The valve train opens the intake and exhaust valves at the right moments by a camshaft that turns at half the crankshaft speed, because each valve opens once in the four-stroke cycle of two crankshaft revolutions. The valve clearance, the cooling path through the valve seat and the spring strength decide whether the hot exhaust valve lasts, and a valve that cannot seat burns.
Part of Internal Combustion
The camshaft, followers and valves of a four-stroke engine: how they are timed, why clearance is needed, and how their failures are diagnosed.
One Valve Event Per Cycle Means The Cam Runs At Half Speed
A four-stroke cycle takes two crankshaft revolutions, 720 degrees. In that time each valve opens and closes once, so the camshaft that opens it turns once in two crankshaft turns, at half speed. It is driven from the crankshaft by gears, chain or belt. The carrier describes the strokes the valves serve: both valves closed for compression and power, the intake open while the piston goes down, the exhaust open while it comes back up.
In practice each valve opens a little before and closes a little after its nominal stroke, so the open periods overlap and are longer than a half-turn each. The exact angles belong to the engine, and this page gives none. The principle, from the corpus steam engines, is the same as for Valve Gear: a small number of timing events per revolution, set by the shape of the part that drives them. An engine uses lifted poppet valves and cams; the steam engine uses a sliding valve, described in Slide Valve.
Clearance, Seats And Springs Are The Three Things That Go Wrong
- Valve clearance (lash). A small gap is left in the train when the valve is closed, so that when the parts heat and expand the valve can still seat fully. Too little clearance holds the valve slightly open when hot, so it cannot pass its heat to the seat and burns, and the engine loses compression. Too much makes the engine noisy and shortens the valve’s open time. The clearance figures belong to the maker.
- Seat contact and cooling. A valve loses its heat mainly through the seat. A seat that is pitted or narrow, or a valve that does not meet it squarely, leaks and overheats. The exhaust valve runs hotter than the intake, which is cooled by the incoming charge.
- Spring strength. A spring that is too weak lets the valve float at high speed, so it does not follow the cam; a broken spring drops the valve into the cylinder.
The materials matter. An exhaust valve sees very hot gas and needs a heat-resistant steel; Thermal Properties explains why unequal expansion cracks parts long before they melt. A settlement cannot easily make such valves, which is another reason to salvage.
What You Need
- Feeler gauges or strips of known thickness to set clearance.
- A way to turn the engine by hand and find top dead centre on the compression stroke.
- A valve spring compressor or a lever, a lapping stick and grinding paste for the seats.
- The maker’s timing marks or data, or a marked timing wheel taken from the original.
Method: Set Valve Clearance
Step 1: Turn the engine to the compression stroke of the cylinder, where both valves are closed, usually with the timing mark at TDC.
Step 2: Slide the feeler between the valve stem and the rocker or tappet. The clearance is right when the gauge slides with a light drag.
Step 3: Adjust, lock and recheck. Adjusting nuts can shift when tightened.
Step 4: Repeat for each valve. Do it on a cold engine unless the maker says otherwise.
Step 5: Listen and check compression after starting. When it is wrong, ticking points to too much clearance, and a loss of compression with a hot exhaust points to too little.
Worked Example: Event Counts At A Given Speed
At 3,000 rpm, an assumed speed:
- Camshaft speed: 3,000 ÷ 2 = 1,500 rpm.
- Each valve opens once per camshaft revolution, so 1,500 times a minute.
- That is 1,500 ÷ 60 = 25 openings a second.
Each valve and spring therefore works 25 times a second and about 90,000 times an hour (1,500 × 60). A part that is slightly weak or slightly loose does not stay that way for long.
Common Mistakes
- Setting clearance with the cylinder on the wrong stroke. The cam is on its lobe and the reading is false.
- Setting zero clearance “to be safe”. The valve burns.
- Lapping a seat until it is wide and pitted. The valve loses its seal.
- Fitting a timing belt or chain a tooth out. Valves can strike pistons.
Safety
Valve springs store energy and timing errors destroy engines
A compressed valve spring that slips can fire a retainer or a collet across the room, so compress it with a proper tool and wear eye protection. Turn the engine by hand and watch for resistance after any timing work before it is ever started, because a wrongly timed valve can hit a piston and bend or break.
What’s Next
- Learn next: Fuel Systems — what the intake valve admits
- Combine with: Ignition Timing — the second timing system in the engine