What is the Cause of Engine Noise? Is it the Cylinder Head or Crankcase?

A well-maintained motorcycle with an engine that has reached the point of being over-riding can be very quiet, with little mechanical noise. Comparing the odometer readings, there are many cases where the readings are almost the same, but one is quiet and the other is rattling with mechanical noise. Let's take a look at the source of mechanical noise in 4-stroke engines. What is the cause?

Mechanical noise heard from around the head

The cylinder head of a four-stroke engine plays a "cerebral" role, as the name implies.

1: DOHC = Double Overhead Camshaft (Twin Cam) Specifications
The intake and exhaust tappets and valves are operated in conjunction with the two camshafts. In some models, the intake and exhaust valves are actuated via rocker arms.

2: OHC = Over Head Camshaft (also called SOHC = Single Over Head Camshaft) Specifications

The rocker arms move in conjunction with a single camshaft to operate the intake and exhaust valves.

3: OHV = Overh  Head valve specification
The camshaft, located on the crankcase side, is driven directly by the crankshaft, and in conjunction with the camshaft, the tappets and pushrods push in on the cylinder head rocker arms.

There are three main types of 4-stroke engines in use today, and the root of mechanical noise is often found in the "operating parts" and "sliding parts" where parts rub against each other. When engine oil is supplied efficiently and in good condition to these moving parts, lubrication is achieved, and wear of the parts is prevented, thus preventing mechanical noise. Many motorcycles that have not been ridden for a while have engine oil that has fallen from the cylinder head, causing the oil curtain to run out. When starting such an engine for the first time in a long time, or when the engine is in the process of being assembled, it is necessary to apply engine oil in key places. A high-performance oil sprayer is a convenient way to efficiently spray oil over a wide area.

Importance of Tappet Adjustment

The basic condition to satisfy such a desire is to use engine oil that is always in good condition. The tappets are the parts where the intake and exhaust valves are pushed in. The tappet is the part where the intake and exhaust valves are pushed in, and on OHC engines, the rocker arm has a tappet adjustment function. If the engine is equipped with an adjusting bolt, insert a thickness gauge into the tappet gap when the piston is at the top dead center of compression and adjust the screw while feeling the gap with your fingertips. Then fix it with a lock nut. If the tappet clearance is too wide, the mechanical noise will become louder. If the clearance is adjusted correctly, and the mechanical noise does not disappear even if the clearance is adjusted narrower than the standard value, the wear of the tip of the adjusting bolt can be suspected.

Is there any wear on the camshaft holder?

The part that supports the camshaft is called the cam journal. Since it is the part that supports the rotating camshaft, if this bearing part is worn out, it will cause mechanical noise. When disassembling and assembling the cylinder head, tighten the cam holder and camshaft to the cylinder head and check the rotation and sliding condition without oil. If the camshaft rotates smoothly without any rattle, it is good, but depending on the rotation condition, mechanical noise and frictional resistance may cause power loss. If you are using a mini motorcycle race engine, you may want to cross-cut the oil outlet hole in the cam journal to allow for a wider oil supply.

The reason why the tappet sound does not disappear

Even if you adjust the tappets over and over again, you may notice noise from the head when the engine is performing. One of the causes of such mechanical noise is the wear between the camshaft and rocker arm slipper. If the oil condition is not good and you keep performing, the slipper surface that rubs against the camshaft will wear and cause the tappet noise. If the tappet noise does not disappear after replacing the tappet adjustment screw, suspect the slipper surface of the rocker arm is worn. There have been cases in the past where the slipper surface was prone to wear due to insufficient oil supply to the slipper area depending on the model. This was addressed several times with each model change, but in the end the slipper was made with a super steel tip to prevent noise. In the case of a twin-cam engine, a direct tappet system in which the cam mounts directly push the tappet is often used. If the outer circumference of the cylindrical tappet and the tappet holder on the cylinder head side exceed the current level of wear, the body of the cylindrical tappet dances in the holder when the tappet operates, and mechanical noise is often generated.

Be careful of rocker shaft wear.

If the rocker arm is compared to a balance, one side where the weight is placed is the cam sliding surface and the other side is the tappet. The fulcrum of the balance is the rocker shaft. This shaft also needs to be lubricated in good condition, but if you continue to use deteriorated engine oil, the shaft and rocker arm itself will wear and make noise. The condition of the engine oil is also important here. Once the metal parts start to wear, the speed of the wear increases rapidly. The most important thing is to minimize metal wear by changing the engine oil regularly and using high-quality engine oil.

The clutch area also has a lot of noise factor.

Whether it is a wet clutch or a dry clutch, a large friction force is generated between the concave groove of the clutch outer and the convex part of the clutch friction disc outer circumference. In the case of wet-type clutch, if the clutch is repeatedly disengaged intensely in poor oil condition, the concave grooves of the outer will be dented and stepped wear will be caused by the repetition. This wear will cause poor clutch operation and sharpness, and at the same time, mechanical noise will be generated. When the damper wears out and the surrounding parts are worn, mechanical noise such as "roll, roll, roll" can be heard from the clutch area during idling. When the noise is reduced when the clutch lever is depressed, it is suspected that the noise is coming from the primary damper or its surrounding parts.

Cam chain extension and tensioner function

Cam chain extension is also greatly affected by the condition of the engine oil and can cause mechanical noise. It is not the plate of the roller chain that extends, but the chain rollers and bushings are worn and the clearance increases, resulting in an extended chain. The chain extension affects the cam sprocket and wear occurs next. When disassembling the engine, if the cam chain is clearly extended, not only should the chain be replaced, but the cam sprocket and crank side timing sprocket should also be checked for wear. The cam chain tensioner guide rollers are made of rubber parts, and on larger vehicles, rubber dampers are often installed in the tensioner rollers and idle gear mounts. If these rubber parts deteriorate and lose their elasticity, cam chain noise will increase and this will also affect chain extension. In the case of a 4 mini engine, a rubber damper is also installed on the pusher end which presses the tensioner arm. When you disassemble the engine for overhaul or bore up, you should replace these rubber parts with new ones.

I'll visually check the camshaft chain tension.

When assembling an engine for a 4-stroke mini motorcycle, it is important to visually check the cam chain tension to ensure proper tension adjustment. Chain tensioners are generally self-adjusting with spring tension, but with a horizontal 4-stroke mini engine, the cam chain tension can be visually checked by removing the parts even with the engine installed. In fact, there are many cases where the tensioner arms and pushrods are not working as they should and the cam chain is not being tensioned correctly even though the adjustments were made.

Regardless of whether you have a two-stroke or four-stroke engine, mechanical noise is always a concern. There is always a cause for engine noise, so let's start by listening to the engine in operation and find the source of the noise. Instead of just vaguely listening to the engine noise, try to find out "where" the noise is coming from in the engine. It is also a good idea to start by identifying the area of the noise.

Depending on the condition of the engine, various mechanical noises are generated. The most common sources of mechanical noise in 4 stroke engines are the cylinder head, cylinder & piston, and crankcase. In the case of the crankcase area, the crank noise is the rumble and sizzle of the bearings, and the piston skirt slap noise is the sound of the piston skirt dancing when the piston passes through top dead center or bottom dead center and the direction of motion is reversed (slap sound), etc. In addition, when you hear the sound of the clutch hitting from inside the clutch cover, you should repeatedly disengage and release the clutch lever to find out the source of the noise. By experiencing such listening and response, you can grow up to be an experienced mechanic.

In some cases, the mechanical noise is caused by wear of the operating sliding parts due to oil management, while in other cases, the mechanical noise is caused by hardening of the rubber parts due to aging and the inability to absorb the operating vibration of the parts. In any case, the most important maintenance of a 4-stroke engine is to change the engine oil regularly. Using a high-quality engine oil, changing the oil and oil filter regularly and as required will ensure that the ideal lubrication is maintained. Motorcycle maintenance and essential servicing and oil changes will minimize the occurrence of mechanical noise. To enjoy your motorcycle life to the fullest, make sure you do what you need to do and become the undisputed Sunday mechanic.