I have a problem that I can not resolve. I punched my 2000 XR400 into a 440 with a Hotcam about 2 years ago with nothing but good results. A few months back during an easy ride, my transmission countershaft sheared just inboard of the right side bearing. When the shaft broke I was decelerating and the engine was at mid to low rpm. Well, $800 in parts and 2 months later, I just finished reassembly. When I had the engine apart I checked every part I could item for signs of damage or wear and replaced anything which was outside usable limits. No significant finding except for the countershaft and a galled piston and scratched sleeve. The major items replaced this time were:
New 11:1 89 MM piston
New overbore Sleeve
New Countershaft and bearings
All new seals
New sprockets and chain
I am confident that the engine is properly assembled.
During my first easy break-in ride I was going along at about half throttle in 4th or 5th and the engine came to an immediate stop (locked up). The back tire skidded until I pulled in the clutch. I pulled over and found no signs of any external damage and then started it on the first kick and was back riding again. No repeat for the next few miles home. No abnormal engine noise, no erratic shifting, no other signs of anything wrong.
Has anyone out there ever heard of an intermittent engine lock-up on the XR400? Any thoughts on what could have been the root cause of the original countershaft failure? Would love to hear any advise you might have.