Cranky crank

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7/11/2022 12:51 PM

After changing pistons on my TM 250 and 300, I noticed they sounded quite different.
My 250, all of a sudden, had a knocking sound that sounded similar to an old YZ250 with a broken big end bearing I had.

Eliminating the piston, wrist pin and wrist pin bearing I bought a whole new set but that didn't help. Only thing it can be now is the big end bearing right?

The 2 engines can be heard here, first heat cycle.

I guess I should send the crank away to get rebuilt but can I keep running on this, even for a few more weeks I wonder?

Cheers/ Fred

7/11/2022 7:28 PM

Could be the big end or the crank bearings. Its something you need to check when you have the cylinders off


7/12/2022 6:11 AM
Edited Date/Time: 7/12/2022 6:13 AM

Correct piston size ? A B C D to account for cylinder wear ?
Top of the old piston will be marked with the size that was in previously
Alot of dealers only stock D pistons


7/12/2022 7:51 AM
Edited Date/Time: 7/12/2022 8:00 AM

I just did a top end on a KX100 and it exhibited a similar sound. Figured maybe it'd go away if I rode it for a while.

That strategy ended in a catastrophic failure costing well into the 4 digits to fix after only a few minutes of riding. Crank was seized.

I wouldn't recommend that route.

I wonder if it's not incorrectly sized piston. But in my case eventually the big end rod bearing blew up. Could have been both, based on how incredibly mangled the piston was.

It also is a good way to destroy your aftermarket heads.


7/12/2022 7:57 AM



7/12/2022 10:31 AM

Did you install the piston correctly with the arrow facing the exhaust port?