Piston skirt question

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Piston skirt question

Post  JBR-3 on February 10th 2016, 6:14 pm

1.) Some pistons have a skirt 360 degrees around, including around the sides
under the ends of the pin.
2.) Some pistons don't continue the skirt around the sides.

What are the advantages of either design ?

Less skirt = less weight. Also, I could see more windage splash to lube the pin
if the ends of the pin are more open to the crankcase.

But, with a full 360 degree skirt, maybe this helps keep more oil off the cylinder
walls, making less work for the oil scraper, I don't know.

Any more reasons for either design ?



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Re: Piston skirt question

Post  gt350hr on February 10th 2016, 7:55 pm

Piston companies felt that adding the lower band kept the piston skirt from collapsing. In more recent times some companies feel the lower band can contribute to cracking in the corners of the skirt because that area is too stiff and doesn't flex like the skirts do. Through the use of FEA ( finite element analysis) this has been proven and that band can be reduced to a small rib . Other pistons are designed from the start without that area . Those ( in the industry ) are referred to as "boxed" or "strutted" style pistons. These are sometimes tricky to use because the strut ( area between the skirt and pin boss) angle ( or lack of angle) determines the rigidity of that area and also the expansion.
That lower band has little to do with oil control since it doesn't touch the cylinder wall to remove oil. To me it that band is a waste of material and only necessary if aluminum pin buttons are being used instead of conventional pin locks.

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Re: Piston skirt question

Post  JBR-3 on February 11th 2016, 1:01 am

Thanks much, GT350

From what you say, sounds like it would be a bad idea to remove the side bands
for lightening or clearance, it would screw up the piston body structure.

On "extreme" strokers, you have to watch closely what the the skirt and crank counterweights
are doing at BDC. Also, the skirt side band (if they exist) can't pull out of the machined cylinder bore
and they cannot ram into the top of the main web at BDC (this would happen, on one side of the
bottom of the cylinder).

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Re: Piston skirt question

Post  DaveMcLain on February 11th 2016, 2:52 pm

JBR-3 wrote:Thanks much, GT350

From what you say, sounds like it would be a bad idea to remove the side bands
for lightening or clearance, it would screw up the piston body structure.

On "extreme" strokers, you have to watch closely what the the skirt and crank counterweights
are doing at BDC. Also, the skirt side band (if they exist) can't pull out of the machined cylinder bore
and they cannot ram into the top of the main web at BDC (this would happen, on one side of the
bottom of the cylinder).

An area to really watch is to make sure that the block is bored deep enough so that the piston isn't "pinched" as it approaches the main webbing as it gets close to BDC.


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Re: Piston skirt question

Post  gt350hr on February 11th 2016, 6:26 pm

JBR-3 wrote:Thanks much, GT350

From what you say, sounds like it would be a bad idea to remove the side bands
for lightening or clearance, it would screw up the piston body structure.

On "extreme" strokers, you have to watch closely what the the skirt and crank counterweights
are doing at BDC. Also, the skirt side band (if they exist) can't pull out of the machined cylinder bore
and they cannot ram into the top of the main web at BDC (this would happen, on one side of the
bottom of the cylinder).


No I'm saying the opposite! "I" ( and others in the industry) routinely remove the lower band to the point where it is a small "rib" remaining. This ( again in my experience) smooths the load distribution substantially and reduces skirt corner cracking.
I too have seen the bottom band "clip' the bottom of the cylinder on long stroke engines. that obviously is a really bad thing.

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