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Piston measurement helps to determine various aspects. It provides values for piston size, piston cylinder clearance, piston deck clearance, and several other dimensional specs. This is important when reinstalling pistons or even ordering new ones.

But the process to measure a piston is not known to many. If you are rebuilding your engine, that can be a problem.

This article explains piston measurement procedures in easy-to-follow steps. It will help you to carry out the process on your own.

Also, using readily available tools.


Piston Measurement Process

You will need the following materials.

Micrometer, feeler gauge, a work bench, and the engine manufacturer’s service manual. The manual contains the piston specifications chart. The specs include clearances (maximum and minimum), piston diameter, and others. The idea is to ensure the measurements you get do not deviate from those in the manual.

Here are the piston measurement steps to follow. We divided the process into parts, each covering a particular measurement.

Measuring Piston Size

This involves finding the piston diameter at the widest section, or the part below the piston skirt and piston rings. Pistons are not made in a uniform round shape, but larger as you move away from the crown. The design prevents seizing due to the expansion caused by heat in the combustion chamber. Here is how to carry out the measurement.

position the micro meter

Step 1

Position the micrometer across the piston at the gauge point or thrust surfaces. Piston manufacturers usually indicate this point. If in doubt, it is advisable to refer to the manual. You do not want to end up with the wrong dimensions.

Step 2

Take the reading. Measure the piston from other points, too, so that you do not miss collapsed piston parts. These include piston skirt diameter, both top and bottom, and at the ring lands.


Measuring Piston-Cylinder Clearance

This piston measurement step checks the fit of the piston, if it is within specifications. To determine that, you need to have your engine’s manual with you. Before proceeding, check the piston diagram below that shows the gap you will be measuring.

measuring the piston cylinder clearance
Source: http://mymoto.com.ng

Step 3

Put the piston in the cylinder bore, upside down, and the strip of a feeler gauge. The strip should be oiled slightly to facilitate movement. It should also start at just above the piston pin opening and stretch all the way.

Step 4

Use different thicknesses of the feeler gauge strip to measure piston cylinder clearance. Doing so helps to increase precision.

Alternatively, you could use a spring scale to determine piston clearance. The scale measures the amount of force needed to pull the strip. Using the dimensions of the strip and the spring’s readings, determine if the clearance is too tight or not.


Checking Piston Rings

Piston rings need to provide the right cylinder seal, or it will affect compression. If reinstalling existing rings, check the amount of clearance first. Also, if you have had the cylinder re-bored, and the space between piston and cylinder increased. A piston kit to rebuild an engine usually comes with replacement rings. These also need to be checked for correct dimensions as explained in the next piston measurement steps.

checking piston rings
Source: http://www.oocities.org

Step 5

To measure the ring end gap, Insert the ring in the cylinder bore.

Step 6

Using the piston head, push the ring down the cylinder, preferably to the furthest section reached by the piston. This is the part that has not experienced much wear and will provide accurate results for clearance.

Step 7

Use a feeler gauge to measure the clearance left by the split ends of the ring. Avoid measuring ring clearance at the top of the cylinder since the split ends will close.

After you have obtains the readings described above, check them against those shown in the service manual of the engine. This will help you tell if the piston can be installed as it is.


Piston Measurement Tips

Correct measurements when sizing a piston ensures proper piston engine operation. It ensures every piston stroke happens right,  helps prevent seizing problems, loss of compression, and various other issues. Follow these tips when measuring pistons and piston rings.

piston measurement tips
  • Use the right measuring tools. You need a micrometer and feeler gauge for the different piston measurement processes. Never rely on a ruler or similar measuring tool such as tape.

  • Always obtain measurements, never assume anything. A piston may look undamaged visually, but measurements show otherwise.

  • Your engine service manual should be your reference. It contains the exact specs for the pistons clearly laid out in a chart.

  • If you do not have the engine manual, consider looking it up online. Automotive piston manufacturers often have a lot of information on their websites, including piston sizing, piston depth chart, and even piston price.

  • If you have to replace the piston set in your engine, ensure you buy the right types, both in design, dimensions, and piston material. A piston kit will have values for piston size as well as the installation procedure.

  • To ensure the right piston deck clearance, have information regarding how tall your replacement pistons should be. Automotive piston design options are many and that can affect the effective piston height.

  • Apart from ensuring correct measurements, aim to keep the costs to rebuild your engine low. Piston price varies across automotive piston suppliers. Scout for offers to make the project inexpensive. Find an automotive piston catalog online, find the right piston, then look up its price.

  • The automotive piston market is flooded by many different pistons. In as much as measuring the pistons is important, finding a quality piston is what will make a difference. Look for the best by comparing the features and company reputation.


Conclusion

An automotive piston should fit the cylinder correctly. The right dimensions mean a piston that operates properly and better engine performance. With the information contained in this article, piston measurement should not be daunting anymore. You only need to ensure you have the required tools. For the installation procedure, read our next article on piston installation.

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