On 4-stroke internal combustion engines, setting the valve timing is critical. Different engine designs have different methods of achieving the same objective-precise, reliable operation of the inlet and exhaust valves.
The experienced mechanic will approach each engine design to ascertain the correct method for setting that engine's valve timing. He may consult a shop manual for any special considerations, but in general he will need to know:
- Crankshaft position
- Camshaft position
- Timing marks
- Direction of rotation
- Relative cylinder number (on multi-cylinder engines, each cylinder has a number applied to it. For example, on the four cylinder Suzuki GS 1000 engines' cylinders are numbered one through four from left to right--number one cylinder is on the left or clutch side)
Knowing the timing system before disassembling or reassembling an engine is critical, but one aspect of timing comes before all others: crankshaft position.
Number One Cylinder
When a mechanic approaches an engine to ascertain the crank position, he first must identify the position of number one cylinder. The vast majority of engines have timing marks on their ignition flywheel and often an arrow to indicate the running direction of the engine. However, if the mechanic is unsure of the direction of rotation, he should remove the spark plug/s, select 2nd gear and rotate the rear wheel in a forward direction noting the direction of rotation of the flywheel.
Once the engine's direction of rotation has been ascertained, the mechanic can move on to finding the position of the engine. For example, he must find which stroke the piston is on (inlet, compression, power, exhaust). A visual inspection through the spark plug hole is generally all that is required to determine the stroke. However, it is good practice to find the inlet stroke first; this can be achieved by visual inspection or by removing the inlet valve cover (where applicable) and noting when the valve opens-the piston will begin its downward stroke as the inlet valve opens.
Another method of determining when a piston is on the compression stroke is to use a cranking pressure tester (compression tester). When the gauge shows an increase in pressure, the piston is on the compression stroke. However, this method will not work if any of the valves are damaged or stuck (typically after being incorrectly stored for some time).
When the position of the number one piston has been ascertained, the mechanic should rotate the engine until the piston is moving upwards on the compression stroke (both valves closed). At this point a suitable measuring device should be inserted into the spark plug hole.
The ideal tool for this purpose is a dial gauge indicator. These tools are available from dealers, specialist tool suppliers and on-line retailers, with prices starting at around $30.
The use of a dial gauge indicator ensures accuracy when finding TDC (Top Dead Center). TDC is typically the point from where all timing procedures begin. However, a common drinking straw can be inserted into the spark plug hole to determine, approximately, when the piston is at TDC. When using the dial gauge, the actual point of TDC will be the point at which the dial needle begins to reverse its rotation.
The mechanic should examine the flywheel at this point to locate the TDC timing marks. (Highlighting the marks with an orange paint pen, for instance, will help to see the marks more clearly-something that is particularly important when using a timing light for ignition timing checks).
Camshafts are gear, chain or belt driven. Gear driven camshafts are, as the name implies, camshafts that are driven by a single or series of gears. Typically the gears and camshaft have alignment marks on them. However, occasionally, some gear driven systems will require the use of a degree wheel attached to the crankshaft, to place the crankshaft in a precise location before the gears and camshaft are engaged.
Belt and chain driven camshafts follow a similar location procedure. The crankshaft will be positioned as per the manufacturer's specifications (found in a shop manual), as will the camshaft. The connecting belt or chain will then be fitted at a set number of teeth between the camshaft alignment marks and the crankshaft alignment marks.
Rotate Slowly to Check
Whenever a mechanic has re-timed an engine, it is good practice to slowly rotate the crankshaft by hand (a wrench on the flywheel center bolt works best). This rotation must be done slowly and halted if the mechanic feels any resistance, as this may indicate a valve is hitting a piston due to incorrect timing.