YZ400 Rebuild

I am planning on rebuilding my 98 YZ400 soon and was just wondering if anybody had suggestions on what to look for or any insight to what else I might need to do.

I have only had it for about 7 or 8 months and it runs great except it has overheated the last few times out and it uses a lot of oil. I plan on putting a new piston in it, new clutch, carb rebuild & valve job. Anything else I should do?

I also was thinking about putting a crankcase filter on the breather tube that just vents out, has anyone ever done this?

Watch for the cams binding. On the 400's when the valves are adjusted, the cam caps tend to start binding, and the cams don't spin freely. Set the head on a bench (with a towel or pad under it) and reassemble it, cams, caps, etc. just like you would in the bike, torqueing everything. Then check your valve lash. This is much easier than trying to do it while it's in the bike. When my cams were binding, I sent my head to Engine Dynamics in California, they are super friendly and very knowlegeable. They also replaced valves, valve seals, seats, lapped the valves, and set the lash for me for about $300.


The 426 cylinder from 2000 (only 2000) will bolt on your 400 making it a 426. You will have to split the cases and have the crank machined to accept the 426 rod and journal bearing and have to add the 426 piston. (you will need 426 cylinder, rod piston) This is many times more reliable than trying to bore the original cylinder.


Replace your timing chain, it is cheap insurance. (about $30.00)

There are many other things that can be done to the 400. The aluminum subframe from a 426 is a direct bolt on and will shave a couple of pounds.

The radiators from an 04 - 05 450 will also bolt on and will add cooling capacity. (you will need the shrouds and fins from an 01 - 02 426)

These are a few of the things I have done to my 99 400 and they all work.


ericyzf4 pretty much summed it up. It might be cheaper to just buy a complete 426 crank assy than to get yours machined. The TT store has everything you need at really good prices. I'm looking to rebuild my 400 after the next desert season and am probably going with the 440 stroker kit from Powroll. Check your cylinder head, especially around the shim buckets. Lots of them are mushroomed out there and make it hard to remove the shim buckets. Mine is doing that, probably due to excessive torque on the cam caps. It still runs fine but I will need some machine work done. I have an extra head that I will build up to use and hopefully rebuild the original one as a spare.

Watch for the cams binding. On the 400's when the valves are adjusted, the cam caps tend to start binding, and the cams don't spin freely. Set the head on a bench (with a towel or pad under it) and reassemble it, cams, caps, etc. just like you would in the bike, torqueing everything. Then check your valve lash. This is much easier than trying to do it while it's in the bike.

This method has its advantages, but some pitfalls as well. Most of the time it works out fine, but you should always recheck the clearances once the head is on the engine and fully torqued up. Things can tweak just enough to change something.

Also, in setting the clearance on the bench, it is not necessary to torque the cam caps past about 40 inch pounds or so.

Which brings me to the subject of cams binding. This problem is often caused by the mechanic not correctly torquing up the caps. They must be seated squarely before being tightened, and not drawn down with the bolts, and they must be torqued in the correct sequence, and incrementally, in small steps, as a set, rather than tightening one completely, and then each of the others. I tighten them all to 45 in/lb, then to 60, then finally to 75 in/lb, with the threads lightly oiled. The specified 86 in/lb feels excessive, based only on my professional experience, and I have never had a problem with using 75 in/lb instead.

Thanks for the advise. I would like to try the 426 conversion but it's not in the budget right now so I will probably just do the basic rebuild and save up for the conversion later. I will definately put a new chain in it.

Anybody have any experience rmoving the breather tube and replacing it with a breather?

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Similar Content

    • By Zack Ailing
      hey guys just picked up a new project bike. Its a 2008 Yz450f with a big bore kit and a stroker crank. The total cc of the engine is now 499. It runs on race gas and will be an absolute monster when shes ready to rip. I will continue to make more videos on the bike so stay tuned Link to the video: 
    • By High On Octane
      2005 YZ450F, Athena Big Bore, Power Bomb, Q4.
      Ok.  Last fall after a decently hot day at the track, I noticed coolant leaking from the weep hole on my water pump after pulling into the pits.  Ordered an Athena re-seal kit, put it all back together and didn't really get a chance to ride it.  3 weeks ago, went out to the track, and after about 20 laps, it started leaking again.  Kept filling it with water to get thru the day, never ran anywhere close to dry or empty, jut a cup or 2 low.  Ordered a Hot Rods re-seal kit with bearing and also ordered a new w/p shaft.  Did it all again and got it back together, ran it, and as soon as it got to temp, it was leaking out of the weep hole again.
      So, what the heck am I missing?  I'm totally stumped on this one.  Is something wrong with my case cover?  Do I need to replace the w/p itself?  I'm feeling like I should've just ponied up for the Boysen in the first place, and now it's biting me in the butt.  Any help or suggestions is greatly appreciated.  Thanks!
    • By Awheeler25u
      Alright guys, my 04 YZ450f runs fine minus some slight backfires when decelerating (need to adjust fuel screw). The other day when I was done checking my valve clearances I changed the oil and filter and put the bike back on the stand, kicked it a few times without the ignition coil in to get some oil flowing got off and noticed a little drip spot on the ground, it was no bigger than a dime so I thought nothing of it. Started the bike with no issues took it for a quick 10-15 min ride, came back to the garage to find out my whole case was soaked with oil and it started pooling in the low spots on the clutch basket side. I cleaned all the oil off and took it out again for 5 mins to try and see where it was coming from, here are my three possible places but nothing directly leads me to believe it’s any of the three either.
      1. Decompression plug area
      2.  Valve cover gasket leak (not really sure because the amount of oil leaking Looks to be a large amount
      3. Oil tank (bad weld or cracked weld somewhere
      what are you guys thinking?
    • By bofar
      Hey guys. I could really use your help diagnosing an issue that came up riding today. Bike is a 2013 YZ450F. Well maintained and haven't had any issues until now. Both the oil and coolant are new and levels are to spec. 
      After riding for about an hour in the sand dunes, I noticed a grinding going on on the right side of the bottom end (from what I could tell). When I looked to see what was going on my kickstarter was vibrating and grinding really loud. This was a different sound than if you engaged the kickstarter while running. It sounded less "clanky" and more "vibrating/grinding" if that makes sense. Grabbing the kickstarter and moving it didn't help the issue.
      When I noticed it, I shut the bike off and gave it a second. Being a few miles deep into the dunes, I had to get it out of there so I started it back up and it seemed fine- no grinding. I headed back to the car and after a few minutes, it came back so I shut it off again, started it back up and nothing. That process happened one or two more times before I made it back to the truck. Seemed to run fine otherwise. 
      Once I got it back to the truck I started it up and tried to pay more attention to what was going on but it wasn't doing it. I did seem to have more of a metal on metal noise coming from the motor. I started and stopped the bike a few times and after the motor stopped there was a distinct metal clank/grinding noise that happened right after the motor shut of for maybe half a second. This doesn't happen every time. 
      I tried doing some research and have seen things that make me think it could be the spring that connects to the kickstarter, or the kickstarter stopper plate- but really I have no idea.
      Seems strange that the issue is off and on.
      Thanks for the help!
    • By Goof331
      Does anyone know if there is any differences between the 2 radiators? Will the yz450fx radiator bolt on a yz450f? What I am wanting to do is wire up a fan to my 2016 yz450f. I know how I am going to do it I just don’t want to use a “universal” fan kit. What I would like to do is get a 450fx radiator and the direct bolt on Trail Tech fan for that radiator to have a good clean look.