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Sons friend wanting to buy his 1st bike. Due to lack of funds I suggest he look at something that needs rebuilt. He finds a mid 90's YZ 125 that the owner claims needs a new crank bearing. Has a new top end, but needs a crank bearing. I know that means they put a new top end on a bad crank. I suggested he pass this one.

Nope. He buys it for $350. I help his tear into it to find a new top end wasted because of a bad bottom end. I split the cases and am waiting for him to get parts.

I'm so tired of idiots seeing / knowing the bottom end is bad but putting a new top end on only to cause more damage. If there is play in the bottom end, please stop and get it fixed before throwing money away. It's cheaper in the long run.

Thanks guys I feel better now.


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People have a crazy tendency to act as if believing/strongly wanting something to be true will make it so. I have given up trying to help any of my friends with work/advice on their bikes. No one ever wants to spend money when it would be cheap to fix and then they carp and whine when it inevitably costs them more in the end. Like doing a top end before worn piston/rings eggs out your cylinder and requires you to spend even more getting a replate. Save $100 spend $300. Infuriating stuff.

Reading this got me to thinking about my bottom end. While you have the top end off, how can you check the crank to make sure it is good before tearing the whole thing apart? I see in the service manual they talk about checking run outs with the crank in V-blocks. Should I figure out a way to mount the mag base indicator and try to check the run out with it in the motor? What kind of axial play checks can be done at this stage?

1st thing to check for is play in the main crank bearing, often referred to as the big end bearing. Spin the crank so the bearing is on top. Inspect the needle bearing and thrust washers for wear. Grab ahold of the rod, and try to move it up and down, there should be no up and own play. There will be some side to side play, but there shouldn't be any up and down movement. Also listen for any sounds as you try to move the rod up and down. If you feel any play or hear metal hitting metal, it's time to rebuild. While holding the rod, spin the crank, trying to feel for any abnormal movement. In the case of my 91 basket case, there was no movement, the rod was very tight. In the YZ, the up and down play in the rod was quite evident, and you could hear the tink sound of metal hitting metal. Most of its needle bearings were missing, allowing for abnormal movement.

Ok cool, thanks for the info. I know that I noticed some side to side play in the rod when I first pulled the cylinder off and didn't know what to think. I will go back and check for radial movement in the bearings as you described. I was nervous when I noticed that side to side movement... was hoping there was an allowable range of side to side play with that rod that I could check. I guess if there is no up and down movement, I should be OK if they are all needle bearings that I am checking... I know when I spun the crank around in there, I didn't feel anything abnormal or hear any metal to metal contact. I just don't want to turn out like the people you are venting about when I'm all done with my build!

P.S. Every time I see your screen name, I always want to ask you...do you have any relation to the Provenzano's who have the motorcycle shop in Claysville, PA?

Tyl3r wrote: P.S. Every time I see your screen name, I always want to ask you...do you have any relation to the Provenzano's who have the motorcycle shop in Claysville, PA?

Nope no relation... I do need to stop out there and introduce myself though... you never know what comes out of those types of contacts... I asked my Grandfather about our last name once... He said we're like the Smith's we're everywhere...

Haha I gotcha man, not a name I hear everyday, had to ask! They are a very nice family down that shop. I thought for sure being they do dirtibike repairs and seeing the level of detail you get into with your bikes. O well! Let me know if you're ever out that way, I'll show ya my neck of the woods

Tyl3r wrote: I was nervous when I noticed that side to side movement... was hoping there was an allowable range of side to side play with that rod that I could check.

Standard is .40mm-.50mm with a service limit of .70mm

Thanks Tedh98 I didn't have the details so I didn't want to speculate


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Tedh98 wrote:Standard is .40mm-.50mm with a service limit of .70mm

Yea, thanks Ted. Where is the measurement taken? Was that in the service manual? Maybe I just need to take a second look at it...