'99 YZ400F Rebuild

I've got my bike all torn apart, and I'm looking for suggestions on a couple items:

Fork Oil

I went to the Yamaha shop, and he basically said I was a dummy for using anything but his $25 bottle of "01" stuff, which I would need two of. What do yall use: brand, weight?


I'm looking for durability here, I ride alot of hare scrambles and stuff, which promptly trashed the renthal and no-ring chain that my bike had when I bought it (used). I like the durability of steel, but can't imagine a $15 rear sproket being worth a hoot. I was looking at Sunstar, but none of the mail order places I normally use sell them. Sidewinder may be okay, but I've heard nothing but bad things about their service, and their prices seem pretty steep.


I'm looking for a high quality O-ring chain, and I'm open to suggestions

I'm lubing the swingarm, steering head and wheel bearings, changing fork oil, replacing chain and sprokets, and the ultra-soft chain buffer that wore out way too prematurely. Any other suggestions while I've got it apart? I've only had the bike since October, so it's still rather new to me.



for the fork oil I would use Mobil 1 synthetic ATF, it works better than fork oil because the weight is approximately 7.5w (which makes the mid stroke stiffer)! My friend recently did this and his suspension feels ALOT better now! MX tech i think also uses ATF and so do many other places.

Thumper racing sells some good steel sprockets that are reversible, Talon also sells some aluminum ones that susprisingly last a long time! I used to swear by DID chains until I tried the EK chain... this chain is as light as non o-rings (due to weight saving holes)and due to the weight savings HorsePower isnt robbed since the inertia is the same. The model I have is the EK MVK 520 X-ring gold chain. Oh yeah only replace your sprockets and chain as a set or the will wear out prematurely! Hope this helps



I get my kicks on a 2001' YZ426!

Friendswood, TX

I use Renthal all around (front sprocket, rear sprocket and Oring Chain). I have good luck with it.



99'YZ-400 (Mine)

00' TT-R 125 (Hers)

91' KX-125 (Son #1)

93' CR-80 (Son #2)

99' PW-80 (Son #3)

I always use Spectro or Silkolene(hard to find here) suspension oil, yes its expensive (about $12-$15 a qt.) and you have to buy 2 quarts and only use a little of the second one. I run 5wt oil with high level (105-90mm). A little more preload on the springs if your 180-200lbs will help also.

The only penalty a steel sprocket has is weight, they last a least twice as long as aluminum.

Definitely get an X-Ring chain, I run a DID and have just about a year on it with very minimal adjustments.

A couple of tips:

Remove the front chain guard (outer piece) but leave the case protector on, mud won't pack as bad, requires shorter bolts.

Get a flywheel weight (8oz.) if it doesn't have one, its a big plus offroad.

When you fill your forks, fill them all the way to the rim and siphon down to the level you want.

Don't panic when you see the solid lube in the bearings, just pull it out and repack with a good waterproof grease.

[This message has been edited by F-Pilot (edited 03-27-2001).]

Hey Jason,

I have a '99 as well. Be sure to check valve clearance if it is used. Lots of guys were intimitated by the valve train and neglected this: they tend to tighten over time so take a look. When you replace chain guide check look at weld on swing arm under guide. You may want to grind it down a bit as it causes a high spot pushing up guide (a CM mod). Check for play in rear wheel, depending on type and how much riding that has been done, you may want to replace rear bearings, it is easy and relatively cheap (around $45.00). Even a little play could factor into premature spocket wear. I would also change break fluid, especially rear, as this is often neglected and can greatly impact quality of breaking. It is also easy and cheap. Castrol makes a nice DOT4 with high temp range. Check fork seals when changing oil (may want to dissasemble fork for thorough cleaning), it would be easier to do them now with the forks off. Once again relatively easy and cheap. I know I feel better riding mine knowing that I have gone over everthing. Good luck and enjoy the bike. I was thinking of buying a new one but my '99 has been rock solid so I will wait another year (or 2).


Thanks for the suggestions. Does Thumper Racing have a web page? I looked them up on the web, and only found "400s of Australia".

I know that "regular" ATF has seal expanding properties that would seem to damage fork seals, guess that's not a problem with the mobile 1? Anyone else use ATF? I suppose it can't be too bad of an idea, or you already would have had 10 angry replies pouncing on ya...

I'm still looking for a good mail order supplier, who do yall use the most? I've had good luck with rocky mountain, but all they have as far as steel sprokets, as I've said, is $22/set "Primary Drive" units. May be okay, but I hate to risk a high dollar chain, since they have to go as a set.

On suspension setup: as far as a compromise between MX and woods, I'd rather have crappy suspension in the harescramble and good on MX, than vise-versa. As it is, it seems somewhat soft (I'm ~190#). Just thought I'd mention that for anyone suggesting settings (thx F-Pilot).

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