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Piston measurements, accuracy, best practice

Engine Building Fundamentals

Discussion and questions related to the course Engine Building Fundamentals


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I don’t think it can be overstated enough the items and tools must be clean when measuring.

1) What is best to clean with? Is that also safe for the calipers.

2) When measuring a piston, for example to validate piston to cylinder clearance or manufacture variance, is there an easy way to do this with 2 hands.

3) The specs on the box show a gauge spec, which I assumed was the expected piston diameter, is this correct? Picture 5021917a

4) in the supplied piston picture I was messuring as close to center of the window as possible.is that a good practice?Picture 18fe5f25

5) also on the piston is a star, is that also a measuring point? Does it have a purpose? picture 18fe5f25

Attached Files

Hey Charles, you don't seem to have attached any pics here that you're referring to?

1. In terms of cleaning your engine components and your micrometer, I'd suggest either isopropyl alcohol or brakeclean. I always have a lot of brakeclean in the shop and rely on this heavily for most of my engine component cleaning.

2. The technique of measuring the piston skirt is demonstrated in the course. You can sit the piston on the workbench and then use the micrometer with your hands.

3. Yes, the spec on the box would typically be the skirt diameter at the gauge point. Just be mindful that some piston manufacturers actually specify the finished bore size which in turn should provide the correct PTW clearance.

4/5 No pictures to reference here sorry.

Thanks, It looks like I got them attached now.

It looks like the window on the skirt coating is provided so you can measure the piston skirt without being affected by the skirt coating. This however may require a blade-style micrometer as most anvil style micrometers would still contact the coating due to the anvil diameter. The star is branding from what I can see and is unrelated to measurements.