Why Piston Seized?

youwish16
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youwish16
Edited Date/Time 3/11/2014 4:53pm
Backstory:
I am new to the dirtbike world (I mostly race RC cars). I recently bought a used 2003 YZ125. The guy I bought it from said it had a new top end with only a few hours on it. He said he had rode it 1 time in the past 6 months after the new top end but said he would start it about once a week until I bought it. I had about 3 hours on it when it seized up. Just wondering why it seized. I don't think I did anything wrong. 32:1 mixture with 87 octane. Clean air filter and fluids were all ok.

Any info/insight would be greatly appreciated.

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BAMX
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BAMX 3/8/2014 9:35pm
3/8/2014 9:35pm
That is a POS Nakamura Ebay piston. It looks like the ring broke or got caught on the exhaust valve or the edge of the port. Although, it is hard to tell with a pic but by the blow by on the rings, it look to have a lot more than 3 hours.
xnationalwrench 3/8/2014 9:57pm
3/8/2014 9:57pm
BAMX wrote:
That is a POS Nakamura Ebay piston. It looks like the ring broke or got caught on the exhaust valve or the edge of the port...
That is a POS Nakamura Ebay piston. It looks like the ring broke or got caught on the exhaust valve or the edge of the port. Although, it is hard to tell with a pic but by the blow by on the rings, it look to have a lot more than 3 hours.
X2 Even if the piston was new IF the spec (clearance) was off and/or the ports chaffered it could do this. Since it is a 2 stroke you should have taken it down before you bought it especially if you didn't know who you bought it from. Take your time and put it together properly and if ill last.
Parris
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Parris 3/8/2014 10:41pm
3/8/2014 10:41pm
This is not a seized eng. this eng . failed???? the cylinder has shade tree honing and you can see the previous damage to the cylinder.
Paw Paw 271
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Paw Paw 271 3/9/2014 6:05am
3/9/2014 6:05am
It looks like the right exhaust valve failed and caught the piston and broke it . Then the ring broke. The piston looks like it has more time on it than stated. You need to find out why the valve failed.
You will have to do a complete tear down as you may have piston parts and ring parts down in the crank cavity. The cylinder also looks bad and may have to be replaced or plated.
I also suggest that running 87 octane fuel is a bit low, but did not play a part in the damage.

Paw Paw

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youwish16
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youwish16 3/9/2014 6:46am
3/9/2014 6:46am Edited Date/Time 3/9/2014 6:46am
Thanks for the info guys. It is getting rebuilt with new wiseco top and bottom end and new cylinder. It is now a father/son project now for me and my dad.

I probably should have taken the engine apart when I bought it but you live and learn. An expensive mistake but atleast I am having fun with it.
slipdog
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slipdog 3/9/2014 7:30am
3/9/2014 7:30am
DO NOT PUT A WISECO CRANKSHAFT IN YOUR BIKE!

Use OEM first and foremost, a Hot Rods crank can be used, but never a Wiseco. Research Wiseco cranks and you will find horror stories after horror stories about them failing after only a few hours of use.
Parris
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Parris 3/9/2014 7:34am
3/9/2014 7:34am Edited Date/Time 3/9/2014 7:41am
agree
youwish16
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youwish16 3/9/2014 10:37am
3/9/2014 10:37am
Are the Wiseco cranks really that bad? I haven't done anything but take the motor out and take the top end apart. Still have to take the bottom end apart and inspect it. I guess I could sell the Wiseco and get a OEM or Hot Rods.
kevkx125
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kevkx125 3/9/2014 10:48am
3/9/2014 10:48am
Use oem if possible, if cost is with in reason.
Paw Paw 271
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Paw Paw 271 3/9/2014 11:21am
3/9/2014 11:21am
youwish16 wrote:
Are the Wiseco cranks really that bad? I haven't done anything but take the motor out and take the top end apart. Still have to take...
Are the Wiseco cranks really that bad? I haven't done anything but take the motor out and take the top end apart. Still have to take the bottom end apart and inspect it. I guess I could sell the Wiseco and get a OEM or Hot Rods.
Wiseco cranks in 2 strokes seem to work ok as long as it does not have the plastic inserts by the rod and if you have the crank trued before installing it. But, which ever crank you install be sure it is trued.

Paw Paw
DBerg
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DBerg 3/9/2014 1:53pm
3/9/2014 1:53pm Edited Date/Time 3/9/2014 2:01pm
Just for future reference (especially when looking at craigslist), every single bike has never been raced, garage kept, barely ridden, brand new top end with less than 5 hours on it, runs perfect, and is the fastest bike of its kind.
Don't read into the description for anything more than entertainment. When going to look at a bike, basically go into it ready to walk away. Ignore the plastics/graphics too. They are no indication of bike quality. I also wouldn't buy a used bike w/o planning on stripping it down to check anything that pivots, spins, and rotates.
That having been said, you can find some great deals, and while I'm sure this purchase didn't go as planned for you, at least you have a fun project w/ your dad. You should go ahead and check out everything else (suspension, linkage, axles, etc) & when you get it all done you'll have learned a lot and feel that much more connected with your new ride! Smile Enjoy!!!
BAMX
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BAMX 3/9/2014 6:06pm
3/9/2014 6:06pm
Parris wrote:
This is not a seized eng. this eng . failed???? the cylinder has shade tree honing and you can see the previous damage to the cylinder.
That honing is normal on a plated cylinder. I agree with slipdog on the crank, OE then Hot Rods. I have had good luck with Hot Rods but I don't hammer them back together and I wash all of their assembly oil away and replace with the Motorex that I run in my gas. For the piston, I would go OE then ProX. It looks like you will either need to get that cylinder repaired or buy another from Ebay.
Matt Fisher 3/9/2014 8:45pm
3/9/2014 8:45pm
BAMX wrote:
That is a POS Nakamura Ebay piston. It looks like the ring broke or got caught on the exhaust valve or the edge of the port...
That is a POS Nakamura Ebay piston. It looks like the ring broke or got caught on the exhaust valve or the edge of the port. Although, it is hard to tell with a pic but by the blow by on the rings, it look to have a lot more than 3 hours.
From personal experience, Namura = crap.

The good news is that 03 YZ125 cylinders are cheap on Ebay. Avoid the sleeved ones though.
ML512
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ML512 3/11/2014 11:11am
3/11/2014 11:11am
Just a side note: as previously stated above 87 octane is too low, most modern 2 strokes will detonate on the current 91 gas with an abusive rider on it as well. (not saying detonation was the cause of the damage, I just wanted to throw that advice on the fuel choice out there.)
seth505
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seth505 3/11/2014 3:56pm
3/11/2014 3:56pm
slipdog wrote:
DO NOT PUT A WISECO CRANKSHAFT IN YOUR BIKE! Use OEM first and foremost, a Hot Rods crank can be used, but never a Wiseco. Research...
DO NOT PUT A WISECO CRANKSHAFT IN YOUR BIKE!

Use OEM first and foremost, a Hot Rods crank can be used, but never a Wiseco. Research Wiseco cranks and you will find horror stories after horror stories about them failing after only a few hours of use.
x2, I had a Wiseco last 5 hours and then I went back to OEM. I do use Wiseco pistons though.

And when you rebuild it don't use 87 octane fuel!
mx5471
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mx5471 3/11/2014 4:53pm
3/11/2014 4:53pm Edited Date/Time 3/11/2014 4:57pm
I have a lot of experience with the YZ125. Use OEM parts. Yamaha parts are cheap compared to the Suzuki, for example. And are very reliable. Also, do not use Athena gaskets, or anything from Athena. Use OEM or Cometic. But if you run that bike on pump gas, you're only getting one ride, if that. The YZ125 needs high octane gas. VP110 is cheaper than C12. Get some and mix it with the highest octane you can get at the pump at a 50/50 ratio. The crank will last a long, long time if you do the top end periodically according to how much you ride and how hard you push it. You need to change the piston before it starts to crack. They all will fail depending on time and use, but you need to do it before it causes other damage to the cylinder or bottom end. Mix your gas at 32: 1, and jet accordingly. A little rich is better. That's all you need to do to keep the motor in good shape.

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