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84' Goldwing Aspencade, 91' EX500, 98' Ninja 250 w/ 17' 300 engine, 07 EX500
187 Posts
Looks like the head gasket is going to me.

The moisture in the sight glass does have a slight milkshake look to it, but that's from not getting the up to temperature. This happens when one is just starting it up, when the bike is in storage, and/or short rides.

Your friend might what to do a compression test.

Assuming it's just starting, I'd say pull the head, and the cylinder jug, and lapp them both, and get a new gasket of course. Upon taking the head off you should see a bad head gasket. Lapping is a lost art for sure.

If it was me, I'd just get a good used engine, this will save a lot of down time, and then work on the bad engine in my own time, and have a spare engine. If anything I would learn about the engine.

I wish your friend good luck 🤞
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Premium Member
84' Goldwing Aspencade, 91' EX500, 98' Ninja 250 w/ 17' 300 engine, 07 EX500
187 Posts
Lapping valves, jug, and head write-up,

Okay this goes for the Ninja 500, but I'm sure the Ninja 250/300 is the same, if not please let me know.

The intake valves are case hardened to a thin level of hardness and much in the way of metal removal with destroy them.

Why do you think you need to lap them? if they are worn you need to re cut the seats and fit new valves ,no lapping required. If they are worn badly enough to need anything at all they develop a groove that offself is the detriment to flow. Lapping will make that worse.

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 below.

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
Automotive engine gasket Gas Auto part Event Circle

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)

After assembly increase the Head Bolt torque to 40 lbs. ft.

Now’s a good time to degree you cams.

Freshly lapped set
Automotive engine gasket Gas Motor vehicle Engineering Automotive exterior

Shows the process and the lapp plate
Watch Gas Wood Auto part Machine
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