YZ250F Owners...

Combat767

It's all about the dirt..
Joined
Nov 20, 2001
Messages
200
Likes
0
#1
A friend of mine has an 03 YZ250F. He has raced the bike about 20 times this year, with about as much practice time on the bike as well. It has run flawless, and he has done all his regular maintenance. This past weekend the motor seized on him. He tore the motor down. The cylinder looked great except for a mark where the piston stopped. It's just enough to see, but yet not able really feel it or catch a finger nail on. The spark plug is typical brown, the valves are fine, and everything looks hunky doory. The one oddity that he did find was that the piston skirt had expanded and extra .001" in overall diameter. The lower skirt of the piston is .001" over what the service limit is supposed to be. It's not larger at the crown, but down at the bottom edge of the skirt. They put it back together and putzed around the yard for a few minutes and it seized again. Has anyone else ran into this type of situation? Looking for some insight as to why the piston would expand.
 

jmics19067

Lifetime Sponsor
Joined
Jan 22, 2002
Messages
2,097
Likes
0
#2
does sound wierd, do you think it mght be possible that the piston pin bore is egging out ? pushing the skirts out?
 

Combat767

It's all about the dirt..
Joined
Nov 20, 2001
Messages
200
Likes
0
#4
I'm pretty sure it's 1mm all the way around. I'll check with him and see.
 
Joined
Dec 2, 2002
Messages
85
Likes
0
#6
If it is piston pin area is normal and 90 degrees opposite is 1mm less than it is what is comonly called piston slap. Being a short skirt piston it will wobbly from side to side causing sideways movement . Check bore if ok fit new piston. check to see if bore is not scared or worn ,make sure cylinder is circler.
 

Combat767

It's all about the dirt..
Joined
Nov 20, 2001
Messages
200
Likes
0
#7
The piston expanded .001" 90 degrees opposite the piston pin area. You'd think it would shrink or wear, but it grew. Can you say odd! :think:

The cylinder has some small marks, but fine otherwise. He's going to turn the skirt down .002" and see what it looks like. $150 - $160 for a new piston from Wiseco. The local shops haven't heard of this either. I guess we'll see how things turnout.

Thanks everyone!
 

smarttoys

Sponsoring Member
Joined
Apr 29, 2001
Messages
199
Likes
0
#8
The lower skirt of the piston is .001" over what the service limit is supposed to be.
The service wear limits are basically on the edge of being worn out but still serviceable. Measure the piston and compared it to recommended tolerances not service limits. Instead of just measuring the piston, measure the piston and cylinder and find out what your running clearance is and see if its in spec. Also measure the cylinder for wear and out of round using a good dial bore guage.
 

whyz

Subscriber
Joined
Nov 18, 2001
Messages
478
Likes
0
#9
Originally posted by Combat767
The piston expanded .001" 90 degrees opposite the piston pin area. You'd think it would shrink or wear, but it grew. Can you say odd! :think:

Could that be from not thoroughly warming up the bike to operating temp?
I dont know I'm just asking.
Thank You.
 
Joined
Aug 15, 2003
Messages
4
Likes
0
#10
I just had to same problem with my YZ450f and it was not the piston, it was the crank assembly. When it would cool off I could ride for a few minuets before it would seize up again. The crank had one tight spot in it. I change out the crank assembly and it is running fine. A crank assembly from Partsfish.com is about $310.00.
 

Combat767

It's all about the dirt..
Joined
Nov 20, 2001
Messages
200
Likes
0
#11
Well bluerider, I think you hit the nail on the head. It appears the problem is the crank assembly. They found a catch in the rod bearing once every couple turns. (Must have used a cheap bearing) They haven't torned the assy apart yet, but at least they're fairly sure what was causing the problem. I think he is going to have a talk with the dealer. A crank bearing shouldn't go bad the first year you own a bike. Leastwise under normal ride time & conditions.

Thanks everyone for the help!
 
Joined
Jan 10, 2002
Messages
494
Likes
0
#12
Originally posted by Combat767
A crank bearing shouldn't go bad the first year you own a bike. Leastwise under normal ride time & conditions.

Thanks everyone for the help!
20 races with proper maintance is still hard on a bike and remember this is the first year that these bikes have a longer stroke and something has to give. Do you remember all the problems the 250F's had the first year they were released?

How often did your buddy change the oil and what was he using? And did He change the filter every oil change?

Thanks for the insight.
 
Joined
Jan 10, 2002
Messages
494
Likes
0
#13
Originally posted by endosports


20 races with proper maintance is still hard on a bike and remember this is the first year that these bikes have a longer stroke

Nevermind that comment: I had a massive brain fart and thought this thread was about the 450.