Blown Head Gasket

The head gasket's job is to seal the engine's head to the engine's block. It must seal water passages, oil passages, and the engine's compression. When any one of those passages springs a leak - you have a blown head gasket. You can find instructions to replace it in this link Head Gasket Instructions

Why does the head gasket leak? The head and the block on a Yanmar engine are made of steel. As the steel heats up it expands. The engine is designed to tolerate a certain amount of movement and, as long as the engine is kept in the normal temperature range, everything works perfectly. If the engine is allowed to overheat then the steel expands enough that things start to really move around and get distorted. This causes the sealing surfaces to become uneven and areas of low clamping tension eventually develop and things start to leak. Compression is usually first to leak because it has the most pressure (over 1,000psi when under a heavy load). The most common place for the compression to go is into the water ports (because the cylinder is surrounded by water to keep it cool). This is why the most common sign of a blown head gasket is bubbles in the radiator. When the engine's compression is allowed to leak past the metal sealing rings on the head gasket it burns the gasket material (combustion is a very hot and violent thing). This is why when you allow the engine to cool back down - you still have the leak. The gasket is ruined and needs to be replaced.

The head gasket can actually leak in other ways even though the compression leak is, by far, the most common. You can also leak water into your oil or oil into you water if a passage between an oil port and water port fails. You can even leak water or oil outside of the engine as an external leak if a leak develops on the outer side of the gasket sealing area.

How to test for a blown head gasket:

1. Let the tractor's engine cool completely
2. Remove the radiator cap & fill radiator completely to the top with coolant
3. Leave off radiator cap & start tractor
4. Let the tractor run for about a minute and watch for bubbles
5. If you see bubbles or if the water pushes out of the radiator - you probably have a blown head gasket.

Shop Note: be very careful when you remove your old gasket. FOR INSTRUCTIONS CLICK HERE . The LAST tool you should be using is a screwdriver. Use a razor blade and be very careful not to scratch the surface of the block or the head. Scratches and gouges will cause the new head gasket to leak! Be sure both the head & block mating surfaces are completely clean and dry. Install the head gasket dry. Never use silicone on a head gasket. You can use a copper gasket coating spray if you really want to.

Shop Note #2: Use a recently calibrated torque wrench (yes, torque wrenches can get off by a LOT in just a few uses) and it is also a good idea to re-torque your head gasket after a few hours of use. While not 100% necessary in all cases, it is one more piece of insurance against having to do this project again.

FOR REPLACEMENT INSTRUCTIONS CLICK HERE