Replacing Yanmar Tractor Head Gasket

So, you have gone through our "How To Tell If Your Head Gasket Is Blown" article and confirmed that you need to replace your head gasket.

Is this something that you can do yourself or will you need to hire a mechanic? Every situation is different but, in general, these engines are so simple that almost anyone with a set of tools, a mechanical mind, and a Yanmar service manual can follow the steps and successfully replace their Yanmar head gasket.

The instructions below are intended to be a general guide so that you can see what is involved. I would strongly recommend getting a service manual in order to get the full detailed instructions. 

• First you will want to start with a very clean engine. This is surgery and you don't want dirt and grime getting into your patient during surgery. A power washer works best but a garden hose and a brush works too. Products like Simple Green or Purple Power work great at breaking down the old grime. 
• Drain the engine coolant and keep it away from kids and animals. It, apparently, has a sweet taste and is toxic.  
• Remove the muffler so the head is lighter and easier to handle.
• Unbolt or unclamp the intake depending on model. Plug the holes in the intake with a shop rag to prevent anything from falling into the intake. 
• Remove the hoses from the water pump and remove the water pump so the head is easier to handle.  
• Remove the valve cover.
• Remove the rocker arms and pull out the pushrods. It is best practice to store them in order so that they go back into the same hole. Those components have spent a lot of years working together and keeping them with their mate will make sure that the wear patterns are matched and things last longer. Poking holes in a cardboard box and sliding the pushrods into them in order work well. 
• Unbolt the head. Keep track of where the bolts or nuts go since they might be a different length or size. 
• Remove the head. The gasket might be stuck to the head and block. You might need to gently pry on the head with a prybar. Be careful not to scratch the mating surfaces of the head or block.  
• Plug all of the ports, especially the oil ports, with shop towels to keep junk from getting into the openings. 
• Clean the mating surfaces with a razor and steel wool. Be careful not to scratch the surfaces.
• Vacuum all remaining gasket material out of the cylinders on top of the pistons. 
• This is a good time to inspect the cylinders for excess wear or damage. If they need to be replaced then this is a good time to do it. 

You are at the half way point! Give yourself a quick pat on the back and get back to work. We still have to see if you can get this all put back together. :)

• Blow out any of the bolt holes where head bolts will go. If there is dirt or liquid in the holes then the bolts might bottom out before they are tight. 
• Carefully install the new head gasket. They are normally not symmetrical so they should only line up with all of the ports if installed one way. 
• Do not use any adhesive or silicone on the gasket. It should install dry & clean.
• Check one more time to be sure you got out all of the shop rags and that there is nothing on top of the pistons. There is nothing worse than getting an engine all together and then wondering where that missing nut went. 
• Carefully lower the head onto the gasket 
• Brush the head bolts with a little engine before installing. 
• Torque the head bolts to spec (follow recommendation in service manual) and tighten the bolts/nuts starting from the center most bolts and work outwards in a cross pattern. Pro Tip: Get a torque wrench tester and be sure your wrench is calibrated correctly. The testers are very cheap on Amazon. We have a very good torque wrench and it was off by almost 20% after a few years of sitting in a drawer.
• Torque the bolts in 3 stages. If the final torque is 130ft/lbs (made up number) then tighten them all to 50, then all to 100, then all to the final 130. 
• Reinstall the pushrods and rocker arms. 
• Set the valve clearance using this article
• Remove anything from the intake and exhaust ports
• Reinstall the water pump and muffler using a new gasket for both.
• Reinstall the intake or intake tube.
• Fill with coolant & check for any leaks. 
• Check over everything one last time
• Start the engine and enjoy the fruits of your labor. 

You are not quite done yet though. The new head gasket will need to be re-torqued al least once after you get it up to operating temperature and allowed to cool over night.

• Remove valve cover
• Retorque bolts/nuts to correct torque using the same pattern
• Reinstall valve cover
• You are ready to get back to work

You could even repeat this re-torque cycle in a week or so just for good measure. 

So why don't the factory head bolts need retorqued? The factory typically uses metal head gaskets because the head and block surfaces are perfect. 40 years later those surfaces have tiny rust pits and scratches and a metal gasket will no longer seal. This is why new gaskets are usually made from a composite material. Unfortunately, composite gaskets all need to be re-torqued.