Are PISTONS round? OVALITY and TAPER EXPLAINED

If you ever tried to roll a piston across the floor you have probably noticed that it's impossible to roll them in a straight line. No it's not your floor, you can try it on the most level surface on earth and piston will never roll in a straight line. It will always deviate from a straight line and roll in the direction of the crown. If you had enough space you would notice that the piston rolls in a giant arc.
So what does this tell you? It tells you that the piston sides are not straight, it's not a perfect cylindrical shape and you can easily confirm this by putting the piston on a level surface and having a light source behind it. If you rock the piston from side to side you will notice that the sides are actually tapered. From top to bottom the piston incorporates a taper, with the top being smallest and the taper increasing towards the bottom.

So why do pistons have a taper? Why don't they have straight sides just like the cylinder they are supposed to fit in? The reason for the taper is the very nature of the internal combustion engine and the heat that it generates. And pistons being made from aluminum do expand under heat, but all parts of the piston are not exposed to the same amount of heat. The crown being closest to the combustion expands the most, and the skirt expands less. This is why most room for expansion must left at the crown to account for this. If the piston had straight sides we would have one of two negative scenarios. We would either have good ring seal at the top at the expense of loose piston skirt and piston slap at the bottom, or we could achieve a properly fitting piston skirt at the expense of a ring pack that is too tight and has potential to seize or bind in the bore.

What happens when we look at the piston from the top? Surely it must be perfectly round in this case because it's supposed to fit into a perfectly round cylinder. Well, the answer is both a yes and a no. The ring pack is indeed perfectly round because it's trying to achieve the best possible ring seal with the cylinder, but everything from below the bottom ringland isn't round. It's actually egg shaped or oval, and to see why we have to remember what happens to the piston during combustion.
As combustion occurs, combustion pressure forces the piston down the bore, at that point the sides of the piston, which are at 90 degrees to the piston pin, experience the most load. The sides of the piston are the major and minor thrust side. In clockwise rotating engines, which account for the vast majority of engines, the major thrust side of the piston is always the left side of the piston when looking at the engine from the front. On counter clockwise rotating engines the major thrust side is the right side of the piston when looking at it from the front of the engine. The side opposite to the major thrust side is the minor thrust side.

The piston skirt along the major and minor thrust sides must follow a perfectly round contour because these are the load bearing sides of the piston and they need maximum stabilization in the bore. The axis of the piston along which you can find the major and minor thrust sides is of course called the thrust axis. But what about the other axis of the piston, the one along the piston pin? The situation is different here and the skirt along the pin axis does not need to follow a perfectly round contour because the piston pin stabilizes the piston in the bore, and the skirt along the pin axis isn't load bearing. This is why many pistons don't even have any skirt along the bore, and when they do have it they incorporate an egg shaped contour to reduce friction. If the piston skirt had a perfectly round contour along the pin axis it would generate additional friction without any strength or stability benefits.

A special thank you to my patrons:
Daniel
Peter Della Flora
Daniel Morgan
William
Richard
Pepe
Brian Durning

#d4a #enginebootcamp

[luidj]

I got a divine revelation!! My vision of piston will never be the same!!

[monsterwithoutname]

The only channel that deserves to share. Thank you so much . Your biggest fan from Saudi Arabia

[samcoote]

I knew pistons werent perfectly round all the way through their height these days, but didnt realise just how different the geometry is, very cool man :) always learning, and knowledge is POWER

[KjV]

Been building engines for years and still learned a few things here. Great video.

[midniter]

Looks like himself again, back in the days I'd watched him make poly mounts... Super smart dude, I've actually made my own mounts not long after you made your video. Keep on keeping on.

[kostadiamantis]

Im an a apperentice engine builder by trade and dude I CANNOT GET ENOUGH OF YOUR CONTENT its super informative and intriguing and watch every video that you put up as soon as you do please keep up the amazing work so i can keep learning and to keep enjoying your videos

[Garage]

I FORGOT FOR A SECOND! NICE HAIRCUT BRO YOU LOOK BEAUTIFUL!

[zibingotaeam]

the fact that every time i have an engine question which you answer in a video is astonishing

[gerrievanstaden]

I literally could not keep my eyes off the haircut. Looks good man👍🏻 great video as always

[G]

You should have mentioned Honda's attempts at trying to make squarish oval pistons work in a desperate attempt to maximize bore area for a given sized block. That went well...

[jhuntosgarage]

Once again, I learned. And yes, haircut looks good. ;)

[lovesjw]

Pure dry learning material without water. Super! Thank you!

[JazzbLu]

I simply love learning the details of the basic parts of the engine and how complex they really are! Without you not sure I ever would learn these details.

[bobross]

The man teaching me what i didn't get to learn in tech school

[DCrr]

This was a fun video, on a Sunday morning enjoying breakfast.

[owenbassist]

ive learned this at school but never seen it like this good hair style dude

[rogerandersson]

I was expecting a few words on piston pin offset, but it would have gone beyond topic I suppose.
Brilliant as usual :)

[owenmeadows]

Huh, never knew that! Thanks for the clear and concise reasoning my dude!

[sahalin]

That made pretty hard to develop Honda`s NR750 oval pistons.

[Garage]

I'm sure you've thought about it, but I'd love a video on wrist pin height as well. Also, do any pistons have a pin offset from the center? Towards the major thrust side for example? Great vid as always!