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Yz125 decision

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Hey guys, recently I just bought a 2001 yz125, yeah it's an old one. So I got it home and as my luck would have it within 2 weeks it was making a rattling sound in the engine. I am almost positive it is the crank going out. I have no experience doing a bottom end rebuild, I have only rebuilt top ends on my previous bike. The question is, would it be stupid if I attempted a bottom end rebuild? Is it to big of a task for someone with no experience with bottom ends? My other option is to sell the bike but I know I will have to do a bottom end rebuild somewhere down the road, no matter what bike I buy. I want to get the experience now but I am afraid I will screw it up. Any thoughts or suggestions if I should sell it or rebuild it?

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Get a service manual and rebuild it. Pay close attention to details, take a lot of pictures of the process of taking it apart so you'll have the references when you start putting it all back together. RMATV has some videos on rebuild stuff, YT can yield some good items of info too.

It seems daunting now, but after it's done and running, it won't be all that big of a deal.. :thumbsup:

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Hey guys, recently I just bought a 2001 yz125, yeah it's an old one. So I got it home and as my luck would have it within 2 weeks it was making a rattling sound in the engine. I am almost positive it is the crank going out. I have no experience doing a bottom end rebuild, I have only rebuilt top ends on my previous bike. The question is, would it be stupid if I attempted a bottom end rebuild? Is it to big of a task for someone with no experience with bottom ends? My other option is to sell the bike but I know I will have to do a bottom end rebuild somewhere down the road, no matter what bike I buy. I want to get the experience now but I am afraid I will screw it up. Any thoughts or suggestions if I should sell it or rebuild it?



Bottom end rebuilds sound and seem daunting. They really are not as bad as most people think. You will wants tools like an electric impact, crank puller, torque wrenches, and manuals. Watch lots of videos and take a lot of good pictures as you take it apart.


About a year and a half ago, (only have been riding for 2 years) I put a new crank in my 02 cr125 and a 94 kx125. (didn't need them, just playing it safe. Good to know what's new and what's not.) I tore both down at the same time, took lots of pictures, rebuilt them, and put them both back together at the same time without mixing or losing a single bolt!

It's better to learn how to do it rather than avoiding it and over paying someone to do something pretty easy.

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You could always get a rod kit and send the crank out to get it rebuilt in my experience it's easier for someone else to do the work than for me to do it 

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Lol 2001 isn't that old bro. But yeah it could be crank or top end. You gotta tear it apart. It's easy with a service manual. That's the first thing you should buy. They are EXTREMELY helpful. Next is come here with any questions, the guys here know EVERYTHING, you won't go wrong.

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If you get your crank rebuilt Ken O'Connor racing is pretty good, like bettter than OEM good. I've yet to hear a bad review from him. And it's not very expensive either, in the ball park of $120-$150

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If you get your crank rebuilt Ken O'Connor racing is pretty good, like bettter than OEM good. I've yet to hear a bad review from him. And it's not very expensive either, in the ball park of $120-$150

I was looking at those wrench monkey kits for like $400 for a complete top and bottom end rebuild, a new hotrods crank included, is it not a good idea to get aftermarket stuff or should I stick with OEM?

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I was looking at those wrench monkey kits for like $400 for a complete top and bottom end rebuild, a new hotrods crank included, is it not a good idea to get aftermarket stuff or should I stick with OEM?



Some people will tell you oem only, some will tell you after market is fine. I personally like going as oem as I can, but some things are ridiculous. I personally have hot rod cranks and bearings and have never had a problem.

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Some people will tell you oem only, some will tell you after market is fine. I personally like going as oem as I can, but some things are ridiculous. I personally have hot rod cranks and bearings and have never had a problem.

Yeah I will probably just order the kit. I have an uncle who is a auto mechanic and he used to rebuild Harley's so I would assume he has a fair bit of knowledge on bikes. Maybe I'll bring the engine and my service manual to his shop and I can get some assistance.
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4 minutes ago, yamahadude112 said:


Yeah I will probably just order the kit. I have an uncle who is a auto mechanic and he used to rebuild Harley's so I would assume he has a fair bit of knowledge on bikes. Maybe I'll bring the engine and my service manual to his shop and I can get some assistance.

Do yourself a favor and shop RMATV for the following.

Hot rods crank

ProX gasket and seal kit (does not come with main seals GOOD, you want OEM)

OEM main seals

OEM crank bearings

Measure bore and get a piston (OEM, Wiseco, Vertex, NEVER A Namura!)

That will be everything you need to rebuild the entire engine.

Dont forget oil, lube, air filter, carb dip cleaner, carb kit 

Carb kit can e found on ebay for 28 bucks and all seals and brass come in the kit.

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I took my top end off tonight and I checked the crank and it had no vertical play, only horizontal. So I'm starting to wonder if the noise I heard was just the top end needing to be redone. I looked up a video of a bad top end and the guys rattling noise sounded exactly like mine. I have some pictures I took of the piston and cylinder below. Hoping you guys would know what to do next. Whether I should just leave the bottom end alone and do the top end only or could something else be making the rattling noise. The YouTube video I found is below. No that's not my bike. That's just the noise it was making.

 

 

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IMG_1506477420.044258.thumb.jpg.c7bbb0f3c2e0cd1066afc21174f6b2d6.jpgIMG_1506477457.859473.thumb.jpg.e66d69171a01d41995b9614cf98df52b.jpgIMG_1506477503.043796.thumb.jpg.34216e58815da3a146e0322cb6c6d40a.jpgIMG_1506477553.563928.thumb.jpg.28e970ffe0192edb84306079f1553505.jpg

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How do your main crank bearings look? I'm sure they're probably ok but it doesn't hurt to look while you've got it apart. Try turning the crank and listen for any type of noise, there shouldn't be any at all. Also there should just be a little side to side play on the rod, like a few mm tops.

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2 things right off the bat on the photos:

1) Piston is missing two oiling holes in the front on exhaust side to keep exhaust bridge cool.

2) Wipe out the cylinder, and get better lighting so we can see cross hatching existance, also better focus.

Edited by 2 STOKE YZ DOC

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2 minutes ago, ASP1227 said:

How do your main crank bearings look? I'm sure they're probably ok but it doesn't hurt to look while you've got it apart. Try turning the crank and listen for any type of noise, there shouldn't be any at all. Also there should just be a little side to side play on the rod, like a few mm tops.

This is a good thing to do now also. If I was you, I would replace the main seals at a minimum. Once they are removed, you can check any play in the bearings that would indicate they are not long for this world. 

And the video was not linked.

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2 things right off the bat on the photos:
1) Piston is missing two oiling holes in the front on exhaust side to keep exhaust bridge cool.
2) Wipe out the cylinder, and get better lighting so we can see cross hatching existance, also better focus.

So your saying the piston isn't even the right one? And also sorry for the bad lighting, and I'm still not sure if these new ones are good enough. Sorry
IMG_1506478424.987540.jpgIMG_1506478554.334019.jpgIMG_1506478590.213193.jpg

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 Piston was not properly prepped. Some aftermarkets will fit other bikes without a bridge so no holes are there. Yamaha specific will have holes, or you drill them yourself.

Don't be sorry about how and what you post. Be more sorry that that jug is done and you need a new one, or that one repaired. The bridge is burnt and deformed from heat, and the vertical lines have shown it is past it's life and nicasile cross hatching is gone. Look into a OEM new one, or go big with a Althena Big Bore complete top end... LOL, mom and dad will love that.

Edited by 2 STOKE YZ DOC

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 Piston was not properly prepped. Some aftermarkets will fit other bikes without a bridge so no holes are there. Yamaha specific will have holes, or you drill them yourself.
Don't be sorry about how and what you post. Be more sorry that that jug is done and you need a new one, or that one repaired. The bridge is burnt and deformed from heat, and the vertical lines have shown it is past it's life and nicasile cross hatching is gone. Look into a OEM new one, or go big with a Althena Big Bore complete top end... LOL, mom and dad will love that.

Hmm that sucks that the jug is toast but not as bad of news as replacing the bottom end. And I wish my parents would pay for any of my stuff. If they did buy me stuff I wouldn't have a 2001. ;). All the money I make from being an electrician apprentice goes to my bikes. Not the smartest investment but a fun one.
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