Farmer's Weekly (South Africa)
Setting valve clearance
In older engines, valve clearance must be adjusted accurately and by hand using a special measuring tool. Jake Venter explains how to do this.
In most modern engines, the valves operate via one or two overhead camshafts on top of the cylinder head. The camshaft followers are usually fitted with hydraulic valve clearance adjustment devices that keep the clearances at their designed values. Some older designs still employ camshafts inside the cylinder block that operate valves in the cylinder head via pushrods, but even these now have hydraulic adjusters.
So much for modern vehicles. Many thousands of cars and trucks on our roads still have older pushrod engines with manually adjustable valves. If you have such a vehicle, and you’ve never adjusted the valves, you may find it quite daunting. So here is a guide to help you.
rotating the engine precisely
Once you’ve removed the tappet cover, you’ll find some valves open and others closed. A valve’s clearance can only be adjusted when it is fully closed. This means that the engine has to be in a specific position to enable you to adjust a particular valve. There are several ways to do this:
• The first step is to remove all the spark plugs so that it becomes easier to rotate the engine either with a crank handle or a suitable spanner. If you can’t reach the nut that’s usually on the crankshaft pulley, you can always put the gearbox in top gear and nudge the car with your body to rotate the engine.
• A time-consuming but fail-safe way is to turn the engine until a particular valve is fully open and then rotate the engine exactly one revolution. That valve will now be closed and can be adjusted.
• A better way is to get someone to rotate the engine while you put your finger in the spark plug hole. You’ll feel and hear the air escape as the piston comes up on the compression stroke. You can even judge when the piston is very close to top dead centre by putting a piece of stiff wire into the hole. At top dead centre on the compression stroke, both valves are closed and can be adjusted. When using this method you have to rotate the engine 180° every time.
• The valves on a conventional four-cylinder engine, where the firing order is 1-3-4-2, can be adjusted by applying the ‘rule of nine’. This means the number of the open valve plus the number of the closed valve you’re setting must add up to nine. You must rotate the engine until: – Valve number 1 is open. You
can then set valve number 8. – When number 2 is open,
you can set number 7.
– When number 3 is open, you can set number 6. And so on. Four-stroke engines need correctly timed positive opening and closing valves so that the fuel-air mixture can arrive and depart from the combustion chamber at the correct times. These are controlled by the timing and shape of the cams mounted on the camshaft, but the actual timing values are also determined by the valve clearance gap, which is established during the design process.
the dangers of very narrow clearance
The clearance gap has a further role to play. Its value controls the length of time the valve is in contact with its seat. This is the only time the valve gets a chance to cool down properly. If the clearance is too small, the valve will open too soon, run hotter and may even be destroyed.
When the clearance is excessive, the valve will open later. This may affect the power output and even the starting ability. The tappets will also get noisier. But it has the advantage of cooler-running valves.
The valves should be adjusted while the engine is at its normal operating temperature. This can only be achieved quickly by taking the car for a spin. Cold settings are also given in a workshop manual.
• Jake Venter is a journalist and a retired engineer and mathematician. Email him at jacobventer77@gmail.com. Subject line: Auto engineering.