Big Bang Theory

READER: I have a 3020 John Deere diesel that blew a head gasket out of both sides of the engine between the head and the block while I was cranking it to start. The engine was overhauled five years ago, and it has been a jewel of a tractor its whole life. Do you have any idea why it just blew the head gasket with a big sound like a shotgun?

STEVE: I have a good idea what caused it. This is a common occurrence when any liquid gets in a cylinder. Coolant can leak in the cylinder from a small leak in the head gasket, from diesel leaking in from a dripping injector or, the most common way, from rainwater creeping in from heavy rain entering through an open exhaust system.

Fluid can’t be easily compressed, so the compression gets extremely high in the engine when the piston comes up on the power stroke, and the head gasket can’t hold this extreme amount of pressure. The head gasket is the weakest link for holding in compression, but the good news is that it will usually blow before the extreme pressure bends a connecting rod. The before and after photos (accompanying this article) of a blown 3020 head gasket show the power of the shotgun explosion you heard.

If you think none of the fluids I mentioned entered the cylinder, and you have not exposed the engine to an excessive amount of starting fluid, then it might be a good idea to check the protrusion of the cylinder liner tops above the engine block with a dial indicator (where the head gasket seals compression) to engine specifications with your John Deere dealer. If it is out of specs, there are ways to correct it.

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— Email Steve Thompson at mechanic@progressivefarmer.com, and be sure to include your contact information and phone number.

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