After my Honda CB500 was (mis)diagnosed to suffer from a blown headgasket, I trawled the net looking for more information on the topic.

What’s a headgasket

The headgasket is the seal between the cylinder head and the engine block. It ensures the pressure stays inside the cylinder(s), coolant stays inside the water sleeve (and doesn’t seep into the cylinders).

Symptoms of Headgasket problems (”blown headgasket”)

1. Loss of power - because compression from the cylinder(s) is escaping due to a leak of the headgasket.

2. Coolant can leak into the cylinders and evaporate inside or mix with the engine oil.

3. Mixing fluids (coolant and oil) causing either fluid to look contaminated (milky).

4. Compression from the cylinder may escape into the coolant system, causing coolant to bubble and gush out of the overflow tank.

5. Reducing coolant levels will cause the engine to overheat, which may seize the piston and cause further damage.

6. Excessive white smoke coming from the exhaust (caused by coolant evaporating inside the engine), damaging the catalytic converter (if your vehicle has one).

When your headgasket is damaged, it may be that you do not instantly notice a difference in how the vehicle drives. It could run like normal (but leaving behind clouds of white smoke) until all the coolant has gone and the engine overheats.

Repair

Depending on what type of vehicle you have (how much it’s worth to you!) there are two ways of countering a headgasket failure. Either take it to a mechanic and have the headgasket replaced (or if you feel up to it, replace it yourself). This could be quite costly, which may or may not be worth it considering the value of the vehicle.

You may also want to try a less conventional method; Steel Seal. It’s a liquid which you pour into your radiator. It then moves around and (hopefully) seals the damage on the gasket to make it “as new”. Now I haven’t used this, so can only go by people’s opinions in various forum posts and reviews. The crowd is divided. Some people say it’s crap, and some say it’s brilliant. But it’s not that expensive so you might want to try it out before shelling out hundreds of pounds replacing the headgasket….

Prevention

There is no way to prevent your headgasket from blowing. As the engine gets older, the seal may become weak and just give up after a while. All you can do is pay attention to what your vehicle is doing (is excessive white smoke coming out of the exhaust?) and try to limit the damage to just the head gasket rather than driving around until your engine overheats and causes all sorts of additional problems!

A few months back I tried to start the CB500 in the cold, and stupidly switched on the choke before switching in the engine on. This little mistake turned into a big nuisance as the engine started, coughed and stopped and left me standing outside my office, repeatedly trying to start the bike again. However as the engine had got flooded with excess petrol, nothing happened. I ended up parking the bike overnight, in order to get it fixed the following day.

As I told the mechanic what had happened he explained the following: Once the carbs flood with fuel, the spark plugs will get wet and basically become useless. After that, the points of the spark plugs (even if you do take them out, wipe them dry, and sand off the dirt from them) will never be clean enough to give a reliable spark.

It’s not all that bad though. Replacement spark plugs come for around £6, and if you have the original tools that come with the bike, you will already have all required tools to replace the spark plugs yourself.

Here’s what you do. Once you get two replacement spark plugs, take the side covers (shrouds) off the tank. On the top of the engine you will see black rubber plugs which go all the way inside. This is a little fiddly, especially if you have big hands, but it’s just about possible to pull out the plugs from the engine and take them out entirely, put the spark plug socket inside the cavity you just removed the rubber plugs from. Open the spark plug by twisting on the tool with a spanner going anti clockwise. This may require quite a bit of force!

Once the spark plugs are unscrewed, just take them out, if you can reach make sure the area round the spark plugs is clean and dry, and put the new one in. Repeat this process on the other side of the engine. Once you’re done, simply push the plugs back in, make sure they are properly tightly connected - you might feel a subtle click when they push in, after which they won’t move around anymore. Then put the shrouds back on and you should be up and running!

Just wanting to look something up I did a quick search for an online version of a service manual, well here is the full thing:

Honda CB500(S) 93-01 Service Manual