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· Fast Old Guy
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Discussion Starter · #1 ·
Head Gasket repair
Lapping the head

A common failure of the EX engine is, a blown head gasket. The usual cause of this besides the obvious low water or frozen coolant reasons is, warpage of the mating surface between the head and the cylinders. The string of events leading to overheating is:
High pressure gas leaks from the combustion chamber into the coolant system, thereby over pressurizing it and blowing coolant overboard, then the coolant shortage causes overheating which causes even greater warp age, add on infanatum...

As well as the above, any engine that has had the head removed for any reason need to be re lapped. This engine is a very flexible unit because of the cam chain gallery down the middle. The only thing holding the two cylinders together is the very thin wall of the front and back sides of the chain gallery. 1/8” if you’re lucky. This means that the engine works like two single cylinder engines running from a single crank held together by scotch tape. As long as they remain together they are usually fine, but once disassembled they need to be re flattened

The repair:
Unfortunately it is not a simple as replacing the head gasket. Because the “gasket” is not compressible, it is not able to absorb even slight distortion or warp age that may not even be detectable with the usual straight edge type of inspection. The up side of this is that you can re use the gasket.

Tools needed:
A lapping plate; usually a steel flat plate or a pain of flat plate glass. I’ve used my table saw table successfully (the cast iron type)
Lapping compound: Valve lapping compound available at a Auto parts store
The means to wash the parts clean after lapping. A bucket of Kerosene is fine.

The Idea is to use the abrasive to grind down the high spots of the head and the cylinders till they are perfectly flat. See the pictures.

The process move the part over the surface of the Lapp in a figure 8 motion
Clean and inspect your progress often. Shown is a partially cleaned surface

Here are the completed two parts.

You are now ready for reassembly. Clean the old head gasket with lacquer thinner and Scotch brute or steel wool. Then repaint it with 1 coat of spray enamel on both sides (1 coat)
You ought to re- fresh your valve seats now too.
After assembly increase the Head Bolt torque to 40 lbs. ft.

Now’s a good time to degree you cams.

Pi1 shows a freshly lapped set
P 2 shows the process and the lapp plate
p3 showa a partiallycleaned up head with the dark spots still holes in the surface

PS. I have found a good source for a suitable lapping plate is a Granite shop. One that actually cuts the stone for counter tops. They have a pile of sink drops out back that they will give you for free. (usually). these are good 1 time tools

here are some pic to show you how lapping is done
Bottom: the plate
next p: partially cleaned note the rings from the compression rings
middle finished head and cylinders
Top: partially cleaned up you can see the warpage and the why it must be done

Good luck

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