Ask the Mechanic

Find the Knock

(Steve Thompson)

READER: I have a 4010 John Deere tractor I bought for odd jobs. While pulling a grain drill last spring, the engine began to knock a little. Then, it began to knock a little louder. We tore into it this past winter. The underside of the engine looked perfect -- main and rod bearings perfect. Then we removed the head, thinking something was on top of a piston. Nope. We took the engine further down and found a broken crankshaft between cylinders 3 and 4. Got any ideas what could have caused it?

STEVE: I believe the engine damper, sometimes called harmonic or vibration damper, has never been changed. The damper helps absorb the vibration that each piston rod delivers to the crankshaft each time the engine fires. I guess we can say that it helps keep the engine in "harmony." Deere recommends changing the damper every 5,000 hours. I bet the rubber insert in the dampener is hard as a rock. However, if the crankshaft has ever been "turned," this also increases the chances of crankshaft breakage, because a crankshaft is designed to flex a little, and it is heat-treated only on the thin outer surface of the journal allowing for long bearing life. This design allows for this very small flex action to happen.

Note: After visiting with the owner, he told me the crankshaft had been turned .010 on the rods and .020 on the mains. If you have a crank turned, be sure and take it to a shop that works on big diesel engines. How well it is done and inspected is important.

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-- Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email mechanic@progressivefarmer.com.

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