Automotive Troubleshooting  Oil Loss Via the Piston Ring/the Valve Guides Inspection

Purpose of inspection
When burning oil causes white smoke to be seen in the exhaust gas, changing the engine rpm to change the volume of white smoke allows you to judge whether oil loss via the piston rings or oil loss via the valve guides exists. As well, disassemble the engine and judge the condition of the accumulated carbon.

Exhaust gas inspection
• Judge if there is oil loss via the piston rings
After warming up the engine, race it at approximately 2000 to 3000 rpm to check the state of the exhaust gas
Criteria
• White smoke increases when the engine is raced
• White smoke increases when rpm rises
• Judge if there is oil loss via the valve guides
After warming up the engine, let it idle for approximately 5 minutes then race it to check the state of the exhaust gas.
Criteria
White smoke will discharge for approximately 30 to 60 seconds at the start of
racing, but the amount will gradually reduce.
Why the cause of white smoke can be determined by oil loss via the piston rings
(1) The temperature in the combustion chamber is low when the engine is idle, therefore even if oil loss from the piston rings occurs, the oil does not burn. Therefore, the volume of white smoke is small.
(2) When engine speed increases, the temperature in the combustion chamber rises as does the amount of oil supplied to the cylinders. As a result, the amount of white smoke discharged increases.

1. Description

The piston ring is designed to prevent pressure from leaking out of the gap between the piston and the cylinder. There are three piston rings that function to keep the combustion chamber airtight as the top two compression rings serve to disperse the heat from the pistons to the cylinder. Also they act to scrape off excess oil on the cylinder wall in order to create the minimum film of oil necessary while preventing excess oil from entering the combustion chamber.

2. Marks of ring

The name of the manufacturer and the oversize mark

are inscribed on the piston ring.

NOTICE:

Pay attention to the following when assembling:

The surface with the mark should face up.

Do not confuse the order of the compression rings.

When the No. 1 compression ring does not have the mark, it may be located on the side of the ring. In the case that there are no marks in both locations, consult the Repair Manual to judge the difference in shape.

In order to reduce pressure leakage as much as possible, assemble the piston ring end gap at the separated position as in the illustration. Check the ring end gap when using a new piston ring.

3. Ring end gap

The ring end gap must be from 0.2 – 0.5 mm when at room temperature. If the piston ring end gap is too large, pressurized gasses will leak out through the gap. If the ring end gap is too small, the two ends of the piston ring will touch each other due to heat expansion and cause the ring to expand. This may result in scoring of the cylinder wall or the piston ring itself may break.

NOTICE:

When measuring the ring end gap, insert the piston ring in the piston cylinder at the place with the least amount of wear.

The position for measuring the ring end gap differs according to the engine models.

4. Ring pumping effect and ring flutter

(1) Ring pumping effect

The piston ring moves up and down inside the piston ring groove while the engine is running. This acts to pump oil on the ring, helping to improve lubrication. If the gap between a piston ring and piston ring groove is too large, the pumping effect will also be large, resulting in an increase in oil consumption.

(2) Ring flutter

When the piston ring vibrates up and down or side to side in the piston ring groove, ring performance decreases. When this condition continues, the piston ring or piston ring groove will experience abnormal wear and may eventually result in seizing.

1. Description

The piston consists of the lowest part of the combustion chamber. In order for the pis-ton to move, there needs to be clearance between the piston and the cylinder wall. The construction is designed to maintain the proper clearance when the piston expands due to high temperatures during combustion. As the piston boss part is thicker, it is easily affected by heat expansion. Therefore it is made to be slightly oval than the direction of right angle for the piston pin (B) so that the diameter in the direction of the piston pin (A) expands to make a circle during heat expansion. The piston head is exposed to high temperature during combustion, and it is not directly cooled by coolant and air. For that reason, the piston head reaches greater temperatures than the piston skirt. Taking into account the heat expansion during combustion, the piston head is slightly tapered when compared to the diameter of the piston skirt.

HINT:  For the piston diameter, measure the area specified in the Repair Manual. Note that the location for measurement is not the maximum diameter. For that reason, keep in mind that the standard oil clearance of the piston in the Repair Manual is not the actual gap between the cylinder and the piston.

2. Thrust force

When the pressure during compression or combustion acts on the piston, a part of that force acts on the piston skirt and causes it to push against the cylinder wall. This is called thrust force. Thrust force is divided into two typesmajor thrust force and minor thrust force. The former occurs during the combustion stroke and the later occurs during the compression stroke.

3. Piston slap (Side knocking)

Piston slap is the noise that occurs when the piston hits against the cylinder wall. This is also called side knocking. Piston slap occurs when the direction of the thrust force changes from the compression to combustion strokes. Piston slap is affected by the amount of piston clearance. When the piston clearance is large, there is a greater piston slap. In some engines, the centerline of the piston and the centerline of the piston pin are slightly offset in order to reduce piston slap.

4. Operation of offset pistons

In engines with offset pistons, the direction of thrust of the piston changes from the minor thrust direction to the major thrust direction near the end of the compression stroke. In this way, the piston slap is reduced because the piston’s thrust direction changes before the piston receives combustion pressure.

5. Piston size

When the piston and cylinder wear beyond the allowable limit, it is necessary to replace the cylinder block or piston, or to rebore the cylinder block or cylinder liner and use oversize pistons. The piston and piston pin are usually provided as a piston assembly.

The standard size, which indicates the piston diameter, is inscribed.

The piston must be installed facing the correct direction.

The direction is inscribed on the piston head.

The side with the mark represents the front side.

The exact location of the inscription differs according to the engine model.

(1) Standard size piston

When the engine is assembled, each standard size piston is set for each cylinder bore size to achieve piston clearance precision. As the number gets larger, the piston diameter increases incrementally by 0.01 mm (0.0004 in.) units. Recently, engines have only one cylinder bore size and one piston size.

(2) Oversize piston

The size of an oversize piston is determined by the extent of wear on the cylinder. Normally, only the 0.5oversize piston is available as supply parts. The 0.5value in the oversize piston means that it is 0.5mm (0.02in.) larger than the standard size piston. Besides the oversize piston written above, there are engines that require supply parts for 0.75 and 1.0sizes. For some engines, oversize pistons are not available as supply parts.

REFERENCE

Striations on the Piston Skirt

The piston skirt has been striation finished to improve the lubrication properties. The striations on the piston skirt are not a poor finishing. In some engines, the striated piston skirt has been coated with a composite resin to reduce friction.