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Tacoma, WA
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cwtoyota
4/15/2016 2:28pm
4/15/2016 2:28pm
Edited Date/Time
5/13/2016 8:05pm
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?
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?
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.
The Shop
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?
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.
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.
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.
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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