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Nov 4, 2008

Dirt Bike Bottom End Rebuild, CR250 2-Stroke, Part 1

My bike has been getting sick for a while now and on our last outing it got much worse. I know for sure that the right main seal and bearing are bad since I have been burning clutch/gear oil and it has a definite knocking noise in that area. As can be seen this has been producing a great amount of white smoke and oil spooge all over the place. I will be replacing the crank, connecting rod, main bearings and seals. I will also be diagnosing the rest of the engine in the process.


Once you do this a few times you get to know a few tricks. First remove the 3 bolts to the sub frame, loosen the clamp on the air boot, and remove the sub frame and air box as an assembly. Leaving the 2 radiator scoops on the gas tank makes removing the front easier as well. Once these assemblies and the pipe is removed, the engine can be accessed to start to disassemble the engine.

A nice clean work space is a must. This is as good as it gets in my garage. Make sure to inspect the reeds. The carb will be cleaned inside and out before installing it back on the bike. New gaskets will also be used.

As soon as I removed the top end, I could see that my suspicions about the right main seal were correct. If you look closely you can see the pool of gear oil under the crank. A bad seal usually indicates a bad bearing since the bearing usually goes first taking out the seal as the worn bearing allows additional movement. I could move the crank slightly up and down also confirming that it was bad. The entire crank assembly and connecting rod will be replaced as a single unit. This old crank is a hot rods aftermarket crank, so I know that this has been replaced during the life of the motor. On average the bottom end requires attention about every 4 years, so being a 2001, it's right on schedule.

Since I am wanting to diagnose the entire engine for possible problems I started by taking measurements of the cylinder and piston. To measure the cylinder first determine the specifications. On a 2001 Cr250 it should measure between 2.6141 and 2.6144 with the service limit being 2.615. The cylinder should be measured in a number of locations along the cylinder on both x and y axis. My lowest measurement was 2.6143 and my highest was 2.6149 so it was within the service limits.

The piston on this bike should be between 2.6114 and 2.6117 with a service limit of 2.609 with a clearance to cylinder service limit of .09. I got a low reading of 2.606 on my piston, so I will probably end up replacing it.

In part 2, I will be removing the engine from the frame, and removing the parts from the lower end.

PART 4, before and after


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