YZ125 case repaired. (Common problem?)

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4/15/2016 2:28 PM

A few years ago I got a spare '06 YZ125 engine from a friend which had a broken case. I had a 2008 model at the time so I figured it was good for spare parts. From the looks of the damaged engine, it assumed it was from someone running the chain too tight. When the suspension moves and the chain is too tight, something has got to give...

As the hours added up on my 2008, I bought a 2011 YZ125 and decided to rebuild the '08 after just one more ride... That's when the countershaft bearing broke and broke the case in exactly the same way as that spare '06 engine. So I bought a new case, rebuilt the 2008 and sold that bike to a friend.

Two weeks ago I was doing the last moto on my 2011 YZ125 when the same thing happened to it. I already had the bearings, crank, top-end, etc. on the shelf to freshen up the bike. The hour meter was right at 96 total hours on the bike when she popped.

So here I am with three busted cases and a whole load of parts for my favorite bike. I had an idea for repairing these cases so I gave it a try yesterday. The photos below pretty much show how I accomplished the job. I am left wondering if this is a common problem on this bike due to the small bearing size?














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4/15/2016 3:22 PM

That's a beautiful looking piece of work -- now you have me worried about mine!

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4/15/2016 3:27 PM

Ya, not sure if common or not but nice repair!

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4/15/2016 6:18 PM
Edited Date/Time: 4/15/2016 6:21 PM

Repair looks GREAT, please let us know the on track results.
My friend's big brother over the years has bought three blown up YZ125's [2012 and two 2009's] all three had cases busted just like yours. The mechanic who torn down the engines, [I just got off the cell with him] told me all three of the clutch basket nuts were really loose. He assumed that the resulting play in the axle drive [under full acceleration] forced the axle main to put pressure on the bearing breaking the case.
Anyway, he is a genius engineer from MIT with about a million patents.

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4/15/2016 6:22 PM

my buddy busted a 12 the same way first one i ever saw but i guess its common, i dont know how many hrs were on it but he got it from a friend that raced it two seasons in the B class

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4/15/2016 7:00 PM

Had the engineer check out your post and he called me back.
He said in the one case it looks like bearing failure was a huge reason for the case failure. He also said the previous YZ125 engine design, that bearing had a ring around it to stop it from breaking out the cases [even if the bearing failed]. He told me he had the new cases machined to use the old style bearing. He said the three cases he had machined have a combined 1500+ hours on them without another failure.
He likes your repair and was curious if you have the clearance to run an o-ring chain? I hope he does not read this but... believe me a compliment from him is RARE!!! I have my cell laying here and he is still talking.........

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4/15/2016 7:10 PM

Thanks guys! Mark, just keep fresh bearings in your bike and I think you'll be ok. Especially that bearing.

I think this is simply caused by bearing failure. The bearing races in all three that I have were pitted badly toward the rear of the bike. Since the chain is pulling in that direction it makes sense to me. When the bearing gets bad enough it pukes out the balls. Since 2nd gear is right there, the only place for the bearing-balls to go is through the seal. They don't quite fit, so the casting breaks and you're all done for the day.


Pete24, I am B class speed also, and I think somewhere in the 50 hours per season is a pretty good average number for an avid weekend racer. That puts him right around the 100 hour mark too.

Ti, I didn't find loose clutch hub-nuts or any other loose parts in the three I've worked on, but I'm sure that doesn't help these little bikes last.

For the record, I change my oil every single ride day, I think this is just a case (pun) of Yamaha using a bearing just large enough to survive past their service interval. My new bottom end rebuild number is going to be 75 hours on the YZ125. It's a damn good bike with great reliability in my opinion. I have no problem splitting the cases and throwing in a bearing each year to prevent this.


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4/15/2016 7:21 PM

Ti wrote:

Had the engineer check out your post and he called me back.
He said in the one case it looks like bearing failure was a huge reason for the case failure. He also said the previous YZ125 engine design, that bearing had a ring around it to stop it from breaking out the cases [even if the bearing failed]. He told me he had the new cases machined to use the old style bearing. He said the three cases he had machined have a combined 1500+ hours on them without another failure.
He likes your repair and was curious if you have the clearance to run an o-ring chain? I hope he does not read this but... believe me a compliment from him is RARE!!! I have my cell laying here and he is still talking.........

I read this after I typed up my last comment... Looks like he came to the same conclusion I did about the bearing.

As for the O-ring chain, I believe it will clear and here's my deductive reasoning as to why:

I designed this part to have equal chain clearance to the original, although it's on a flat instead of a conical profile.
When I bought my first YZ125 (it was a 2008 model) it was fitted with an o-ring chain. The broken case shown in the 2nd photo above is the 2008 case that had the o-ring chain with a 13 tooth (stock) sprocket.
So if the stock case barely cleared the o-ring, my repair should / hopefully clear. If it doesn't, I'll cut the case down a little more and change the shape of the repair piece.

I have not yet assembled the engine, I'll be riding all weekend so that will probably happen middle of next week.
I was considering offering this as a service to YZ125 owners who were unfortunate enough to have the issue.

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4/15/2016 7:35 PM

Clever! I like it. Let us know how it holds up.

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4/15/2016 8:13 PM

cwtoyota, sounds good.
The three engines the engineer repaired had the whole case broken out by the bearing. Two of the cases had the piece [a ring of case material] still behind the sprocket. He said the Yamaha dealer told him a rock getting sucked in between the sprocket and case would break the case. And he saw at a Hare Scramble a track banner marking the course got sucked in between the front sprocket and case [resulting in a cracked case just like the one he had with him].
Anyway you are right the YZ125's take abuse, my friend has 5 of them at his farm, and a lot of people come up to ride on his tracks. Sometimes the bikes are only ridden in first and second gears for hours on end by people who can barely ride. During the winter they have a 25 second lap time track in the horse arena, meaning the bikes spend the winter being ridden mostly in second gear. On the weekends his friends have races all night long [these races are called slamfests] and the bikes are abused along with anyone who leaves the inside open on a turn. The barn has fans that suck all the dust and fumes out, so its fun for the whole family. Gotta Fly...Time to land.

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4/15/2016 8:47 PM

Ti wrote:

cwtoyota, sounds good.
The three engines the engineer repaired had the whole case broken out by the bearing. Two of the cases had the piece [a ring of case material] still behind the sprocket. He said the Yamaha dealer told him a rock getting sucked in between the sprocket and case would break the case. And he saw at a Hare Scramble a track banner marking the course got sucked in between the front sprocket and case [resulting in a cracked case just like the one he had with him].
Anyway you are right the YZ125's take abuse, my friend has 5 of them at his farm, and a lot of people come up to ride on his tracks. Sometimes the bikes are only ridden in first and second gears for hours on end by people who can barely ride. During the winter they have a 25 second lap time track in the horse arena, meaning the bikes spend the winter being ridden mostly in second gear. On the weekends his friends have races all night long [these races are called slamfests] and the bikes are abused along with anyone who leaves the inside open on a turn. The barn has fans that suck all the dust and fumes out, so its fun for the whole family. Gotta Fly...Time to land.

All of mine broke exactly like the two pictured. I machined the case down flush with the left hand surface of the bearing race.

I also considered increasing the bearing bore to a larger diameter and installing a larger bearing with a higher dynamic load rating.

Another option I've considered is to enlarge the bearing bore diameter and locate the bearing inside of my repair piece. The repair part would slide down inside the case and bolt on with a similar flange to what I've already done. The big advantage is that the bearing could be inspected / replaced without removing the engine from the bike and splitting the cases. The repair pictured above would allow bearing replacement, but there is no easy way to grab onto the bearing to pull it out of the case.

That arena racing sounds like a lot of fun! (assuming all the racers are mutual friends)
I have a 35 second track on the property with some arena-cross inspired stuff. Get's your heart-rate up!
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5/12/2016 11:49 PM

Ti wrote:

Repair looks GREAT, please let us know the on track results.
My friend's big brother over the years has bought three blown up YZ125's [2012 and two 2009's] all three had cases busted just like yours. The mechanic who torn down the engines, [I just got off the cell with him] told me all three of the clutch basket nuts were really loose. He assumed that the resulting play in the axle drive [under full acceleration] forced the axle main to put pressure on the bearing breaking the case.
Anyway, he is a genius engineer from MIT with about a million patents.

Results as promised:

I finally got my big order of YZ125 and YZ250 parts from Yamaha. I replaced every bearing in my poor little YZ125 engine with Koyo stuff from Japan. I also pressed a new OEM rod kit into the crank and trued it to .0007" runout. The transmission was fine, but I had a spare set of gears, shafts and forks so I threw those in for good measure. I found a chipped reed pedal on the VF3, so I threw the stock reed cage in while I wait for some replacement pedals.

It's always best to set squish when the bike has a fresh crank and bearings, so I measured some parts and cut .033" off of the head to bring the squish down from .051" (as delivered stock) to .027" which is as tight as I feel comfortable running on a 125cc.

She started first kick as always and runs smooth. I went to the local track for the Thursday night practice, wrung this little bike out for all it's worth and put 1.4 hours on the meter.

No leaks, no issues, no stress and a big shit-eating-grin shredding on my favorite bike!




The chain doesn't rub (even though it's near the end of it's life) but just like the stock case, the dirt and rocks that go through scratched up the aluminum part. It's a functional repair and I couldn't be happier with the result.





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5/13/2016 12:17 AM

Very nice repair!

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5/13/2016 12:46 AM

Helder wrote:

Very nice repair!

Thanks, after the beating I put on it today, I think it's going to hold up just fine. Only time will tell.

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5/13/2016 4:16 PM

Nice. Sweet repair. I had never seen this on the YZ125 before but it appears fairly common judging by this thread.

Being the owner of 2 YZ's and CRF450, there is a big difference in the correct chain tension. The Hondas have required very little slack for a long time. It doesn't take much over the recommended play and the chain will chew up the mud flap and the inside of the frame and make really annoying chain slap sounds. The YZ's, on the other hand, require a fair bit of play, about twice what the Honda does. I can see where a rider coming from a Honda to a YZ125 could run too little play and end up causing this problem to develop.

Bike sounds great. What fuel are you running now that you cut the head?

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Powerband in every gear !

5/13/2016 8:05 PM

FGR01 wrote:

Nice. Sweet repair. I had never seen this on the YZ125 before but it appears fairly common judging by this thread.

Being the owner of 2 YZ's and CRF450, there is a big difference in the correct chain tension. The Hondas have required very little slack for a long time. It doesn't take much over the recommended play and the chain will chew up the mud flap and the inside of the frame and make really annoying chain slap sounds. The YZ's, on the other hand, require a fair bit of play, about twice what the Honda does. I can see where a rider coming from a Honda to a YZ125 could run too little play and end up causing this problem to develop.

Bike sounds great. What fuel are you running now that you cut the head?

Thanks, the bike runs great, the head is always a big improvement on a two-stroke.
I run Trick 114 and mix it 50/50 with ethanol-free unleaded 92 octane pump fuel. The oil is 32:1 Yamalube 2R.
The porting is all stock (as-cast not even cleaned up). I am running the FMF fatty with a PC 304 (not the short one).

The bike will pick up a little more mid-range when I get the VF3 back in there. It does more for this bike than any pipe.

I always go by the factory service manual on each model of bike for the chain slack. A good way to know your setting is safe is to remove the shock and run the swing arm through the full travel. The chain should have just a little play at the tightest spot in the stroke.


My next project is to make up a set of 4340 billet shift forks for each YZ250 to complete my "works" transmission mods. Then I'll start building up my spare YZ125 engine for the Washougal 125cc Two Stroke Dream Race.

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12/13/2016 8:14 PM

Add me to the list of bikes with this same failure. My son's 2015 with less than 50 hrs had the same failure. It busted the case and destroyed the transmission. Sickening......

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12/13/2016 9:28 PM

MX429 wrote:

Add me to the list of bikes with this same failure. My son's 2015 with less than 50 hrs had the same failure. It busted the case and destroyed the transmission. Sickening......

Sorry to hear that.

That little bearing seems to be right on the edge of strength in that spot.
A rock in the chain or an over-tightened chain could cause it, or just hard use.

I think a 15 model is worth repairing, especially if you can come up with the transmission parts at a decent price.
Once you start adding up the cost of each individual gear, shaft, etc. even Yamaha gets expensive.

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12/14/2016 8:06 AM

Very nice work. Thanks for sharing this.

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12/14/2016 11:27 AM

MX429 wrote:

Add me to the list of bikes with this same failure. My son's 2015 with less than 50 hrs had the same failure. It busted the case and destroyed the transmission. Sickening......

cwtoyota wrote:

Sorry to hear that.

That little bearing seems to be right on the edge of strength in that spot.
A rock in the chain or an over-tightened chain could cause it, or just hard use.

I think a 15 model is worth repairing, especially if you can come up with the transmission parts at a decent price.
Once you start adding up the cost of each individual gear, shaft, etc. even Yamaha gets expensive.

Yep. The bike is so new that it is worth repairing, but the prices add up having to buy each item separately. I wish there was a way to buy a lower assembly. We could cut corners if this was a pasture bike, but if my son is going to trust it to do what he does, it will not go back together being anything less than new quality.

This was just a case of bad luck I suppose. Engine oil or chain tension was not a factor in this case. The bearing just gave up.

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12/15/2016 10:00 AM

This might be a common YZ problem but if you get a bunch of mud behind the front sprocket it will do that as well. The mud pushes the sprocket out from the case and puts stress on that bearing which will then break the case half.

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6/11/2017 9:28 AM

Well I just got added to the list. 2016 YZ125 with 25 hours on the meter since new. Countershaft Bearing just let go and took out the cases and spit ball bearings into the tranny. Don't know what other damage this caused until I get it opened up. Chain was properly adjusted, oil changed every other ride (3 hours), properly filled with 660cc of oil, etc. This doesn't look like an isolated incident. Thing that has me worried is if I fix it with OEM parts what's to say this doesn't happen in another 25 hours? Has Yamaha updated this bearing with a new spec? This is my 2nd new YZ125 in the last couple years. My 2014 had 100+ hours with no transmission issues whatsoever.






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6/11/2017 12:45 PM

I had the same issue a couple weeks ago. Had the side solderd with zinc. I did change the bearing to an C3 bearing and it runs like a champ now....

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4/18/2019 11:49 AM

anyone have a case thats broken like this they would sell. mine is is beyond repair but i could probably fix something like this

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4/18/2019 3:12 PM

David_Milligan wrote:

anyone have a case thats broken like this they would sell. mine is is beyond repair but i could probably fix something like this

I imagine most folks just part the bike or have the engine repaired and the old case goes in the scrap bin.
Check e-bay or try posting an ad in the bazaar section here to see if anyone has a spare.

I did another one of these repairs for a Vital member a few months ago along with a spare seal ring. He said it's working well with no leaks. The original repair is still on my 2011 bike with zero issues as well.

Long term, it seems to be a viable repair as long as you keep fresh bearings in there.

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