Lower Crank Bearing

I have been riding and racing dirt bikes for the past 10 years, and I've never seen this problem before.

 
I own a 2006 Yz125.  For the past 3 years I've been riding this bike, but only recently my problem occurred.
 
The Problem: It all started in April of 2013,
I blew out a transmission bearing, which I had to split the cases to replace.  I ended up replacing the bearing, sealed the case halves and headed out to a local track.  After about 10 minutes of riding, the bike seized up.  
I brought the bike back to the shop, took the jug off and discovered that the liquid gasket maker had melted around the whole entire crankshaft.  I eventually flushed out all of the gasket maker, but nylon on the crankshaft melted which made the crank hard to turn..I ended up having to replace the crankshaft.  (This is what appeared to be the problem) This resulted in me ordering a Hot Rods crankshaft, new piston, etc.  
I learned from my mistake and the bike was running good (engine wise) for around 4 months, up until I accidentally took it through too much mud and water which got sucked in by the piston, and crankshaft.  I took off the jug, cleaned all of the muddy water out of the crankshaft and everything was spotless.  After re-assembling everything, it was running as if nothing had happened. The next day I went to a local track and after a good 30 minutes, the bike seizes up.
Again I took the engine apart, un-seized the crankshaft, and found out that the lower crankshaft bearing was shot, meaning there was too much play in the bearing.  This must have been the result of the bearing being too dry from the mud and water.  (Keep in mind when I flushed everything out, it was as dry as a bone, and then I put 2-stroke oil by the crank)
I ordered up a new Hot Rods Crankshaft and piston, split the cases, put it all together, and broke in the new parts. After the break in process, I was riding for a good 5 hours, and then cleaned up the piston from any carbon deposits, put a new plug in, changed the oil, cleaned the air filter, and she was ready to go! 
 
I went to another local track, and had a good 2 hours of riding on the bike before it seizes up.
I took the jug off, and the lower crank bearing won't budge at all.
When this occurred I was riding at a sand track at very high rpm's. 
 
Any suggestions?  Could it be a Hot Rod piston manufacturer error for the last time the crank failed?  Is there a greater problem that's occurring?  I do run an oil mixture of 32:1  
 
Any help would be greatly appreciated, thanks!

What brand oil?

Just rebuilt my 06 144 with a hotrods crank i got 4 hrs on it at washougal no issues yet running awesome.

Hot rods are decent,  the plastic stuffers are a problem on both the yz125 and yz250 oem cranks. But worst on the 125. Under extreme heat they like to melt.

 

here an 08 i did

photo1_zps2a1162d5.jpg

 

How big are you and what class do you race?  I had to run 25:1 in my full mod rm250 to keep 1/8th inch of residual oil in the crank.  I might suggest something tough like castor 927 at at least 32:1. You sound like you are wringing its brains out (good for you)  Use more oil, maybe go up one size on the main jett

 

 

and for the crank, here is the best way to fix the oem crank and avoid having to use crummy aftermarket cranks. Maybe try a new PRO X crank, or send your oem one to these guys for a rebuild (hot rods is decent, wiseco is junk)

What brand oil?

I've been using Amsoil Intercepter 2-Stroke oil for a good few years now with no problems.

Just rebuilt my 06 144 with a hotrods crank i got 4 hrs on it at washougal no issues yet running awesome.

Sweet man!  What oil and ratio do you use?

How big are you and what class do you race?  I had to run 25:1 in my full mod rm250 to keep 1/8th inch of residual oil in the crank.  I might suggest something tough like castor 927 at at least 32:1. You sound like you are wringing its brains out (good for you)  Use more oil, maybe go up one size on the main jett

 

 

and for the crank, here is the best way to fix the oem crank and avoid having to use crummy aftermarket cranks. Maybe try a new PRO X crank, or send your oem one to these guys for a rebuild (hot rods is decent, wiseco is junk)

 

I'm around 5'9" and 155 lbs.  I was trying to qualify for Loretta's in Schoolboy 1. 

Thanks for the suggestions about the castor 927 and looking at maybe going up one size on the main jet. 

 

Alright, maybe I'll have to give a Pro X crank a try, I agree that hot rods is decent but for some reason it just didn't make it past 7 hours.

I use 927 i got this hotrods crank free so if it locks oem is rebuilt.

In order of likelyhood (my opinion)

 

Oil - A 125 being wrung hard likes a lot of it. I'd suggest something like 24:1. You can run the best oil in the world, but if there isn't enough in the system to get to the parts that need it -> toast. The higher the rpm and the higher the load the faster the oil migration through the engine and the greater the amount of oil in the mix is needed.

 

Crank centering - Is the crankshaft centered between the case halves when reassembled? If left biased to one side or the other it could cause a sideload on the thrust washers, which means a lot of heat and eventual failure.

 

Air filter maintanance - Bearings hate dirt. A lot.

 

Detonation - Isn't just hard on pistons. The shockwave adds a lot of load to the bearings below as well. Do your pistons show any signs of knock (sandblasted look)? If already running it near the edge, a little more load on the bearing from detonation could push it over the edge.

 

 

 

Your first seizure - You said you slapped it together, went to the track, and the case sealant melted. How long did you give it to dry? What sealant are you using?

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now