Category: 

What Is a Piston Skirt?

The piston skirt is the bottom part of the cylinder-shaped pistons.
Article Details
  • Written By: Lori Kilchermann
  • Edited By: Lauren Fritsky
  • Last Modified Date: 23 October 2014
  • Copyright Protected:
    2003-2014
    Conjecture Corporation
  • Print this Article
Free Widgets for your Site/Blog
Sharks hunt by sensing electromagnetic fields produced by their prey.  more...

October 24 ,  1929 :  The Black Thursday crash in the US stock market took place.  more...

The piston skirt is the part of a piston that extends the lowest. It is tasked with keeping the piston from rocking excessively in the cylinder. It is typically machined with small grooves to aid in holding and transporting oil to the cylinder walls to provide proper lubrication. In some high-performance applications, the piston skirt may be coated with a type of chemical which aids in lubrication and prevents scuffs from occurring on the cylinder wall.

In a combustion engine, the pistons are sealed inside of the cylinder walls by the piston rings. The rings, as they are called, are making contact with the cylinder wall while the piston rides up and down, centered by the rings in the cylinder wall. At the top and bottom of each stroke, as the piston is changing direction, the piston rocks. It is then that the piston skirt makes contact with the cylinder wall, setting the piston straight once again to continue its journey.

Ad

Depending on the stroke of the crankshaft and the length of the connecting rod, the piston skirt could be in danger of making contact with the crankshaft at the very bottom of its travel or stroke. This is especially common in high-performance engines known as stroker motors. In a stroker motor, the engine's stroke has been altered by swapping to a longer throw on the crankshaft and also changing the length of the connecting rods. The piston pin location is changed, creating a new location for the piston to connect to the connecting rod.

In the typical stroker application, the engine block needs to have notches ground in the bottom of the cylinder walls to prevent the crankshaft and connecting rods from making contact with the block. Often, the bottom of the piston skirt must also be clearanced to avoid making contact with the connecting rod as it spins around the crankshaft throw. When clearancing a piston skirt, it is wise for individuals to make the same cut on both sides of the skirt. This maintains balance within the reciprocating components. The most minute removal of material can cause the components to be dangerously out of balance, leading to catastrophic engine failure.

One of the telltale signs an engine has been operated while low on oil is a scuffed piston skirt. As the piston travels up and down the cylinder wall, the lack of oil can result in the piston's skirt scrubbing or scuffing the cylinder wall, leaving the telling scuffing sign. When this occurs, it is often time to replace the pistons as well as to bore the cylinders to remove the scuff marks.

Ad

More from Wisegeek

You might also Like

Discuss this Article

Drentel
Post 2

Sporkasia - I don't know what the actual problem with your car was, and your mechanic may have been right about low oil levels causing the damaged piston. However, a scuffed piston is not always a sign of lubrication (low oil) problems.Scuffed pistons can also be caused by over heating. Continual over heating of an engine is eventually going to cause you big problems.

Sporkasia
Post 1

I had damaged pistons in the first car I owned. It was a used vehicle and I had a mechanic check it out before I bought it. Still, less than a year after I purchased the car the pistons needed to be replaced. The mechanic said the damage was probably caused by low oil levels and the piston skirt hitting the cylinder walls, like described in the final paragraph of the article.

One thing I learned from my father about cars was always check the oil, so I was religious about maintaining my oil. I think the damage was done before I bought the car. Anyway, I replaced the entire engine with a practically new one from the salvage yard. The car ran great from then on, and I always changed the oil on time.

Post your comments

Post Anonymously

Login

username
password
forgot password?

Register

username
password
confirm
email