2001 Yamaha YZ426F project

Hi everyone! After a long time lurking and getting a ton of useful information, I decided to join the forum. I have been wanting to put a dirt bike together for a long time. The last bike I owned was an 89 YZ250 and I had a blast on it. A buddy of mine knew I was tentatively looking for a new project and had an 01 YZ426F he had left at the bike shop to be repaired due to grenading the motor. He got tired waiting for the shop to get to it so he picked up and locked it in the garage to live (kinda) the rest of its days. He sold the bike to me for $400 which I felt was more than fair and he needed the money. I know the parts will add up but I like bringing old machines back to life and enjoying them.

So I picked up the bike today and it was actually in pretty good shape! My wife and had a joke that I was buying a motorcycle in a box (which was partly true). I pulled the head and cylinder off and just as suspected the cam appeared to have skipped a tooth sending the valves into the piston. Luckily when the engine failure happened it didn't totally lock up and he was able to shut it down. I pulled the valves out and a couple were bent. The good news on the head is there was no damage to the head itself! The seats look perfect! Also the cams were replaced with the auto decompression setup so that's nice.

The cylinder and piston are total losses... I pretty much expected this (and much worse) when I heard the term "grenaded the engine."

The connecting rod is bent but it amazingly did not damage the case thank goodness! I checked the movement in the crank itself and it feels good. No up and down play and only a little side to side play.

So here's my plan so far:

I am going to replace all of the valves, springs and valve stem guides. One question I have is I know the 01 came with factory factory titanium valves but would be alright to run stainless due to being a fraction of the cost. I understand I might lose some response because of the heavier material but I'm not racing or anything and this bike will most likely have more than enough power and responce for me.

I am going to try to pick a good jug that I can just run a hone through and a new OEM piston.

For the bottom end I'm thinking about buying a new connecting rod kit and having the machine shop set it up for me.

I know this project is going to cost some money but I am going to spread it out over time. I think the bike is worth it and will be a fun ride to just put around on. Besides my son loves helping me out! He's helped me build from the groun up a couple old waverunners and an 80 Arctic Cat sled.

I really look forward to showing my progress on this forum!

Thanks for your help in advance!

The stainless valves will cause no detectable loss of performance. 

Thanks! I ordered my valves today! I'm looking for a set 2000 vz426f valve springs because I've read that I need to run 2000 valve springs with stainless valves.

I'm going to get the head completely redone then I'll move on to replacing the cylinder.

I am trying to decide what I want to do about the bottom end. I'll ether buy a rod kit and rebuild my crank shaft or look for good condition WR complete lower end because I'd like to use this as more of a trail bike. I've read the first gear on the YZ is too high.

 I've read the first gear on the YZ is too high.


Too high for what? 

People made it seem that the first gear is pretty high for slow tight sections of trail. I haven't ridden one of these bike so I have no first hand experience.

That would be true, and if you're going to be doing mostly trail riding, that's not a bad approach.

Well I just picked up a really nice complete bottom end out of a 2002 WR426 so this will check off two more items on my list: A wide ratio gear box and tight working crankshaft/rod assembly.

I'm still looking for a good cylinder and piston.

What piston do you recommend? I've seen several different manufacturers that make them for the 426. Are there any brand I should stay away from? I'd like to stay with the factory compression ratio.

What piston do you recommend?... I'd like to stay with the factory compression ratio.


Yamaha OEM.  There is no better piston for the price with  the stock compression ratio.  (They're forged, you know)

I finally figured out how to post pictures. At least you can see what I'm working with! My new WR bottom end came in and it looks brand new. The crank feels like it was recently changed or never ridden. The connecting rod is straight too which is nice! The one that came on my bike had a nice 45* bend in it. My replacement valves came in (the intakes are only shown in the picture) and they fit so much better than their bent counterparts. I'm going to give the cylinder head a nice soak to clean out all of the carbon. The new valve seals came in as well but I'm still waiting on the valve springs and new cam chain. I'll at least have my cylinder head back to 100% soon!

I'm still keeping my eyes open for a reasonable replacement cylinder because all the used ones I've found are either hammered or the price of gold. I might have to bite the bullet and buy a new one. I did find a factory Yamaha piston kit for only $130 which seems reasonable.

Does anyone know what the little aluminum valve on my handle bars is? I posted a picture of it in the last image.










Where does the other end of the gizmo connect?  Looks like it could be a bar mounted hot start kit, but I don't recognize it.

It is hooked to a clear tube that may have ran to the carb. The carb was out when I picked up the bike. Where would a hot start valve attach? What does it do? I'm coming from two smokes so these are new to me.

On a 400F, the hot start would be an air valve that would connect to the intake port at the head.  On a 426, that device is built into the carb and has a pull knob as does the choke.  A bar mount hot start for a 426 would be a small lever with a cable running to the carb.


The hot start provides an air bleed to create a leaner than normal mixture at idle to facilitate restarting a hot engine, as they tend to "flood" for the smallest reasons when at full temperature. 

That's the Terrycable universal remote hot start. 

Thanks! It seems well made. Now all I have to do is figure out where the line attaches.

The last of my exhaust valves and my Clymar manual came in today! I'm going to get the valves lapped and once my valve springs arrive I'll get the cylinder head back to 100%. I'm still waiting on the right deal on a replacement cylinder.


What about stainless valves? I figured those were ok to lap.

Edited by 98yota

Some are, and some aren't, because some of them are nitride coated in the same manner as Ti.


In the first place, valves and seats don't wear evenly.  Their sealing surfaces must be completely circular and conical, and the interfacing surface must be at a precise angle.  As they wear against each other, the angles drift, they wear out of round, they become eccentric, and instead of being a nice "flat" slice of a cone shape, the seat typically becomes wider and rounded over, and beats a rounded bottom groove into the face of the valve.  So then, along comes Shade Tree Freddy with new valves an some lapping paste, drops his new valves onto his beat up seats. Yes, he will eventually get them top match each other, but the seat will still not be flat.  He will have lapped down about half the wear on the seat, but he will also have transferred the wear pattern of the seat to his new valves.  That, and the seats will be far too wide.


The ONLY right way to replace valves is to have the seats either cut or ground by a machinist using tools made expressly for the purpose.  By doing this, the seats can not only be restored to their correct round/conical shape, the can be narrowed to the right width, and moved up or down the face of the valve by adjusting the 30 and 60 degree angles above and below the seat itself.




My valve springs came in today! I finally have all brand new parts to get this cylinder head back together.

As far as the cam chain goes, is factory the best option or does someone make one that lasts longer?



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