In-Situ Clearance Check with "Plastigage": A Thorough Explanation of Features and Usage

When disassembling and assembling an engine, it is important to check and set the clearance between the crankshaft journals (shaft sections) and the crankcase bearings. Clearance can be calculated from the difference between the journal's outer diameter and the bearing's inner diameter, but it can also be measured using a thread-like gauge called Plastigage. This article explains the meaning and importance of engine internal clearance, as well as the usage and features of Plastigage.

Individual Measurement and Calculation by Subtraction: Micrometer + Bore Gauge

An example of an upper crankcase for a four-cylinder engine. The crankshaft is supported by five bearing points, and bearing shells that contact the crankshaft main journals are installed in these bearing points. This is for an integrated crankshaft; for a built-up crankshaft with ball bearings, there are no bearing shells.

Appropriate gaps, or clearances, are set for the rotating parts of an engine, such as between the crankshaft and crankcase, the big end of the connecting rod and the crankshaft, and the camshaft and cylinder head. If this clearance is smaller than the specified value, it increases frictional loss during rotation, leading to component wear. Conversely, if it is too large, it causes play during rotation, resulting in abnormal noise and damage. Therefore, it is crucial that the clearance falls within the range specified by the manufacturer.

If you have the opportunity to overhaul an engine through major work such as repairs or restorations, you can check the clearances of these rotating parts and, if adjustment is necessary, address it through parts replacement. The essential first step in this process is to confirm the current condition. You cannot proceed without knowing the current state, whether you need to reduce or increase the clearance.

The measuring tools for determining the "outer diameter of the shaft" and the "inner diameter of the bearing" are a micrometer and a bore gauge. A micrometer, used to measure the outer diameter of a shaft, is a tool for measuring length. While a standard mechanical caliper has a minimum reading of 0.05mm, a micrometer can read up to 0.01mm.

In contrast, a bore gauge, used to determine the inner diameter of a bearing, is not a tool that provides absolute dimensions on its own. To explain it simply, it is a tool for measuring the plus or minus difference from a reference dimension.

For example, if you set the reference dimension of the bore gauge to 32.00mm and insert it into the bearing, and the dial gauge attached to the bore gauge indicates +0.05mm, you can determine that the inner diameter of the bearing is 31.95mm. Conversely, if the dial gauge shows -0.04mm, the inner diameter of the bearing is 32.04mm.
Using this method, the clearance can be calculated as: Inner diameter of bearing (measured by bore gauge) - Outer diameter of shaft (measured by micrometer) = Clearance.

While this may seem simple when written down, in practice, using a bore gauge requires skill and experience, and obtaining accurate measurements is not easy without considerable practical experience. Furthermore, a bore gauge itself is not an inexpensive tool, so amateurs may find it difficult to utilize effectively and may end up with it unused.

Plastigage: Clearance Determined by Crushed Width

Plastigage comes in a paper package, and the package itself has printed scales indicating the crushed width and clearance. It was originally an American product with clearance indicated in inches, and since it's converted to mm, the markings are detailed.

When using, cut to the measurement area with scissors or similar.

Wipe off any debris or oil from the crank journal and bearing surfaces, then set the Plastigage.

The tightening force affects the crushed width of the gauge, so tighten to the specified torque. Also, be careful not to rotate the crankshaft during measurement.

While clearance measurement typically requires specialized tools like micrometers and bore gauges, there is a simpler item for measuring gaps: Plastigage.

Plastigage is made of a special resin formed into a thread-like shape. It is placed in the area to be measured, the parts are assembled, and the clearance is determined by the crushed width due to the tightening torque. Here are the differences between measurement with Plastigage and measurement with a micrometer + bore gauge:

● Micrometer + Bore Gauge
・Measures the outer diameter of the shaft (micrometer) and the inner diameter of the bearing (bore gauge) individually.
・Calculates clearance from the difference between the two.
・High measurement accuracy, evaluable in μm (microns).
・By measuring multiple points, roundness and uneven wear can also be assessed.

● Plastigage
・Checks the actual gap with the parts assembled.
・Reads clearance using an analog indicator: the crushed width.
・Accuracy is approximately ±0.01 to ±0.02 mm.
・Measures only at one point, assessing localized clearance.

In essence, even when measuring the same "clearance," the approaches of the two methods are fundamentally different.

Furthermore, with Plastigage, it is necessary to use different gauges depending on the size of the clearance to be measured. This is because if a thin gauge is used in a large gap, it will not crush sufficiently, and if a thick gauge is used in a narrow gap, it will crush too much, reducing reliability.

The Plastigage used in this instance comes in three types, color-coded by measurement range, with the following measurement ranges for each:

Green: 0.025–0.076 mm
Red: 0.051–0.152 mm
Blue: 0.102–0.229 mm

As can be seen from these specifications, when measuring clearance with Plastigage, it is essential to first check the standard clearance in the service manual. The service manual is indispensable for engine disassembly and overhaul.

In the case of the engine measured in this article, the standard clearance between the crankshaft main journal (shaft section) and the main journal bearing shell (bearing inner diameter) is 0.014–0.038mm, with a service limit of 0.08mm. Therefore, it can be measured with the green Plastigage.

Additionally, the clearance between the big end bearing shell of the connecting rod and the crankpin has a standard value of 0.035–0.059mm, with a service limit of 0.10mm. This can also be measured with the green gauge.

What to Do If the Clearance Measured with Plastigage is Too Large?

Loosen the fastening bolts evenly and remove the gauge straight up to avoid dragging it. Then, read the clearance by referring to the gauge on the package.

The narrower the clearance, the wider the Plastigage crushes. The standard clearance for this engine is 0.014–0.038mm, and the service limit is 0.08mm. The crushed width is narrower than the 0.038mm band (meaning larger clearance) but wider than the 0.076mm band (meaning smaller clearance), so it can be determined that it has not reached the service limit of 0.08mm. If you need to know the clearance as an absolute value, a micrometer + bore gauge is necessary.

In this engine, when the manufacturer assembles a new vehicle, they select the optimal bearing from three types of bearings with different thicknesses based on the combination of the inner diameter of the main bearing section of the crankcase and the outer diameter of the crankshaft main journal. If the measured clearance exceeds the service limit after accumulating mileage, it may be possible to reduce the clearance by using a thicker bearing than the current one, if such a part is available.

The thickness of the bearing shells can be identified by paint marks on the sides. In this engine, the blue bearing shells are the thickest of the three types, and the black bearing shells indicate intermediate thickness. If the current black bearing shell has excessive clearance, changing to a thicker blue bearing shell will reduce the inner diameter and thus the clearance.

When Plastigage is placed on the shaft and the bearing is tightened to the specified torque, the gauge crushes. By comparing the crushed width of the gauge to the width of the lines printed on the Plastigage package, the clearance can be determined.

Whether the crushed gauge width matches the printed width is somewhat a matter of chance, and in reality, you cannot know the exact clearance down to the 0.001mm level. If the crushed width is wider than the printed 0.051mm but narrower than 0.038mm, you can estimate it to be around 0.04mm.

While a micrometer + bore gauge can measure at multiple points around the circumference, Plastigage can only measure the clearance at the point where the gauge was placed, i.e., a single location. If foreign matter is present on the journal or bearing and is measured, it may result in a narrower clearance than the average. Also, the thickness of the oil film can affect the measurement. Therefore, when using Plastigage, wipe off the oil and work in a dry state.

As wear progresses with accumulated mileage, engine components expand their clearances. So, what should be done if the clearance exceeds the service limit?

Depending on the vehicle model and age, parts availability varies. However, "bearing shells" installed on crankshaft main journals and connecting rod big ends are available in different thicknesses. These are parts used to adjust the clearance to an appropriate level by correcting for tolerances in the outer diameter of the main journals of mass-produced crankshafts and the inner diameter of the main bearing sections of the crankcase during new vehicle assembly. They are also used in overhauls and repairs.

It is necessary to check the types and thicknesses of bearing shells in the service manual. By using bearing shells that are one or two grades thicker than the ones currently in use, the clearance can be reduced. However, if the clearance has increased due to problems with the crankshaft itself, such as the peeling of hard chrome plating on the surface of the main journal or crankpin, the crankshaft must be replaced, not just the bearing shells.

Furthermore, in cases like camshafts, where there are no bearing shells in the cam holders and the cylinder head directly supports them, if the cause of increased clearance is on the cam holder side, the cylinder head may need to be replaced.

When is Plastigage Effective?

The clearance of the connecting rod big end bearing can also be measured with Plastigage. Since the connecting rod is more prone to movement during measurement than the crankshaft, hold it in place and tighten the cap nuts.

The clearance of the connecting rod big end bearing is 0.035–0.059mm. Checking the Plastigage package, this closely matches the 0.038mm band width, indicating it is at the lower end of the standard range, meaning there is no wear at present.

For Sunday mechanics and amateurs measuring the clearance of engine rotating parts, the decision between using a micrometer + bore gauge or Plastigage can be based on the cost of acquiring the tools and their frequency of use.

A micrometer + bore gauge is essential for precise clearance measurement, but acquiring both can easily cost over 10,000 yen. If this is a "once-in-a-lifetime" task in your motorcycle life, Plastigage, which can be purchased for around 1,000 yen, is sufficiently practical.

Do the clearance values obtained from both measurement methods match? In conclusion, they do not match perfectly, but they should be close approximations within the acceptable range. The reasons for the discrepancy are as follows:

1: Differences in Measurement Conditions
・Micrometer + bore gauge measures individual components under no load.
・Plastigage measures under actual conditions with applied tightening torque.
・Even with proper torque, distortion from tightening can affect the measurement.

2: Differences in Measurement Position
・A bore gauge can measure at different locations to determine the average or minimum value.
・Plastigage only indicates the local gap where the gauge was crushed.

3: Differences in Resolution
・Micrometer + bore gauge can calculate values in 0.01mm increments through calculation.
・Plastigage relies on visual comparison with printed band widths.

If accuracy is paramount, a micrometer + bore gauge is the choice. However, the standard values set by motorcycle manufacturers themselves have a range. The standard value for the clearance between the main journal and bearing in this engine is 0.014–0.038mm, a range of 0.024mm. The service limit is 0.08mm, leaving a margin of 0.042mm from the upper limit of the standard value to the service limit.

On the other hand, the green Plastigage has numbers from 0.025–0.076mm printed for four different band widths. Based on the relationship between these, the crushed width of the Plastigage used here falls within the standard range if it is within the gauge's range, indicating that bearing replacement is not necessary.

If the goal is to standardize the clearance of all bearing shells (or at least bring them close to a uniform value), or to maximize clearance to reduce friction loss, then a micrometer + bore gauge, which measures actual dimensions, is more reliable and useful. However, for routine maintenance, Plastigage is sufficiently practical.

Measuring clearance with Plastigage is an interesting task that can enhance curiosity and a spirit of inquiry about engines. However, wear on the crankshaft, connecting rods, and camshafts is most often caused by poor engine oil management. Therefore, it is important to diligently perform regular oil changes to avoid the need for clearance measurements during engine repairs.

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