I have a 99 YZ400 and whilst riding the other day I was in 5th just cruising the bike locked up.

I tore down the engine yesterday to find the conrod has failed just below the small end. It has gouged out the cylinder, destroyed the piston and on splitting the cases I have found 4th gear (input and output) shattered on a couple of teeth, a shift fork rod has also come loose causing the lugs for the rod in both cases to break off. I imagine this would mean both cases are now junk.

On further inspection the crank around the big end area has gone blue either side of the big end crank pin.

My question is, what does anyone think has happened. What has failed first and what the hell should I do. Repair or New Engine.

Just for info, I don't race the bike, It is regularly serviced and looked after extremely well. It hasn't got many miles on it also. Has anyone heard of this happening?

From your description, it appears that you have a tendency to rest your foot on the shifter. At the very least, I suspect that you tend to "slam" the shifter. Think about you tend to "jab" hard with your foot when you shift? If you do, you probably have bent a shift fork. Some riders are gentle with their shift levers while others are brutes. The "brute" types have problems with their tannies.

Boit - where the heck have you been hiding?

Didn't you do a rebuild this past year?

I recall some input from you pertaining to bent shift forks, gears, ect.

I have that ticking noise inside the left side cover right at the C.S./sprocket, so I'm planning to have a look this Winter to see if I can figure out what's happening.

Also, I think I better replace the CB key - was there ever a confirmed material / source for the harder stuff?

I have an '02 - very low hours.

Happy Holidays!



A blued crank points to severe overheating of that part of the metal. I would not think that this is normal for this motor. The only thing that I can think of which would allow that part of the crank to get so hot is lack of lubrication. Bad gas or lean jetting can contribute to overheating in general, and maybe even blueing of the wrist pin and top of the connecting rod, but for the buttom end I would guess that lack of lubrication would be the most likely cause. Maybe the next step would be to look closely at the oil system and see if you can find anything wrong. I'm not a pro mechanic so hopefully someone who is can provide more info on this.

If the crank turned blue its definitely an oil related problem, how was the oil pump? filter? level? condition?passages to crank? If the casees are not damaged in a bearing area you may be able to get the cases "arc welded". Good luck man . Hey that Hayabusa kicks ass!!!! :)

Well I do shift hard I suppose, all those years of road racing. How does this theory sound :

4th gear tooth snapped off during hard shift or landing, piece of gear takes out shift fork causing this to snap off lugs from cases, all debris some how fouls oiling system overheating big end and ultimately whole con rod overheats and fails at weakest section (just below big end). Result - BANG. Any other suggestions.

I just don't want to put it all back together and do it again.

And yeah the Hayabusa is incredible, for any of you guys that haven't ridden one I strongly recommend it, even if you hate road bikes just try one.

Boomer: I'm still around annoying people. I haven't had to do a rebuild yet as I've never experienced any mechanical problems....yet. My counter balancer key hasn't even caused a problem.

How the heck did you find BOTH of the blue bike faults at one time?? After breaking the rod in my bike (small end failure), I did a lot of calling around. The general feeling was just fatigue. The 400's had a flawed design, a rod that after time, and not much time, will break. That is why they redesigned it in 2000.

Here is my theory on your bike...while under a great load, a transmission gear broke. Just enough debris got between gears to cause the engine to ubruptly stop turning, and snap goes the rod. At 10,000 rpm it would not take much.

As for the rebuild, I was advised by everyone I talked to to go with 2001 or 2002 parts(crank, rod, piston, cylinder, head, valvetrain, ect...). At the desert races around here, fast guys on 2001's were still breaking transmissions. I decided to scrap it and bought a KTM 520 instead :) . It (the YZ) still sits in my garage in full desert trim.

Talk to the fine folks at Montclair Yamaha, I'll bet no team had more experience blowing up YZF's than they do. When I mentioned my bike blowing up on this board, many questioned my mechanic. After speaking to Montclair Yamaha, and Yamaha themselves I question the design.

'Busa, I don't think your scenario happened. If the pieces of tranny bits broke off, you'd know it way before there would ever be a lubrication related failure. There is a big screen right at the oil pick up point in the bottom of the cases. I'm sure you saw this when you split the cases. When you say the rod failed, do you mean it seized on the crank big end? Or did it break halfway up? How often do you change the oil? What oil do you use?

The cases should be able to be heli-arced. I'd definitely go with a rebuild instead of a new motor.

Two other options you need to think about. A 426 rod has a 1mm bigger diameter than a 400. The rods are interchangeable. Since the cylinder needs to be repaired, a big bore kit would be a feasable option at this time. A 2mm overbire is known as the 420 kit (416cc actual) and makes a big improvement in overall power. If you go with teh 426 rod, you can go with a new 426 cylinder and piston (bolts right on a 400 bottom end). And a 3rd option would be boring the 426 cylinder 2mm and making it a 450 (444cc actual). This is what I did with my WR 400 and it is similar to driving a V8 powered Pinto. Incredible boost of power from idle on up. It is then a *race fuel only* motor but, like Toyota says, "oh, what a feeling!!!"

Just presenting a couple upgrade options that would make sense right now.

Thanks guys for responding.

Just to answer a few questions I use Mobil 1 15-50 and change without fail every two rides so bad oil shouldn't be an issue.

The conrod has snapped just below the small end. The big end has NOT seized onto the crank pin but the crank is noticeably blue either side of the crank pin (I haven't taken the conrod off yet).

As for all this happening at once, I'm just lucky I guess. After all I wrapped my first Hayabusa around a pole and couldn't even salvage small bolts. :)

I give the heli-arc a shot, this should save me big $$$$, is is reliable?

MX Tuner - Thanks for giving me some options, I like the 426 idea. In fact I think that would be the go with the bigger rod. Thanks again everyone.

??? About the rod failure, was the small end seized or galled?

Was the cylinder seized or scored?

Could the loose end of the rod have stabbed the transmission and caused that damage?

Got any pictures?



The small end wasn't seized or scored. The gudgeon pin was still in good condition however it had broken free from the piston lug (piston lug cracked).

The cyclinder wasn't seized however was badly scored (due to broken rod) I believe by looking at the length of the rod it couldn't reach that far over to the transmission to hit the gears.

Don't have pictures yet but if you think it might help I will go to the shop and take some.

Thanks again

Yes mx tuner is right all 426 parts will work. You can use your cylinder and sleeve it like mine. You can fix the cases to.

yz454 bored and stoked long rod.

A sleeved cylinder isn't as reliable as a non-sleeved cylinder. When Thumper Racing first came out with the 450, they had a lot if issues based in this. A new cylinder isn't very expensive,plus a plated cylinder will last much longer than a sleeved cylinder. When your sleeve wears out, what do you do? There isn't an oversized piston for a 97mm bore.

YZ454, is yours a 97mm bore? 97mm figures to a 444cc, not a 454cc.

Originally posted by MX Tuner:

YZ454, is yours a 97mm bore? 97mm figures to a 444cc, not a 454cc.


The big problem with the 426 cylinder is there is no hot start attachment, it is on the carb on the 426.

Originally posted by hayabusa:

The big end has NOT seized onto the crank pin but the crank is noticeably blue either side of the crank pin (I haven't taken the conrod off yet).


This is an interesting post, I wish I’d seen it sooner. My ’00 had third gear explode, this spit a shift fork into the crank, resulting in a bent rod and damaged crank. It sounds like a slightly more extreme version of the same thing may have happened to you. In my case the rod did not remain lodged under the crank, I could still spin the motor (but not the tranny), but it was easy to see where they got together.

When I got the new crank it already had those same marks you describe on either side of the con rod. Maybe they heat the crank when they press it together at the factory.

My total parts bill was about $1,000. I got a new crank assembly, cases, piston, four gear wheels, the fork, and (almost) every bearing and gasket in the motor. I also went with a 97 mm piston and bored/replated cylinder. I think I paid Eric Gorr like $350 (not included in the above $1,000) for the boring and plating work. You will need all the help you can get to install new bearings in new case halves if you go this route, I used a ten ton press, my torch, & my freezer, and still had a hard time coaxing all the bearings in there.

I also replaced both fourth gears as they had chips on the load-bearing face of each tooth. I thought this was a ’00 issue only, but it sounds like your ’99 has the same problem.

The expensive thing is the crank assembly, I paid $300 for a ’00 part, but remember this includes the rod and bearing (and it is already installed).

Also, Tuner makes a good point about the 426 parts, especially the cylinder. Unlike a two stroke cylinder with its all-important ports a four stroke is just a hole with water jackets, they are only around $250, brand new. Cheaper than boring and plating your 400 cylinder.

Sorry for your misfortune, good luck with the repairs.

I need to add that I'm not sure if the $350 for the bore and replate is accurate, maybe it was $450. But the price included a 97mm Wiseco and rings.

So the $1,000 in parts I mention did not include a piston.

Eric Gorr 920-356-0741

Thanks again people for your info.

I am looking at buying 426 parts after the new year so thanks to you guys I might have a better bike after all this S**T.

What do I do with the hot start then if the 426 doesn't have it on the cylinder because I will be running the 400 carb (I imagine).

The hot start isn't part of any cylinder. The hot start on a '98 and '99 YZ is part of the air boot and intake manifold. The '00 and newer is only in tha carb. The hot start is a non-issue when thinking about swapping cylinders.

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