Broken Chain... Busted Case '03 WR450F

Well sometimes shit happens and this time it happened to my bike.

While down in Ocotillo during the holidays I sheered a pin on the chain and it wadded up on the counter sprocket and broke the case at the ‘ear’ of the clutch shaft.

Now the question is what to do about it… I haven’t pulled it apart as yet so I’m not completely sure how extensive the damage is.

For those that may have experienced this before, or those with suggestions, I’d like hear what options may be available. :lol:

Here are some I am considering;

There is a 426 case at the local shop, see if it’ll fit and rebuild. :confused:

Take all the goodies off and find an ’05 that could use em (mine not yours). :crazy:

Go sit on an ’07 and pull out the credit card. :confused:

Thanks in advance for any help.


The 426 case will not work. I know you haven't pulled it apart yet but visually how bad is the damage (oil leaking out, etc). It doesn't sound like it's in a structural area so it may be able to be welded. The case is a matched set so you will have to replace both right & left halves if it comes to that.. I paid around $380 for the set (comes as one part#) about a year ago.

Thats exactly why I made this chain guard. A buddy of mine had the same thing happen to his YZ450. I didn't want it to happen to mine so I made it out of a piece of 3/4" cutting board. I made it long enough to prevent the chain from slapping the clutch actuating arm and breaking the case and starter cover.


what cracked the actual engine case or the case cover??? If its the case cover then take it off and jb weld it. If its the engine case, well your probably screwed...

A good weld shop can work miracles with the cases but they have to be good. The case needs to be cleaned properly (no dirt or oil) for the weld to take properly. Welding alloys is always tricky and more art than science. The problem now is that all the old welders that I could trust with a job like this are now gone.:lol:

Are there any good protecting covers to prevent this from happening?

replace your chain more often..

You have my condolences. Same thing happened to my '06 450 about 6 months ago. Look at the left upper side of the sprocket, between the teeth.



I did a lot of reading trying to figure out the least expensive way to fix it. A few people posted that they tried JB weld on similar holes or cracks and it worked. Most people said I would have to remove the engine, split the cases and have it welded. Or just replace the cases altogether.

I decided that I was going to try and JB weld it first because that did not require removing the engine and it would only cost a few dollars. The hole in the case was in an oil passage so I had the added stress of not packing too much JB weld in the hole because it would starve whatever the passage was feeding. I used an aluminum can lid cut to size for a cover over the hole and put the JB weld over that. I was pretty impressed with the results and it worked... for about 5 minutes. Then it started dribbling oil. Here is the pic with the JB weld.


If not for the placement of the hole over an oil passage I would have had it welded. Of course, the engine has to be removed and cases split and cleaned to do this. But I ended up buying new cases. I removed the engine with the help of some friends and just sent that to the dealer to have the cases replaced. Saves some money removing the engine yourself.

I've been told that you can get used cases on Ebay for pretty cheap, especially for older bikes. I'm still trying to forget how much I paid for the new cases.

Hey Rooster Wrooster,

Why couldn't you have got it welded and the oilway redrilled.

Replace the bike, its an 03? cases cost big bucks.........and alot of them on ebay are hot...............bad carma

Well sometimes shit happens and this time it happened to my bike.

Thanks in advance for any help.


I have a spare DR350 motor you could drop in the frame, (I thought your WR was a little underpowered!)


First of all.... thanks for the input, it was actually quite helpful (except of course for Dave who has the spare DR350 motor.. he's just likes to tweek me... damn Limey). :lol:

PDBLUE Thanks for the tip on the 426 case. Didn't think it would work so I won't go there.

msgbean... very nice and I will have a chain guard next go round.

New Case or Weld is the most likely choice(s) and I'll post some pics and information as I learn more.

Again thanks for the input.

BTW this was the 3rd new chain (D.I.D. ERT2) I have put on the bike because chains are just an expendable item. Oh well...


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