ill have to purchase. Thanks!

You can use your stock rod just make sure its within specs still. The stock carb is also fine with a little jetting/ tuning. Search for jetting specs on here as others have done the 444cc kits before and said what they have as far as jetting, but your elevation and temp/ humidity might be different from theres. I would suggest going with the stainless steel valves from a 2000 YZ426 as they are far cheaper and will last probably as long if not longer then titanium, search for info. on that as well. You will 00 yz springs and keepers as well along with spring seats. Do some searching on ss valves and 444cc kits and you will find plenty of info. in this forum and the yz400/426/450 forum. Definitely replace the timing chain as well!!!!

Thanks alot. I can just get springs and use the same valves as long as they are in good shape and I get them re-seated correct? and how bout just getting a larger piston and gettin the stock cylinder bored rather than a kit with a new cylider. I know thats what I did with my other bikes, is it the same with this one?

Do not lap titanium valves whatever you do. When I rebuilt my top end I just used my stock ti valves because they were well within specs so I just cleaned off the carbon as much as possible with degreaser and a scotchbrite pad. I tried not to scratch too much in fear of going through the ultra thin hard coating. It's worked so far. You can bore the cylinder but you have to get it replated as well with Nikasil coating. Not sure what size pistons are offered other than the 444's 97mm I believe it is.

If you are worried about the top end, replace the valves. A good indication of when to replace the valves is when you have to re-shim them the third time, it's time to replace 'em. The SS valves, springs and keepers are cheaper and more durable than the Ti valves, just be sure you replace everything. Using springs made for lighter Ti valves will cause the heavier SS valves to float. As Cali says, don't lap the Ti valves, it will scratch off the hard coating on them and they will mushroom in about one good ride. If you do replace the valves, be sure to get the seats recut. The new valves will last a lot longer if you do!

As far as the big bore kit, Lukes Racing and Eric Gorr's forward motion are the hot ticket. You will spend more trying to do it all your self (buy piston and gasket kits, bore the cylinder and have it replated) than either Luke's or Gorr charges for the kit. I have a Luke's kit on my 01 WR426 and love it.

Just out of curiosity....

I have always heard/been told NOT to lap Titanium valves, but the funny thing is, my manual for my 04 450 actually tells you to, and even how to do it?

When I did my top end, the valves were good, and I just cleaned them up.

Is the manual wrong?

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