Measuring cam height:
The lobes of the camshaft can wear due to age or lack of lubrication. As a result, the cam height decreases. Complaints caused by worn cam lobes can include:
- The fuel trims show a negative percentage: due to a shortage of air, the injection quantity is corrected (reduced);
- Reduced power: due to a shortage of air, the maximum torque can no longer be achieved. In the case where all lobes are worn by the same amount, the engine can still idle smoothly and regularly;
- Irregular engine operation: this mainly occurs if only one or a few lobes show wear while the others are still in good condition.
We can determine the cam height with a vernier caliper. Alternatively, a micrometer screw gauge can be used instead of a caliper for higher accuracy. For one lobe, we take two measurements to determine the cam height, see the image.
- A = base circle diameter;
- B = total cam height;
- C = actual cam height / lift.
The cam height (C) of the camshaft is the difference between dimensions A and B.
The cam heights of the intake and exhaust camshafts are different. Cam heights on a single camshaft must not differ from each other. However, we do have to deal with tolerances. Only when the measurement falls outside the tolerance is the camshaft rejected.
Example:
The camshafts of a petrol engine are measured. According to the factory data, the minimum cam heights must be:
- Intake camshaft: 45.82 – 45.85 mm;
- Exhaust camshaft: 45.50 – 45.53 mm.
We take eight measurements and fill in the table below. In the table we see that almost all measured values are higher than the minimum values. Only at lobe 4 of the intake camshaft do we see a deviation: this lobe is 1.03 mm lower than the minimum value. For this reason the camshaft is rejected and must be replaced.
Measuring main bearing journals:
With a vernier caliper or micrometer we can measure the outside diameter of the main bearing journals. Just as with cam height, the minimum values can be looked up in the factory data and compared with the measured values.
Wear of the main bearing journals can occur after the bearing caps have been swapped during a previous disassembly and assembly. When two bearing caps are interchanged, or rotated 180 degrees, both the bearing caps and the camshaft will wear at the point where the clearance between both parts is smallest.
If during this measurement we obtain a value that is smaller than the minimum, there is a chance that oil loss will occur at that point: the oil flow can pass this bearing (too) easily, so the flow past this bearing is greater than at the bearings at the ends of the camshaft. The result is that at the last lobes there is a lack of lubrication and a high risk that the lobes will wear down.
Measuring camshaft runout:
Camshaft runout must be measured with a dial gauge at the centre bearing section. When measuring with the dial gauge, we mount the gauge in a stand and place the camshaft in supports so we can rotate it. The stylus of the dial gauge contacts the rear side of the camshaft; here we set a preload of at least 2 mm. By rotating the camshaft, the deflection of the needle (from the set preload to the maximum value at the cam height) shows whether the camshaft is bent.
For certain engines, the runout must be less than 0.02 mm. The maximum permissible value is 0.1 mm. If 0.1 mm is exceeded, the camshaft must be replaced. Always consult the factory specifications.
Measuring axial bearing clearance:
We can measure axial bearing clearance with a dial gauge. For this, the camshaft bearing caps must of course be tightened with the specified torque.
For the camshaft in this example, the axial clearance may be between 0.090 and 0.150 mm. When we measure a clearance of 0.120 mm, this is therefore acceptable.
Measuring camshaft sprocket runout:
An engine with a timing chain has a chain sprocket. If runout occurs in the sprocket due to a defect, damage or installation error, the timing chain will follow this movement during every rotation, resulting in increased wear or even breakage.
Again, the runout of the sprocket can be measured with a dial gauge and stand. The stylus of the dial gauge presses against the sprocket with a preload so that the stylus follows every movement.
For the sprocket in this example, the runout may not exceed 0.25 mm.
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