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Help 128: Valve Gap

Generally, Triumph, BSA, Norton valve clearances gets tighter as they wear. They can get looser if something else becomes loose in the cylinder or with cam lobe wear, but that is rare. Why Do Exhaust Valves Get Tighter? As a valve wears, it sinks into the valve seat, thus making the gap clearance smaller. Not all engines act this way.

Why Is Valve Clearance Important?

Valve clearance is much necessary for a well performing Motorcycle. When the intake valve is opened, the air-fuel mixture moves through the valve opening into the cylinder. Next the intake Valve closes and the piston compresses the air-fuel, the spark ignites the mixture and then power is delivered to the crankshaft.

The exhaust valves close to allow compression, then open to allow exhaust to exit the engine. The valve timing or the exact second each valve opens and closes has effect on the power.

If the valve gap is too small, the valve will open too soon and close late, maybe even staying open in extreme cases. How long a valve stays open will effect the amount of air-fuel mixture, compression, and temperature of the engine (Valve, piston, spark plug, etc). If the valve gap is too large the valve will open too late and close early thus effecting the same engine elements and likewise performance. Set the valves to close, the valves will hang open and a burnt valve may result. Set them to loose, and the engine may self-destruct. In some engines, the valve could hit the piston.

As a engine warms up, the valve clearance gets smaller. The Metal in valve stems, push rods and the entire valve train expand as they heat up and this expanding closes the gap a small amount. If the clearance is too tight, the valve may hang open after the engine warms up. This expansion is taken into consideration in the valve gap measurements. Cold an engine with "solid lifters" (british Bikes) sound "noisey" but warmed up, the engine "quiets" down. If you change the metal in the push rods the expansion of the metal will change, thus a different valve gap is requied. Joe has steel valve rods that require a smaller gap. They were manufactured by STEEL BREEZE RACE TEAM for old T150s, T160, T140s and T120 Triumphs. Steel push rods will improve performance. They maintain a smaller gap during all temperatures causing less valve noise. The Metal Rods flex less during operations. Better performance is a result of inserting these in your engine. If you would like a set, contact Joe's Garage at joes_garage@yahoo.com. They have the exclusive rights to sale of these rods.


It is one of the dangerous symptoms of explosions. When clearance is too low, less gap in the valves, air passing becomes difficult into the cylinder chamber. When enough space is there, heat can flow out from the exhaust, but in this case, heat remains inside the engine and makes it hotter. Also, less clearance affects compression, and such low compression effects performance. If you keep ignoring this issue, it causes the valve to break. So, if you see overheating even at a cold place, better check valve clearance.

Premature Combustion-

Premature combustion means that the spark plug gets enough heat to fire before completion of the combustion process. When you start your Motorcycle, the engine takes a few minutes to heat up, as its temperature rises, valve clearance is reduced. Less clearance seals the heat properly and helps in firing the spark plug. But, in case of less clearance, the engine heats quickly, and as a result, the bike starts earlier, even before the spark plug fires. It’s not a good sign. If you face the same problem, be sure there is something wrong with the valve clearance. Ignoring it may produce unpleasant sounds and even damage the valve soon.

More Fuel Consumption-

The intake valve allows an air-fuel mixture into the cylinder. If the valve clearance is too tight or loose, the exhaust valve may remains open, and at the end, whatever fuel goes inside the cylinder can directly reach the exhausting system and collect in more & more amounts. Backfires can happen due to extra fuel present in the engine. The engine will also use more fuel.

Knocking Noise-

Suddenly you hear a knocking noise coming from the head of your engine. It’s a sign of a loose valve. If you haven’t set the valve clearance at a suitable space, it disturbs the whole valve train, and that string develops an annoying sound. You should take such sounds seriously and at least have a look towards the clearance. As you know, loose clearance can cost you alot. It will affect the acceleration and later performance of motorcycle. If you have any of those symptoms, don’t ride a bike. Overheating is the most common symptom, if it happens to your motorcycle; shut it down and check the valves.

Can Valve Clearance Cause No Start?

If the valve clearance is too tight, less power will be passed to the crankshaft. The bike will be harder to start, especially with kick starting. All over, an inadequate valve clearance can cause no start and also can wreck your motorcycle.

How Often Should Check Valve Clearance?

It’s best to inspect after every 5000 miles. If you keep maintaining your motorcycle, you should also include valve clearance. This good habit will elongate the life of your bike and its parts. And the important thing is to check the intake clearance should be 080 inches, and for exhaust, it would be 100 inches and keep an extra exhaust valve as it expands a lot due to heat transfer from the combustion.

Any Question E-Mail Mike care of Joe.
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