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Calculating Piston Clearance

MAHLE Clevite Team Doc AvatarDoc here,

This week I want to talk to you about piston clearance. A piston is almost always a bit smaller than the cylinder bore because you’ve got to get it in, correct? Yes, that’s true but also because the vast majority of pistons are aluminum and most blocks are cast iron.

Aluminum expands about 1.7 times the rate of cast iron so there has to be room for that expansion. Now MAHLE engineers, who have been designing pistons since the 1920’s, have various means of controlling piston expansion as well as sophisticated software to predict piston expansion at engine operating temperatures. When it is all said and done, they build a piston that is smaller than the final bore size by just enough to allow for expansion.

As you can see in this picture of a MAHLE Original® piston for the Dodge Sprinter, MAHLE uses Grafal® skirt coatings on most of their pistons. Grafal is an anti-friction coating but also forms a membrane to cushion the skirt of the piston as it contacts the cylinder wall. MAHLE engineers calculate the piston to bore clearance before the Grafal is applied, making it almost impossible for you, the engine machinist, to get the same measurement that MAHLE used!

Here’s a picture of the top of this same piston. 88.439 is the metric diameter of the piston and 0.07 is the desired clearance in mm. Add the two together and you’ll get: 88.509 mm. If you finish your bore to 88.509 mm, the piston will fit with perfect clearance. That is, by the way, more accurate than trying to measure the piston with a micrometer.

FYI, if you’re old school like me and need your computer to convert this to inches so you can set your dial bore gauge, it converts to 3.4846”. I have a great app on my iPad that converts metric to inches!

Sometimes you’ll see a MAHLE Original® piston with a little window in the Grafal like this one fitting the Chevy Cruze. Certain OE customers for MAHLE use a sophisticated electronic measuring tool to get the piston diameter before finishing the bore. This gives them perfect piston to bore clearance every time. As I mentioned, MAHLE calculates that clearance by measuring the piston before applying the Grafal, and for these special OE customers, a little window is left so they can do the same thing MAHLE does.

MAHLE Clevite provides a wide range of replacement pistons for cars, trucks and Heavy Duty engines equal in every case to the parts that come from the factory! That’s our business, but sometimes you need more like this guy:

Our friends at MAHLE Motorsports specialize in just this kind of need! Give them a call and say that Doc sent you!

Also, there is going to be a time when you need help or have questions that I didn’t answer in my short articles that you read here. You can always get more technical help at http://askmahleclevite.com/ . Good luck with your piston installation!

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3 Responses to Calculating Piston Clearance

  1. Dave Arce says:

    Hey Doc….thanks for the piston clearance article….keep up the great work! :)

  2. Seth Flanders says:

    Hey Doc,
    Seth here. You mentioned how pistons are aluminum and most blocks are cast iron. For aluminum blocks (such as my Honda), do those pistons have to expand faster/further than the pistons made for cast iron blocks? I assume the aluminum block will expand faster than an iron one, so the pistons would need to play “catch-up” to reduce blowby.

    Seth

    • Hi there Seth,

      I’m glad you asked. Most Aluminum block engines, like your Honda, have cast-in- block, cast iron liners so there is really no difference in expansion from that to a cast iron block. There are some engines like the Porsche that use all aluminum blocks and they would, upon the original piston design phase, require different calculations for thermal expansion. Great question and check back on Friday for an article on Bearing Crush.

      -Doc

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