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.
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[@luidj9675]

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

[@samcoote9653]

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

[@gregeconomeier1476]

Haircut looks good.

[@d4a]

Thank you 😊 I ran out of patience with the long version

[@natejohnson6269]

Did you mean: I like ya cut G

[@gregeconomeier1476]

@@natejohnson6269 Uhhmmm. Huh

[@markospehar584]

Alonso accent: much better than before, amazing

[@christianmeeks4430]

Excellent cut my dude.

[@Kj16V]

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

[@kostadiamantis5637]

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

[@d4a]

Thank you for this amazing comment

[@kostadiamantis5637]

And also please send me a link to your merch page as I'm struggling to find it and would like to buy a couple of pieces of merch thx

[@d4a]

It's always at the bottom of the description 😊

[@802Garage]

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

[@d4a]

I'm a beautiful short haired butterfly 🦋!

[@gerrievanstaden3416]

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

[@jhuntosgarage]

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

[@d4a]

Thanks my man!

[@midniter2001]

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.

[@eTiMaGo]

talks a lot more though ;)

[@zibingotaeam3716]

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

[@bobross6677]

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

[@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.

[@lovesjw]

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

[@4G12]

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...

[@michalwilk4748]

what engine are you talking about ?

[@jth000888]

@@michalwilk4748 Honda NR Engine. They wanted to make it to pass the regulations.

[@Roarman57]

Looking sharp with that hair cut!

[@d4a]

Thanks man

[@rogerandersson279]

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

[@tdogg2535]

I love your content please keep it up!!!! you are good at what you do!

[@DC73rr]

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

[@owenbassist5049]

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

[@michaelvinciguerra4369]

I’m digging the new hairstyle. Also, another awesome video. Cheers!

[@owenmeadows5202]

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

[@dawidsnombrski6524]

New haircut. Looks great and as allways great video ☺️

[@d4a]

Thank you 😊

[@Dumbo3.1428]

Very detailed and informative video. Could you please answer the question: why is it that the main bearing axis is symmetrical below the cylinder bore axis. Couldn't you counteract the main thrusts loadings on the pistons through a asymmetrical design or a flat mounted engine?

[@802Garage]

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!

[@d4a]

That's a great topic suggestion! Thank you. There's definitely offset pins out there, many new engines have them. It reduces friction, reduces NVH too. I'll definitely cover it in the future.

[@tanzaniacarschannel6975]

@@d4a are waiting

[@TurboHappyCar]

Really good stuff man! Dropping a comment to help out the algorithm.

[@omorkhan3437]

what an explanation! you just got a subscriber here.

[@4BillC]

Never knew this one! Very interesting!

[@stevensims3342]

I feel like Western English even though not the most efficient language, is able to convey the most acute, detailed and understandable level of communication between people trying to explain things to each other.

[@georgegherghinescu]

Awesome content, thank you! great channel, hope you get more subscribers and supporters!

[@richardcaldwell6159]

Hi guy. Offset pistons can equalize major and minor thrust, reducing total friction while increasing power, efficiency, and torque. My guess is that they aren't universally used because offset pistons complicate balancing. And adding a thermal break while using the appropriate metal for each sub-part of a piston is far superior to the kluge that is currently standard in obsolete designs. This sort of design also simplifies each part, which makes forged parts practical, greatly improving all aspects of performance without breaking the bank. The above improvements make your engine wayyyy more efficient. They are also some of the several improvements by which a truly modern engine protects its oil from heat and blowby. Oil changes ought to be very, very infrequent. They are needed so often because of contamination and degradation, both of which are glaring indicators of poor design. All in all, a fun little video. Thanks and have a good one. :-)

[@Jupiter__001_]

If your solution weren't far more difficult than the current one, it would already be used. If you believe that to be untrue, start a piston company, and make the automotive world a better place! :)

[@johnanderson5500]

Looking good my man! I like what you did with your hair!

[@bennyz1971]

Very cool video. Is there also a video about the zylinder, the bore and the hone coming??

[@simontist]

0:35 is not the taper (a cone shape would have flat sides and not let light through). This effect is because of the "barrel" shape (barreling). This allows the piston to better handle the side loads from the connecting rod. Pistons are pushed sideways into the bore by the connecting rod, and the barrel shape helps with this so the top and bottom edges don't dig in.

[@porterarthur659]

Great content and superb video. A lot of engine builder rotate the piston in the bore because of the pin offset, what does this do to the major and minor thrust surfaces? And wear and tear?

[@GaurdianAzzar]

Thanks for another great video. I have thought about the things you covered but have a hard time finding good sources of accurate information online. Do you have any books or other sources you would recommend that goes into depth on cylinder and piston geometries as well as the combustion chamber and valve configurations?

[@twotone_ra6558]

Why don’t we have dorito shaped pistons 🤔

[@cavalierliberty6838]

Because braaaaaaaap

[@Randy_bosss]

because it will *chip*

[@Folderq]

Bro, Rotary ?

[@EmmaCross94]

some people just can't handle real power when they hear it...

[@stevend4544]

We do its called a wankel :)

[@lovestar6622]

Thanks for the great explanation

[@sahalin12345]

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

[@d4a]

The rings were also a big problem on those.

[@josefkrakel9136]

Unstable skirts have their good points

[@killerdinamo08]

Looks like motorists got caught up in technical stuff 😂.

[@MM_MAXQ]

Awesome haircut!

[@michaeldavis2531]

Very informative and useful video. Thank you very much!

[@gennarodellacorte8478]

Another great video! Thank you very much!!

[@Folderq]

Gr8 material mate, thank You!

[@leneanderthalien]

A piston is a very complicated object: not only like explained here, but the pin bore from the piston is not cylindrical : it as special forms (double cone like+ oval) , to compensate the flex from the pin and from the piston+ lubrication...and much more hidden secrets...(i had work by Mahle and Kolbenschmitt...)

[@natangburek8979]

So how to make perfect piston

[@todd5082]

Great video. What is this guy’s background? Degree?

[@autumnjeserich2689]

It also makes it harder to measure the piston to wall clearance accurately especially when the piston manufacturer doesn't provide a gauge point. Sure the piston alloy must also effect these geometries. Another show of the engineering marvel that is the 4 stroke ICE

[@valentinuiliqnow6198]

thank you great video happy new hair cut

[@ammarrashid3535]

Thank you so much once again bro

[@TheOverclock]

my dude i like the new hair style haha

[@mannyr.2756]

Thank you for the Piston explanation I always thought it was a little simpler than that but apparently there's a lot of math in it, congratulations on your haircut signs of growing up There's Hope for you yet.....lol

[@VBshredder]

You may have convinced him to grow the hair out again! I enjoyed the piston explanation as well - more to it than I knew.

[@laochek]

U look refreshed!!!

[@d4a]

Thanks 😊

[@joshuaharrison9331]

missed the oval of the crown to correct for the exhaust side thermally expanding more.

[@asharma9345]

Nice video and keep it up.

[@jocrp6]

Did'nt go into off set wrist pin locations?

[@alessiocarlevaro6934]

do you know that the hole for the pin is usually not in the center but slightly offset (we're talking 1mm max) toward the exhaust? i don't remember why tho!

[@MinecraftNickTV]

The pin is slightly offset, so when the full force hits the piston at Top Dead Center, the rod isn't standing straight on the crankshaft, but rather starting to go down, thus converting more force into rotary motion, rather than just smacking your crankshaft

[@alessiocarlevaro6934]

@@MinecraftNickTV Thank you!

[@lancecooper4646]

@@MinecraftNickTV ...THAT'S an excellent explanation !! I couldn't get my head around before 👍

[@fernandoanatomia]

Thank you!

[@pabitrikalita3940]

Wow Man, you are looking handsome with that cool haircut

[@d4a]

Thanks man!

[@KoenigsTiger957]

Cool, thank you!!!

[@tharindulakshitha4017]

Thnx

[@TheCabillaud94]

Great video

[@dougn2350]

Good time to explain piston slap.

[@latenightleiferiksson]

How long does it take that the Piston cools down while a brake with shout of engine and how long does it take to get again to perfect expansion when you start the engine again? Does a 15 min break with 1 minute time to warm up again before full load (on the highway for example) means a lot of stress for the piston and the cylinder bore?

[@The_Man_who_lifts]

great stuff

[@samcoote9653]

Hair is looking great dude, couldnt stand the maintenance anymore? :P i feel that haha

[@d4a]

Definitely, ran out of patience 😅

[@MFDOOMgr]

also the pin hole isnt on the y axis. it has a offset

[@leulekalzeleke7490]

What does it meany by skirt? I don't understand what skirt is

[@aco2518]

do pistons change shape as they get hot?

[@813195]

Thank you, fascinating.

[@killerdinamo08]

The whole explanation isn't valid without a reason for the new haircut 😄👌.

[@jth000888]

Is 4 stroke taper and ovality same as 2 stroke engine pistons?

[@jozsefizsak]

Nice

[@akashssmenon]

Do you have any good engineering source where to calculate piston dimensions like bore, height, taper, ovality, etc.? Would love to get any lead

[@pieterdewit9081]

What kind of piston is that @ 5.10?

[@Mekanik.b]

So why can't you pull pistons through the bottom of a block

[@d4a]

It can be done on some engines, but on many engines the mains overlap the bore bottom or there's simply some sort of lip or something in the block. But even when you can remove them from the bottom there's no benefit to it, because you can't install them from the bottom.

[@jimmyh2100]

Let us know this whole debate on turning pistons 180 degrees for more power

[@chrisbenard7509]

Did it hurt? Getting your hair cut?

[@КлиментЦонков]

I like ya cut, G!

[@dustinhess8666]

I like ya cut g

[@chrisspears247]

You are wrong about the reason for the shape it is narrower across that section because of thermal expansion, it expands more across that area because it has more mass under operating conditions the piston is infact round

[@maddog9659]

lol, i can't believe how good it ran with the pistons in backward.

[@jamescaley9942]

So is it possible to install a piston the wrong away around?

[@d4a]

Yes, but most have markings on top that tell you which way it needs to be directed.

[@giuseppesuriani2263]

So where does the piston measure its nominal size?

[@d4a]

Most measure it 90 degrees from the pin at a predetermined height from the bottom of the skirt. The height depends on the piston and manufacturer, but it's usually close to the bottom.

[@irfank20a29]

why the haircut. really admired your golden majestic locks

[@d4a]

I did too. But not when they got in my eyes, and needed an eternity to dry, and annoyed me in my sleep. This was my second time with long hair, I lasted half a year longer this time 😂

[@irfank20a29]

@@d4a pros still outweigh the cons haha. Will stay with your content till your third time.. Looking forward to new episodes of boost school. Keep it up man. :)

[@claudioparedesz5381]

Hey buddy, I am following you long time. I have a few questions for my car... I will ask on IG

[@goodcritics1410]

Damn what I've done I've put a piston inside the engine cylinder without a piston ring, to check the bore, cause i dont i have a tools try to rotate it and got stuck all of four cylinder and i think the problem is a bore so i go to the machine 😞 Co'z i thought a piston is rounded

[@ezzatluqman]

Nice haircut there

[@charleslowe522]

Hair looks good, but I thought the long hair was cool too!

[@omegaelixir]

The only place that my pistons stay outside the engine are in bubble wrap or an oil bath

[@joe_moms_lover]

Congrats?

[@tomop5603]

You look so different with short hair :)

[@spinnetti]

logic is off. The mass is concentrated in the crown, and the diameter at the crown is smaller diameter.

[@ERUtube]

Did not know that pistons had a side

[@PetAmer599]

ohhhh where are the hair bro? haha

[@omegaelixir]

Why don't we have oval VALVES?

[@kappel92]

I'm a huge fan of your job but today I disagree with some explanations you give. I'm pretty sure that the barrel shape is done due the fluidynamics of the oil in the cylinder wall, in particular to have a meatus of olil and an oleohydraulic lift that keep the skirt apart from the cylinder wall and avoid metal to metal contact

[@d4a]

That's pretty much what I said, I just didn't use the words "meatus" and "oleohydraulic". Friction reduction is basically what it's about. By the way, Google "meatus", I'm afraid it's a bit out of place here 😉

[@kappel92]

​@@d4a I traslate italian meato in meatus, I agree with you that's not used in the same way I use it. I'm sorry. btw if you know it's a pity that you don't explain deeply like you have get used us. But don't worry man it's only a whim of mine

[@jamespeterson4125]

Pistons are not round because they don't expand evenly. 'Cam Ground' was the term used in the past, because of this characteristic.

[@Mekanik.b]

Haircut shock

[@petterandersson7429]

Finally....man bun free!!!

[@ivrgn1720]

WHO ARE YOU???

[@TheAlfaowner]

Oj osisao se

[@d4a]

Oj jesam