YZ426 Catastrophic Failure


22 replies to this topic
  • MNellis

Posted February 15, 2011 - 06:03 PM

#1

Let me digress for a minute. Back in late 2005 I bought a used YZ426 from my endurance racing team mate who kept meticulous records and the bike appeared to be in great condition. I wanted to get back into dirt riding to race scrambles and improve my road race fitness.

Half way through the first day of riding the motor started smoking heavily so my riding was done. I tore the motor apart and found the piston scuffed, the cylinder heavily worn and the valves bent. The rod was shot and allowed the valve/piston to contact each other. I put in new piston/rings, crank and cylinder in and went riding. About 10 hours of riding later the motor dropped a valve and shattered a shim. *crap* Stuff happens I guess. I bought a new head, valves and piston and got it going again. Two races later in early '06 I had another catastrophic failure. I couldn't tell what happend but there was a big hole in the front of the case. I was so irritated and pissed I stuck the bike in the corner to focus my energy, time and money on road racing. I didn't even take it apart and it has languished there ever since.

Well, a lot of time has passed. I stopped road racing about 2 years ago and got into playing with the little colledction I have now. I've still got the road racer ('04 GSXR 1000) for track days but riding around on old bikes, while fun and relaxing, is a little mundane.

I decided to pull the YZ426 back out and give it another go. My wife bought me a complete lower end for Xmas (with crank) that appears to be in good condition (rod bearing wise). My intent was to move over all the good pieces and get the bike running again.

I pulled the motor out of the frame yesturday and here are some pictures of what can be seen from the outside. Initially I didn't notice the smaller hole in the cylinder.

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Through the hold in the case I could see that the rod broke in half. It was time to pull the head and cylinder and see what was going on. Here is what I found.

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Everyone keeps telling me these things are bullet proof but I'm not seeing it. I've blown rods on old car engines back in the 70's and even had a Norton shear a wrist pin on me once. This is the first bike motor where I've seen the rod break in half. It was a brand new crank/rod with only about 10 - 12 hours on it.

I'll keep you posted.

Edited by MNellis, February 15, 2011 - 06:52 PM.


  • chopp123

Posted February 15, 2011 - 06:19 PM

#2

that looks like a pic of my motor all four times in one season before i caught it! check your oil line coming out of the frame. make sure oil is flowing freely through it from top to bottom. there is a screen in there that my clutch fibers clogged and identical results happened.flush it out real good. if thats not it you have an oil clogg somewhere or your oil pump is junk. every once in a while i crack my oil line to my head and check for pressure.

  • howard113

Posted February 15, 2011 - 10:42 PM

#3

it could be a bad oil pump and...rod failure is caused from living life on the rev limiter man.if you rode it easier i bet that wouldnt have happened.

  • Birdy426

Posted February 15, 2011 - 11:05 PM

#4

What kind of crank was it...OEM or Wiseco? Wiseco had some QC problems with their rods and crank kits a while back...

  • MNellis

Posted February 16, 2011 - 12:25 PM

#5

What kind of crank was it...OEM or Wiseco? Wiseco had some QC problems with their rods and crank kits a while back...


It was an OEM crank and had about 10 hours or so on it.

  • tech24

Posted February 16, 2011 - 02:22 PM

#6

It was an OEM crank and had about 10 hours or so on it.


That should have lasted well into the 3 digits no matter how you ride it. Man that sucks. Gotta be a lube issue somewhere.

  • brentn

Posted February 16, 2011 - 05:09 PM

#7

Is that calcium build up in the third photo on one of the ports for coolant??

  • skull1971

Posted February 16, 2011 - 05:54 PM

#8

look on ebay, I complete engines there all the time.

Edited by skull1971, February 16, 2011 - 07:18 PM.


  • MNellis

Posted February 16, 2011 - 08:21 PM

#9

Is that calcium build up in the third photo on one of the ports for coolant??


Yes it is but, remember, this bike has been sitting in the corner of the garage since the failure about 5 or 6 years ago.

  • coreyl

Posted February 28, 2011 - 11:24 PM

#10

looks like what my 450 did...

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  • crf450319

Posted March 01, 2011 - 12:47 PM

#11

Man, that really sucks ! I'm not trying to be a jerk, but why would you spend any more time and money re-building a bike for the 4th time ? If I'd have something like that happen I'd fix it the first time, and if it happened again I'd part the thing out or sell it as a complete unit with a blown engine. For $5000 you can get a new '09, minus the cash you'd get from your 426.

Just seems like bad money after bad money to me.

Good luck with whatever you do with it though, here's hoping it'll stay together if you get it rebuilt again.

:thumbsup:

  • drumbrake

Posted March 01, 2011 - 01:37 PM

#12

if you buy another bike you will learn nothing about engines, go fix it. also im wondering if your a bit rev happy after those 4 cylinder bikes.

  • crf450319

Posted March 01, 2011 - 03:35 PM

#13

if you buy another bike you will learn nothing about engines, go fix it. also im wondering if your a bit rev happy after those 4 cylinder bikes.


The OP is 54 years old and in his first post referred to rebuilding engines in the 70's, both in cars and in an old Norton. I'm guessing he's mechanically inclined and knows his way around an engine. I'd also make the assumption that he knows how to ride and doesn't cause unintentional damage by over-revving his 426. I started road-racing before I'd ever owned an MX bike and knew what was good/bad for an engine.

If a noob (not referring to the OP here) has rebuilt the same engine 3 times and it blows up again, it's either time for a new bike.. or a new hobby.

  • fxrs

Posted March 02, 2011 - 03:26 AM

#14

I would bet the rod had a defect. I don't see any other reason for it to break in that part of the rod.

  • chopp123

Posted March 03, 2011 - 04:42 AM

#15

have seen it twice with in a month on my 426. clutch fibers clogged the screen and no oil was getting in the motor.i wish i caught it the third time befoe it blew again but that was just a seizer that time no snapage. always make sure oil line from the frame is freely flowing oil and crack the bolt going to the head and check for pressure when running

  • mdkcrf250r

Posted March 03, 2011 - 04:52 AM

#16

Either the consecutive blow ups are coincidence (go buy a lotto ticket) or you never fixed the problem that caused the first blow up. Chances are you overlooked something on the first go around. Unfortunately there really isnt much worth salvaging off that one and your prolly better off buying a used motor in good shape.

  • baxterj787

Posted March 03, 2011 - 10:42 AM

#17

Wel, i haven't had a good look at my 426 crank, but I did see something on a 400EX ATV motor that came through my shop last summer: The motor was on it's third rebuild when it came to me. 400EXs have an oil port cast into the RH side of the crankshaft axle. When it had been rebuilt previously, the RH crank axle's inner diameter had been mushroomed slightly. This blocked oil flow. The rod did not last long.

Again, I have not looked at a 426 crank to see if it uses a similar oil pathway. THis would be as good a place as any to look. Also, and I am sure it has been mentioned elswhere, how is the oil pump? The failure(s) described by the OP seem like a lack of lubrication.

The Peanut Gallery rests it's case.

  • MNellis

Posted April 10, 2011 - 07:31 PM

#18

The OP is 54 years old and in his first post referred to rebuilding engines in the 70's, both in cars and in an old Norton. I'm guessing he's mechanically inclined and knows his way around an engine. I'd also make the assumption that he knows how to ride and doesn't cause unintentional damage by over-revving his 426. I started road-racing before I'd ever owned an MX bike and knew what was good/bad for an engine.

If a noob (not referring to the OP here) has rebuilt the same engine 3 times and it blows up again, it's either time for a new bike.. or a new hobby.


As another posted mentioned, I probably should have cut my loses but I'm stubborn like that. I had a desire to win this war. It's kind of like Kevin Costner in the movie "Tin Cup" where everyone tells him to lay up on one of the holes but he pulls out the "big dog" and tries to drive the green only to put it in the water. He hits another ball instead of taking the drop and does the same thing, again and again and again. If nothing else, he was persistent and consistent.

As the above poster mentioned (thanks for pointing out what others failed to read), I know my way around engines as I've built and road raced big bore bikes for many years and I currently have 4 old classic, late '60's early 70's, Honda's and 1 Suzuki that I've rebuilt/restored and many others in the past. This is the first big single that I've owned and my intent on posting the pictures was to add data to an already data rich website. Since I try to never stop learning, maybe someone reading my sad tale of woe would have some advice on an admitted neophyte big single mechanic. But, I digress even more.

I got the motor back together easy enough but after messing around it for quite a few days, but it failed to start. A week or so ago I took it with me to Texas World Speedway to watch the road races for the first time in a few years and figured I'd tap some of my very smart racing friends about ideas on why it won't start. They quizzed me on this and that and most of the items I'd already checked and re-checked, but nothing new came up. Lots of young guys who race motards kicked and kicked but other than the occassional "pop" nothing was working.

In my world when it "pops" it's usually a timing issue. Since these bikes don't have adjustable ignition timing it must be something else. I had drained the carb before storing it so I took the pump apart, cleaned it and shot some carb cleaner through the jets and passages. I didn't check it before, but when I was done, the pump was shooting fuel just fine. Still no joy getting it started.

So, lets think about this. The plug is firing and at the correct time. The carb is putting fuel where it needs to go, so that leaves compression.

New piston, new rings, new cylinder and new gaskets. I had checked the valve seating with some acetone and they were tight. When I removed the head I never even pulled the buckets off the valve springs so that should have been fine.

I'm using an '03 decomp cam so doing a compression check didn't make a lot of sense and I had checked cam timing numerous times. In a final attempt, I decided to check the valve clearances even though there was no reason they should have been off.

To my surprise, the left and center intake valves were so tight I couldn't get a .0015" feeler guage in there. The right intake was fine at .25mm and the exhaust valves were fine.

I removed the intake valve buckets and shims for inspection/measurement and they looked fine. On a whim, I reinstalled the shim and bucket and torqued the cam cap in place. Surprisingly, the valve clearance was now in spec at .25mm. Well, isn't that just peachy.

The only thing I can figure is that when moving the head around it might have gotten jarred or turned on it's side and those two shims got cocked in the valve retainer and didn't allow bucket to seat properly. Regardless, it was fine now.

I quickly put it all back together and it fired on the 3rd kick.

Excellent, now there is nothing left to do but finish up the details and go riding again. I'm having a problem removing the countershaft sprocket nut from the old output shaft that is still in the old crankcase. Even with my 1/2" drive impact wrench and 150 psi I can't get the nut to budge. I suppose I'll have to take it to the local shop and let them have a go with it. My impact wrench is almost as old as I am and was passed down from my dad who was an auto mechanic. Like me, it's probably gotten weak with age. :cheers:

Thanks for listening.

  • grayracer513

Posted April 11, 2011 - 07:01 AM

#19

To my surprise, the left and center intake valves were so tight I couldn't get a .0015" feeler guage in there. The right intake was fine at .25mm and the exhaust valves were fine.

I removed the intake valve buckets and shims for inspection/measurement and they looked fine. On a whim, I reinstalled the shim and bucket and torqued the cam cap in place. Surprisingly, the valve clearance was now in spec at .25mm. Well, isn't that just peachy.

That's all cool, except that the spec for intakes is .10-.15mm. Typo?

  • Hooch33

Posted April 11, 2011 - 07:52 AM

#20

if you buy another bike you will learn nothing about engines, go fix it. also im wondering if your a bit rev happy after those 4 cylinder bikes.


Lol the guy is 54 and roadraces a liter bike, pretty sure he knows his shit by now.





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