Understanding Boat Engine Rotation: Standard Or Reverse?

Boats with twin engines are usually set up so that the torque reactions of the engines cancel each other out. This means that one engine will be rotating in the standard direction, and the other will be rotating in reverse. The direction of rotation is determined by the camshaft and crankshaft, which can be checked by looking at the engine from the front or rear. It's important to know the correct rotation of your boat engine, as using the wrong rotation can cause problems and even damage the engine.

How to identify reverse rotation or standard rotation engines

There are several ways to identify whether an engine is a reverse rotation or standard rotation engine.

Firstly, it is important to note that engine rotation is viewed from the flywheel of the engine. If the flywheel turns clockwise, the engine is a Right-Hand (RH) rotation engine. If the flywheel turns counter-clockwise, the engine is a Left-Hand (LH) rotation engine. When viewed from the front of the engine at the belts and pulleys, the direction of rotation is opposite. That is, if the belts and pulleys turn clockwise, the engine is a LH rotation engine, and vice versa.

Secondly, the crankshaft and camshaft can provide some clues. Some reverse rotation cranks have oil holes drilled symmetrically opposite, so this should be checked closely. Additionally, the camshafts are not interchangeable between standard and reverse rotation engines, as the lobe timing and/or distributor drive gear angle are different.

Thirdly, the distributor and oil pump turn in the same direction regardless of the crank rotation. This is achieved by making the angle of the drive gear on the cam and its mating gear opposite when the cam turns in the opposite direction.

Finally, the firing order of reverse rotation engines is always 12756348, and the pistons need to be installed in the correct direction to prevent piston slap. The wrist pins are slightly off-center to compensate for side loading at the top of the power stroke, so the pistons must be installed with the notches facing the rear on a Right-Hand engine.

It is important to note that the propeller rotation should not be used to determine engine rotation, as all I/O engines are LH rotation.

The firing order of reverse rotation and standard rotation engines

The firing order of an engine is the sequence in which cylinders are fired. It is important to remember that the numbering conventions for engine cylinders differ by manufacturer.

The firing order of a standard rotation engine is 1-8-4-3-6-5-7-2. The firing order of a reverse rotation engine is 1-2-7-5-6-3-4-8.

For a standard rotation engine, the distributor rotates counter-clockwise when viewed from the flywheel. For a reverse rotation engine, the distributor rotates clockwise.

An easy way to remember the firing order of a reverse rotation marine engine is to run the standard rotation firing order backwards (counter-clockwise) around the distributor.

• AMC (most V8 engines): Clockwise 1-8-4-3-6-5-7-2
• Buick (most V8 except HEI): Clockwise 1-8-4-3-6-5-7-2
• Chevrolet: Clockwise 1-8-4-3-6-5-7-2
• GM LS: 1-8-7-2-6-5-4-3
• Small Chrysler: Clockwise 1-8-4-3-6-5-7-2
• Big Chrysler and Hemi: Counter-clockwise 1-8-4-3-6-5-7-2
• Most Ford V8: Counter-clockwise 1-5-4-2-6-3-7-8
• Ford (5.0L HO, 351W, 351M, 351C, 400): Counter-clockwise 1-3-7-2-6-5-4-8
• Most Ford modular (4.6/5.4L): 1-3-7-2-6-5-4-8
• Ford 5.0L Coyote: 1-5-4-8-6-3-7-2
• Oldsmobile (1967 and up): Counter-clockwise 1-8-4-3-6-5-7-2
• Pontiac (most 1955-81 V8 engines): Counter-clockwise 1-8-4-3-6-5-7-2 (Note: 307 Pontiac V8 engine rotates clockwise)

The impact of engine rotation on the distributor gear

The distributor is a crucial component in the ignition system of older spark-ignition engines, responsible for routing electricity from the ignition coil to each spark plug at the appropriate time. Its rotating arm, known as the rotor, receives high-voltage electricity from the ignition coil and transfers it to the output contacts for each cylinder as the rotor spins. This process ignites the air-fuel mixture in the combustion chamber.

When modifying an engine's rotation, it is essential to consider the distributor gear's direction. In standard rotation engines, the distributor gear typically turns in the same direction as the camshaft. However, in reverse rotation engines, the distributor gear may need to be changed to ensure it rotates in the opposite direction of the camshaft. This change in gear direction is necessary to maintain the proper rotation speed and direction of the distributor shaft, ensuring accurate timing of the ignition system.

Additionally, the distributor gear's angle plays a crucial role in determining the distributor's direction of rotation. Altering the camshaft's rotation direction may require changing the angle of the distributor gear to maintain proper functionality. This adjustment ensures that the thrust of the gears is directed appropriately, preventing potential issues with stock distributors.

Furthermore, the distributor gear's compatibility with the camshaft is essential. In some cases, the camshafts in standard and reverse rotation engines may have different lobe timing or distributor drive gear angles. Therefore, it is crucial to use the correct distributor gear for the specific camshaft and engine rotation configuration to ensure proper ignition timing and overall engine performance.

The direction in which the camshaft and crankshaft turn in reverse rotation and standard rotation engines

Marine engines can have standard or reverse rotation. The camshaft and crankshaft work together in a "well-choreographed sequence" for the engine to operate smoothly. The camshaft and crankshaft turn in different directions depending on the type of engine.

The Camshaft

The camshaft is located in the "top end" of an engine and is a critical part of the valve train that allows air and fuel to enter the combustion chamber and exhaust gases to exit after burning. The camshaft uses egg-shaped "cams" to open and close engine valves (one cam per valve). Driven by the crankshaft, the camshaft transfers motion from the cams through various parts of the valve train to open and close engine valves.

In reverse rotation engines, none of the cams are interchangeable. The lobe timing and/or the distributor drive gear angle are different.

The Crankshaft

The crankshaft is located in the "bottom end" of an engine and harnesses the force of combustion by converting the up-and-down motion of the pistons into rotational motion. Connecting rods attach pistons to the crankshaft. Combustion, controlled by ignition and valve timing, exerts massive downward pressure on the pistons, allowing the crankshaft to maintain its rotational momentum.

In reverse rotation engines, some of the reverse rotation crankshafts have oil holes drilled symmetrically opposite.

Working Together

Camshaft and crankshaft gears are connected by a timing chain, timing belt, or mesh gear set located in the "front end" of an engine. To control combustion, they must be indexed (aligned to the manufacturer's specifications) to work in perfect harmony. This is called valve timing. During the four-stroke combustion cycle (intake, compression, power, and exhaust), the crankshaft turns twice, while the camshaft turns once. This results in each valve opening one time for every two crankshaft revolutions in relation to the piston.

The impact of engine rotation on the oil pump

The distributor and oil pump in a boat engine turn in the same direction, regardless of the crank rotation. This is achieved by making the angle of the drive gear on the cam and its mating gear opposite, so when the cam turns in one direction, the thrust of the gears is in the opposite direction.

The oil pump is a vital part of an internal combustion engine, circulating oil under pressure to the rotating bearings, sliding pistons and camshaft. This lubricates the bearings, allows the use of higher-capacity fluid bearings and assists in cooling the engine. The pump is usually mounted low down, either submerged or around the level of the oil in the sump, to avoid the need for priming.

The type of pump varies, with gear pumps, trochoid pumps and vane pumps being the most common. Mechanical pumps are generally used for simplicity and reliability, driven by mechanical geartrains from the crankshaft. It is also possible to use electric pumps, but these are less common due to reliability concerns.

The oil pressure generated in most engines should be about 10 psi per every 1000 rpm, peaking around 55-65 psi. Low oil pressure can cause engine damage, with the cam carrier bearings being the first to fail if the vehicle is OHC. Other issues that can arise include piston/liner nip and the crankshaft and connecting rod bearings seizing.

When priming the oil pump, it is important to turn it in the correct direction, which is usually clockwise when looking down at the pump drive shaft. Priming the oil system ensures that oil is circulated through the filter and oil cooler lines and can be done using a drill or a cutoff oil pump drive attached to a drill.

Frequently asked questions