Cranky crank

Freddyfred
Posts
110
Joined
3/23/2017
Location
Columbia, MO US
7/11/2022 12:51pm Edited Date/Time 7/12/2022 8:00am
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.

https://youtu.be/WBw2cgxe284

https://youtu.be/mRwwIQ4IC_A


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
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kb228
Posts
6170
Joined
1/31/2018
Location
Mansfield, OH US
7/11/2022 7:28pm
Could be the big end or the crank bearings. Its something you need to check when you have the cylinders off
tzmike
Posts
437
Joined
2/22/2014
Location
Warwick, NY US
7/12/2022 6:11am Edited Date/Time 7/12/2022 6:13am
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
garasaki
Posts
147
Joined
1/20/2021
Location
Mount Vernon, IA US
7/12/2022 7:51am Edited Date/Time 7/12/2022 8:00am
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.

The Shop

btroxler
Posts
113
Joined
5/22/2017
Location
Saltillo, MS US
7/12/2022 10:31am
Did you install the piston correctly with the arrow facing the exhaust port?

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