What is The Rule of Nine ?

The "Rule of Nine" is a practical method used to simplify the process of checking and adjusting valve lash on a 4-cylinder engine, particularly for engines with a pushrod design or adjustable rocker arms. 

It’s a shortcut that helps mechanics identify which valves are in the correct position (fully closed) for checking and adjusting the valve lash without needing to track the exact position of each cylinder in the firing order. 

Below, I’ll break down this method in detail, focusing on how it works, why it’s used, and how to apply it, with examples specific to a 4-cylinder engine.

What is the Rule of Nine?

The Rule of Nine is a technique that applies to engines with a 4-stroke cycle, where the crankshaft rotates twice (720°) to complete one full cycle of all cylinders. 

It’s based on the principle that when one valve is fully open (at its maximum lift, typically on the exhaust or intake stroke), another valve in the engine is fully closed (at the base circle of the camshaft) and ready for lash adjustment. 

The "nine" comes from the sum of the valve positions in the engine’s firing order or valve sequence that ensures the correct valve is checked.

For a 4-cylinder engine with a typical firing order of 1-3-4-2 and two valves per cylinder (one intake, one exhaust), there are 8 valves total. The Rule of Nine helps you systematically check all 8 valves by rotating the crankshaft and using the relationship between valve positions.

How Does It Work?

1. Valve Position Relationship:

In a 4-cylinder engine, the camshaft rotates at half the speed of the crankshaft. When one valve is fully open (e.g., during the exhaust or intake stroke), another valve elsewhere in the engine is fully closed (on the compression or power stroke).

The Rule of Nine uses a numerical system to pair valves. If you number the valves in a specific sequence (usually following the firing order or cylinder layout), the valve that is fully open plus the valve that is fully closed will sum to 9.

2. Valve Numbering:

Valves are typically numbered based on their position in the engine and the firing order. For a 4-cylinder engine with cylinders numbered 1 through 4 (front to rear) and two valves per cylinder:

     - Cylinder 1: Exhaust (E1), Intake (I1)

     - Cylinder 2: Exhaust (E2), Intake (I2)

     - Cylinder 3: Exhaust (E3), Intake (I3)

     - Cylinder 4: Exhaust (E4), Intake (I4)

  

The valves are assigned numbers 1 through 8 in a sequence, such as:

1: E1

2: I1

3: E2

4: I2

5: E3

6: I3

7: E4 

8: I4

When valve 1 (E1) is fully open, valve 8 (I4) is fully closed because 1 + 8 = 9. Similarly, when valve 2 (I1) is open, valve 7 (E4) is closed (2 + 7 = 9), and so on.

3. Crankshaft Rotation:

You rotate the crankshaft to position one cylinder at a specific point (often Top Dead Center, TDC, on the compression stroke for cylinder 1 to start).

As you rotate the crankshaft (typically in 180° increments for a 4-cylinder engine), different valves become fully open, allowing you to check the corresponding closed valves.

Applying the Rule of Nine

Here’s a step-by-step explanation of how to use the Rule of Nine to check valve lash on a 4-cylinder engine:

1. Set Cylinder 1 to TDC (Compression Stroke):

Rotate the crankshaft until cylinder 1 is at Top Dead Center (TDC) on its compression stroke. At this point:

     - Both the intake (I1) and exhaust (E1) valves for cylinder 1 are fully closed (ready for lash adjustment).

     - The camshaft is positioned such that other valves are open, and their corresponding paired valves (per the Rule of Nine) are closed.

2. Identify Open Valves and Their Pairs:

When cylinder 1 is at TDC compression, certain valves elsewhere in the engine are fully open. For example, in a 1-3-4-2 firing order, cylinder 4 might be on its exhaust stroke, meaning its exhaust valve (E4) is open.

Using the Rule of Nine, if E4 (valve 7 in the sequence) is open, check the valve that pairs with it to equal 9:

     7 (E4) + 2 (I1) = 9, so check I1 (intake valve of cylinder 1).

3. Check and Adjust Lash:

Use a feeler gauge to measure the clearance between the rocker arm and valve stem (or camshaft and follower) for the closed valve (e.g., I1).

If the clearance is incorrect, adjust it by loosening the locknut, turning the adjusting screw, and retightening the locknut, then recheck with the feeler gauge.

4. Rotate the Crankshaft:

Rotate the crankshaft 180° to bring the next cylinder (e.g., cylinder 3 in the 1-3-4-2 firing order) to TDC or the appropriate position.

Identify which valve is fully open (e.g., E3 or I3), and use the Rule of Nine to find the corresponding closed valve (e.g., if E3 = valve 5 is open, check valve 4 because 5 + 4 = 9).

5. Continue Through All Valves:

Repeat the process, rotating the crankshaft 180° each time to check all 8 valves. The Rule of Nine ensures you check each valve when it’s fully closed.

Example Using the Rule of Nine

Assume a 4-cylinder engine with a 1-3-4-2 firing order and the valve sequence as above (E1=1, I1=2, E2=3, I2=4, E3=5, I3=6, E4=7, I4=8). Here’s how you might proceed:

1. Position 1: Cylinder 1 at TDC Compression

   - E1 (valve 1) and I1 (valve 2) are closed and can be checked directly.

   - At this point, cylinder 4 is on its exhaust stroke, so E4 (valve 7) is open.

   - Rule of Nine: 7 (E4) + 2 (I1) = 9, so I1 is already confirmed closed (you can check it).

   - Check E1 (valve 1) as well, since it’s closed. Pair it with valve 8 (I4): 1 + 8 = 9. If I4 is open, E1 is closed.

2. Rotate Crankshaft 180°: Cylinder 3 at TDC Compression

   - E3 (valve 5) and I3 (valve 6) are closed.

   - Cylinder 2 is on its exhaust stroke, so E2 (valve 3) is open.

   - Rule of Nine: 3 (E2) + 6 (I3) = 9, so check I3.

   - Also check E3 (valve 5). Pair it with valve 4 (I2): 5 + 4 = 9.

3. Rotate Crankshaft Another 180°: Cylinder 4 at TDC Compression

   - E4 (valve 7) and I4 (valve 8) are closed.

   - Cylinder 1 is on its exhaust stroke, so E1 (valve 1) is open.

   - Rule of Nine: 1 (E1) + 8 (I4) = 9, so check I4.

   - Check E4 (valve 7). Pair it with valve 2 (I1): 7 + 2 = 9.

4. Rotate Crankshaft Another 180°: Cylinder 2 at TDC Compression

   - E2 (valve 3) and I2 (valve 4) are closed.

   - Cylinder 3 is on its exhaust stroke, so E3 (valve 5) is open.

   - Rule of Nine: 5 (E3) + 4 (I2) = 9, so check I2.

   - Check E2 (valve 3). Pair it with valve 6 (I3): 3 + 6 = 9.

Why Use the Rule of Nine?

Efficiency: It reduces the need to track every cylinder’s exact position in the 4-stroke cycle. Instead, you focus on which valves are open and use the rule to find the closed ones.

Accuracy: It ensures you’re checking valves when they’re fully closed (on the camshaft’s base circle), which is critical for accurate lash measurement.

Simplicity: For older engines with adjustable valves, it’s a time-tested method that doesn’t require complex tools or calculations.

Limitations and Notes

Not Universal: The Rule of Nine is specific to certain engine designs, particularly those with pushrod systems or older overhead cam setups. Modern engines with hydraulic lifters or non-adjustable valvetrains may not require this method.

Check the Manual: Some engines may have a different sequence or firing order, so always verify with the manufacturer’s service manual. The Rule of Nine assumes a standard valve layout and firing order (1-3-4-2).

Precision: Ensure you correctly identify when a valve is fully open (maximum lift) to apply the rule accurately.

Alternative Methods: Some mechanics prefer other methods, like checking valves cylinder by cylinder at TDC compression or following a specific sequence in the manual, which may be more straightforward for beginners.

Practical Example Clarification

In the original example: “If exhaust valve 1 is open, check intake valve 4 (1 + 4 = 5, but follow the sequence in the manual).” This was a simplification, and I’ll clarify:

If exhaust valve 1 (E1, valve 1 in the sequence) is fully open (cylinder 1 on its exhaust stroke), the Rule of Nine says to check valve 8 (I4, intake valve of cylinder 4) because 1 + 8 = 9.

The mention of “1 + 4 = 5” was an error in the original phrasing, as the correct pairing must sum to 9. The manual’s sequence should always guide you, but the Rule of Nine provides a reliable fallback for standard 4-cylinder engines.