cause of lower bearing seizure?

[kx851]

What are the causes of the lower bearing on the connecting rod seizing? On my '93 KX 125, I recently had a sleeve put in the cylinder, put in a new piston, rings and upper bearing. When I replaced the top end, I checked the connecting rod for wear. There was no up and down movement and the clearances were all within specification. I rode the bike about 7 times and then the lower bearing seized. The cylinder and piston still look brand new. The connecting rod and bearing can move (so they're not completely seized), but only with some force. The lower half of the rod is discolored. I am going to have it rebuilt. What could I have done to prevent this? Or is it going to happen every 7 rides?

 

[fow350]

No that is not going to happen every seven rides. Usually the reason for a bearing (of any kind) to seize would be from insufficient lubrication. You could have dropped some trash in the crankcase and not have known it, although that would be wierd because you rode it several times before it seized. What ratio were you running? Are you sure that everything was in specs? If I had to guess though, it sounds like there was some trash in your bottom end.

 

[marcusgunby]

maybe it was just due to go-they dont last forever, life of a crank according to oem is about 25hours only.

 

[kx851]

I was using 40:1 premix. I had been using 32:1 for the first 5 or so rides, then I changed to 40:1 for the last couple rides. At first I thought about the premix when it seized, but I had ridden it a few times before with the same mix. The day that it seized, I had been out casually riding for about two hours straight. I took a break for lunch, then went back out to practice wheelies. I was doing wheelies in sandy dirt, so traction was not good. I was in first gear, revving it hard and dropping the clutch. I did that for about 15 minutes, then I took a couple high speed passes in about fourth gear and the engine started to bog and then died when I pulled in the clutch. Maybe the way I was riding caused the seizure. The combination of high revs with slow speed might have prevented the engine from cooling properly while practicing wheelies. But it was the lower bearing, not the top end that seized. Maybe it was trash in the bottom end. The top end had blown and that is why I originally had to rebuild the top-end (earlier this year). It's funny now, but the top-end blew up the same day I bought the bike. Anyone ever done that before?

 

[dirt bike dave]

My only low end failure was from riding the bike at high speed, then rolling off the throttle without pulling in the clutch. Basically, this deprived the very hot motor of lubrication and cooling fuel while it was still turning high rpm.

My guess is you got your motor good and hot doing the first gear riding in the sand, then you took it up to speed and rolled off the throttle, letting the hot engine coast with the clutch out. No fuel or oil and a hot motor turning lots of rpm could result in a bearing failure in a matter of seconds, even if the motor was healthy and properly jetted.

 

[Rcannon]

I like what dirt bike dave had to say. That sounds reasonable to me.

However, the bike is 12 years old. I think you have used up all that poor bearing has to offer. Let us know if you discover anything else.

 

[GhostRider32]

FYI.... I work in an industry where we use all sorts of bearings. It is not uncommon for a bearing to show no signs of wear and then go suddenly for many different reasons.......metal fatigue, poor lubrication, high vibration, heat, end of useful life, ect....... That said, I agree with Dave that the most likely reason was because of the heat and lubrication issue and possibly end of life for the bearing.

 

[fow350]

I have never though twice about riding down a long hill and pulling the clutch in. I sure will now though... Thanks Dave

 

[darringer]

dirt bike dave does make a very good point. I have used compression to help while braking down a long downhill and then at the bottom opening the throttle I have noticed the bike detonating because of the spike in the engine operating temperature. It's a very good habit to pull in the clutch while coasting or on a long downhill as the engine will heat up very quickly and have very little lubrication.