Blew crank bearing in KX125...save crank?


13 replies to this topic
  • andrzej

Posted 02 August 2010 - 04:37 PM

#1

Yesterday, had my nephew out at a track for some afternoon fun on his '05 KX125. He ended up seizing the engine up solid at the end of the day so I gave him my YZ250 to use this week at MX camp and brought the bike home.

Got the cases split today.....and it was the flywheel side bearing that gave out. At least 3-4 of the bearings have vanished....the retaining band was mangled with little bits all over. Fortunately, the bearing race in the case is still smooth, with only some cosmetic scoring on the inner face of the case, so that should be OK.

Problem is that when I pulled the crank, the blown bearing came out on the crank. Got the outer bearing ring off (and the bits of metal and remaining bearings), but the inner bearing ring is firmly on the crank.

The crank looks in decent shape, barring a bit of minor scuffing. Rod seems fine...no vertical play. But I'm worried that the metal fragments may have gotten into the rod bearings and such. Plus getting the inner bearing ring off is proving problematic (haven't tried heat yet).

So I figured I would poll the collective wisdom....is it worth trying to remove the inner bearing ring off the crank and try to rescue the crank...or is it just not worth the bother and potential other issues, and just spring for a new crank assembly?

Recommendations on that score?

I'm thinking that since it's split....probably better to spring for a new crank assembly...that way the bottom end is pristine and should give him a few years of good use. Not worth saving the couple of hundred bucks, IMO...but I'm curious what the experts think?

Thanks!

BTW...first time I've split a set of cases....wasn't that bad actually, though I do have the Tusk tools to pull the flywheel, hold the clutch, split the cases/pull crank, and reinstall a crank, without which it would have been basically impossible. Bought all the tools when my bottom end went on my YZ250, since I figured it would be faster and cheaper to DIY the next time. Seems the next time has arrived with my nephew's KX125! :banana:

  • hi_im_sean

Posted 02 August 2010 - 05:42 PM

#2

the bearing stuck on the crank alone doesnt make it bad, you can get a bearing puller and pull it off. but i agree with you, some metal may have gotten in there and you already have the cases split, so it may be a good idea to replace the crank. i always replace cranks if ive gone through the trouble to split, unless i have a really good reason not too(like its fairly new and im the one that installed it). its just so much work to not spend the $150. but.... theres also a good chance no damage was done and the crank may be fairly new, ultimately its up to you, as you can inspect it better than any of us. 0 play sounds like a good number to me. feel smooth, any crap in the big end? just remember, 125's are of the higher rpm variety

  • FPMXer

Posted 02 August 2010 - 09:35 PM

#3

Clean the cases very good. Forget about that crank it's roached no matter how good it looks. You can get a whole rotating assembly off e-bay for around $350-$400. That includes the crank, bearings, top end and gaskets. If you reuse the crank it's a ticking time bomb!

  • eyesky2002

Posted 03 August 2010 - 04:10 AM

#4

Clean the cases very good. Forget about that crank it's roached no matter how good it looks. You can get a whole rotating assembly off e-bay for around $350-$400. That includes the crank, bearings, top end and gaskets. If you reuse the crank it's a ticking time bomb!


Good advise :banana:

  • MrBlahh

Posted 03 August 2010 - 04:27 AM

#5

why would the crank be roached? I would test it, service manual has all the wear limits in it, 350-400 $ is not exactly chump change if the crank is fine

I'm almost sure the kx cranks are rebuildable too, MrCrankshaft has the rod kits for KX125-$89.99 and 50 $ labor to rebuild it

  • eyesky2002

Posted 03 August 2010 - 05:13 AM

#6

why would the crank be roached? I would test it, service manual has all the wear limits in it, 350-400 $ is not exactly chump change if the crank is fine

I'm almost sure the kx cranks are rebuildable too, MrCrankshaft has the rod kits for KX125-$89.99 and 50 $ labor to rebuild it

I can't see using parts from a engine that detonated, balls are missing and the race was mangled. Metal finds it's way in to the smallest places and to me using the crank to save a buck would be like only replacing the blown main bearing and leaving the other side. Just got a quote on a 03-05 KX125 crank. New hot rod for $139.00 shipped, no reason to deal with the bearing race and rebuilding the possibly bad crank.

  • MrBlahh

Posted 03 August 2010 - 05:16 AM

#7

the cases hold the crank bearings tight, there's no way a ball could get up into the crank, and I would doubt any metal did, it sounded like it went and locked

I used a wiseco crank on my last crank failure, they run about the same price as hotrods

check the runout on a hotrod, I've heard bad things lately, possibly shipping damage, it does not take much of a hit to hurt them

  • eyesky2002

Posted 03 August 2010 - 05:26 AM

#8

Got the cases split today.....and it was the flywheel side bearing that gave out. At least 3-4 of the bearings have vanished....the retaining band was mangled with little bits all over. Fortunately, the bearing race in the case is still smooth, with only some cosmetic scoring on the inner face of the case, so that should be OK.

I was referring to the little bits he describes, very apparent that a 3/8 ball would not find it's way into the lower rod bearing.

And since the cases hold the bearings tight and the balls can't get out, where did the missing balls go?

I just built a KX103 stroker using a new Hot Rod crank, packaged good and the engine runs smooth :banana:

  • AdvCoop

Posted 03 August 2010 - 10:28 AM

#9

When I put the Wiseco crank in my nephew's KX60 and people posted and said Wiseco cranks are bad. I read some uses a Hot Rod's crank and then somebody else posts that those are bad, LOL.

Honestly if I didn't know the history of that crank (meaning when was it last replaced?) I would change it. I just don't see the sense in taking the risk with a 125 race bike.

  • MrBlahh

Posted 03 August 2010 - 10:30 AM

#10

When I put the Wiseco crank in my nephew's KX60 and people posted and said Wiseco cranks are bad. I read some uses a Hot Rod's crank and then somebody else posts that those are bad, LOL.


25 $ dial indicator from northern tool or summit or harbor freight is all it takes to check the runout, I think they get damaged in shipping, it does not take very much to put them out of round

  • hi_im_sean

Posted 03 August 2010 - 03:21 PM

#11

i built a kx103 yesterday with a hot rods, went together great, smoothest rolling one yet

  • andrzej

Posted 23 August 2010 - 07:39 PM

#12

I figured with all the effort to split the cases, and all the shrapnel from the bombed crank bearing, it was not worth risking reusing the old crank, so I got a new Hot Rods crank assembly.

Finally started putting my nephew's KX125 bottom end back together last night....the reason it's taken so long to get to it is that he broke his arm on a hard landing on an 85' tabletop a couple of weeks ago...he rode it out, but overshot and musta really hit hard. He was riding my YZ250 at the time and was consistently clearing that jump in 2nd gear. So no rush to get his bike back together...he's done till the cast comes off, at least for track riding.

I tried the "heat the case/freeze the bearings" trick...didn't work...the crank bearings didn't want to drop in at all. They kept getting stuck only a bit of the way in. Did some searching on the here on theKX 2-stroke...turns out others have found that KX's need to be pressed...the tolerances seem to be too tight for the heat/cold trick in many cases, as was my experience.

So....an excuse to go to Princess Auto! Yay! I picked up a 12 ton shop press there this afternoon, along with some other goodies (wrenches to match the axle nuts on both bikes)....and was surprised to find when I got it home that it was $60 off of the regular price. So it was only $119 for the press. Sweet! The garagemahal is even better equipped now!

Tonight, I put the press together, which only took about 15 minutes. Then I pressed in the seals, which was a piece of cake. After that, I was ready to tackle the crank bearings. Used a large socket that was just a shade smaller than the outer race of the bearings. Didn't heat the cases, but I did lube the races with a bit of synthetic premix oil, just to make it a bit slicker. The bearings were still in the freezer, so were rather cold. I figured that wouldn't hurt things, if they were a bit smaller than at room temps. The popped in easy as you please with the new press. Should have gotten one of those a long time ago! Sometimes having the right tools for the job just makes things so much easier, ya know?

So next step will be to use the Tusk crank puller to pull the crank in the cases and the case halves back together...but that will have to wait for another night this week.

New tool in the garage....bearings in the cases....cold beer in the garagemahal fridge. Life is good! :bonk:

  • andrzej

Posted 23 August 2010 - 07:44 PM

#13

the bearing stuck on the crank alone doesnt make it bad, you can get a bearing puller and pull it off. but i agree with you, some metal may have gotten in there and you already have the cases split, so it may be a good idea to replace the crank.


I tried getting the inner race off the crank using a bearing puller. Even heated the race...no dice...that sucker seems firmly glued to the crank shaft!

And anyway...I figured with all the work to split the cases, and us having bought the bike used earlier this season, I have no idea how much wear the crank had on it...probably the original crank. And with all the metal cage fragments that were stuck to one side of the crank, I figured trying to save the cost of a crank assembly was silly.

Good advice all round...thanks guys!

The engine it's on it's way back together now...should be running again by the time my nephew's broken arm is starting to knit enough for some light trail riding in a few weeks. :bonk:

  • FPMXer

Posted 23 August 2010 - 07:47 PM

#14

I bought a 12 ton press from HF years ago and it's paid for itself 20X's. I put both the crank and bearings in the freezer for about 4 hours. Heat the bearing race with a map torch until it starts sweating then pop the seal and then bearings in. Then using an old bearing and other metal to press them in if needed. Then I have a brass plumbing fitting that slips into the inner race and rests on the bearing. I heat that with the torch until it sweats the bearing race. Grab the crank from the freezer and making sure the correct shaft is aligned the crank usually drops right into the bearing. Then get the other case halve set with sealant and heat the bearing race again and using my hands push the two together and then using the case halve bolts I draw the cases together alternating the bolts and concentrate on the bolts around the crank and then outer bolts and never had a problem.





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