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Piston to wall clearance, during operation.

Piston to wall clearance, during operation.

Piston to wall clearance, during operation.


On car engines it is common practice to run about 0.05mm clearance betwern piston and cylinder.

But what does this clearance become when at operatimg temps, like full load ???

Some person says that the pistons becomes a slight press fit but i dont believe it.


RE: Piston to wall clearance, during operation.

Press fit = seized engine.

The only possible answer is "it depends". It's going to be tighter than static clearance ... aluminum has a higher coefficient of thermal expansion than cast iron, and even if you are talking about an aluminum-block engine, the pistons operate at higher temperature than the coolant jackets.

Pistons generally don't have a single clearance number unless you specify a single place to measure it. The shop manual specifies a location which is usually some distance down the piston towards the bottom of a skirt. The clearance at the top of the pistons is normally much higher ... because the piston tops operate at higher temperature than the skirts. So, the pistons are machined intentionally tapered, with more clearance at the top.

Bottom line ... no single answer. "Tighter than cold" is about all you can say.

RE: Piston to wall clearance, during operation.

Pistons are made from a high silicon low expansion aluminium alloy. They also have a complex geometry when cold to compensate for the differential expansion of varying material sections.
At least they were when I was in the auto engine industry.

Politicians like to panic, they need activity. It is their substitute for achievement.

RE: Piston to wall clearance, during operation.

"Because I could" ... I measured the diameter of a piston that I have in the shop downstairs that came out of a Honda single-cylinder motorcycle engine. Piston diameter measured down near the bottom of the skirt 57.92mm. Piston diameter measured just above the top compression ring 57.70mm.

RE: Piston to wall clearance, during operation.

If the "roundess" is measured it is a "Figure of eight" with the waisting around where the gudgeon pin fits. And tapered from skirt to ring section.

Politicians like to panic, they need activity. It is their substitute for achievement.

RE: Piston to wall clearance, during operation.

In the heavy duty world articulated pistons were common for a while. The steel crown was separate from the aluminum skirt so the skirt could be made round. EMD put the wrist pin in a carrier as well so the piston could rotate about the carrier. Now many have moved on to a one piece steel piston.

RE: Piston to wall clearance, during operation.

Bit of a misnomer calling them "crosshead" pistons. Sure - one part of the piston acts as a linear bearing but it still uses the cylinder bore as a reaction surface. A true crosshead mechanism uses a seperate linear bearing to guide the connecting rod small end, leaving the piston and cylinder bore completely free of side loads.

je suis charlie

RE: Piston to wall clearance, during operation.

Not to nitpick but calling modern pistons round, cylindrical, or tapered is a bit of a misnomer as well. Due to the largely hollow structure, save for the crown and surrounding the pin bore, cross sections viewed down the crown are rarely round. The taper above and below the pin bore also varies as you move up/down, and some also vary on either side of pin. To my mind, pistons are more of a smushed egg-shape than anything, hence the comedy of folks measuring them and discussing custom fits within a given bore.

RE: Piston to wall clearance, during operation.

But the modern piston is doing a lot to work towards a more round geometry. Steps to separate the crown from the skirt reduce the taper required and separating the wrist pin bosses from the skirt allow for a more round piston.

RE: Piston to wall clearance, during operation.

@CWB1, have to disagree with your last comment having measured very accurately probably 1000+ petrol road car pistons. From 999cc vw units, to 500hp Bmw V10 items. All are ''round'' just tapered towards top.

To even go as far as F1 pistons, which Ive had a few through my hands..these are also ''round''

Aftermarket High compression billet pistons from many makers also ''Round'' (I know this because Ive designed at least 40 of for different companies)

Perhaps your argument holds through for heavy diesel items, or composite CI/Al units, but Ive yet to see it in anything other than text books from old.

As for the OP question, clearance depends on piston material, and liner material. Broadly speaking, this means a cast aluminium piston/billet piston, and CI/Steel or AL Nik coated block.


RE: Piston to wall clearance, during operation.

Modern pistons have done a lot to isolate the skirt from the wrist pin bosses. The pin bosses have a lot of material which has to grow. Air gaps between the pin bosses and piston skirt eliminate the need for barrel grinding.

On an interesting side note, Electro Motive developed heat dams in the pistons for their 567 series engines. The put an air gap between the crown and ring pack. This allowed them to make substantially more horsepower without overheating the piston rings.

RE: Piston to wall clearance, during operation.


@CWB1, have to disagree with your last comment having measured very accurately...

Yard stick or tape measure?

I have encountered my share of pistons that were round at room temperature, but so far as I recall they've all been in very early combustion or steam engines. Most every recip engine piston since the second war has been profile ground and is noticeably elliptical. I've released my share from 50-300mm for most every application and type of combustion, as well as machining a few for restoration projects.

RE: Piston to wall clearance, during operation.

I've never come across a "ground" piston and I replace a lot of pistons... Off the top of my head I've done a 1948 Hamilton, Enterprise R5, MAK 23. EMD 645, Detroit Diesel 71, Cat 3500, John Deere 4045, Honda CR250, Yamaha YZ250, Yamah TT225, Honda CT90, Honda XR650R, Cox .049, Chebby 350, Quincy QR-25. That last one is an air compressor but... That covers diameters from 1/2 inch to 17 inches. Not one of them was ground, all lathe turned.

RE: Piston to wall clearance, during operation.

@CWB1..very expensive Mitutoyo equipment...

I feel some of your advice in this thread is old wives tales to be honest.

Ive stood beside at least 50 cnc lathes making motorsport pistons, and the same making oem items in cast - they are round - I assure you - please stop spreading false info.

https://www.youtube.com/watch?v=oteRRECMeSo&t=... said lathe here, skip to 2.30 approx.

Taper yes, oval or otherwise, nope,


RE: Piston to wall clearance, during operation.

I hate to add more fuel to this fire.. but some pistons are most definitely NOT round. I'm not a piston DRE, but assembled engines where the piston skirts did not have a constant radius.

They are ovalized in complicated ways by some manufacturers so that they are round at operating temperature.

Lots of companies make pistons, and they clearly don't all do it the same way.


RE: Piston to wall clearance, during operation.

Hi Briangar,

"All are ''round'' just tapered towards top."

Just to be clear,are you saying modern engines' piston skirts are completely round/cylindrical/circular at any point vertically along the skirt, but are larger at the bottom of the skirt, and taper to a smaller round, circular "diameter" at the top of the skirt below the ring lands ?
In concept as shown in the attached exaggerated image ?

RE: Piston to wall clearance, during operation.

A piston 'roundess' can not be measured on a standard roundness checking machine. The variation exceeds the normal roundness range capability.

When I was in the auto business, pistons were tapered with a figure of eight form from from the skirt up to the ring pack.
Used to use pistons from a uk co. called Hepolite who made pistons for just about everybody in the uk. We used in excess of 30k/week.
Also they were ground to finish prior to anodising on special purpose cam following grinders. The high (abrasive) silicon content meant turning was precluded.

All the above is not conjecture. I was there!

Politicians like to panic, they need activity. It is their substitute for achievement.

RE: Piston to wall clearance, during operation.

Still sticking to my word on this guys - as a stubborn Irish man. Modern materials, I've never once seen in anything non round upto 105mm, billet or cast, upto 13,500rpm in production engines - way above in F1 stuff. I've even seen the lathe gcode run...

Bowing out of this thread now as I gave up bickering on the internet about 10yrs ago.


RE: Piston to wall clearance, during operation.

The video I linked above shows piston production at Omega, an extremely well known and respected piston company which produces product for high end motorsport of all types- and they have a machine, shown in that video, which has no other purpose but to take round piston blanks and make them not round anymore.

Just because you've never seen it doesn't mean it doesn't exist.

RE: Piston to wall clearance, during operation.

If the out of round were meant to compensate for thermal expansion, wouldn't it be less expensive to simply machine the piston hot vs using expensive multi-axis machines?

RE: Piston to wall clearance, during operation.

An aluminum piston being machined 'hot' would need to be sprayed with coolant to deliver the required surface finish, IE it would remain 'hot' for maybe the first 5 seconds of the operation.

RE: Piston to wall clearance, during operation.

The coolant would be heated as well.

RE: Piston to wall clearance, during operation.

Even so, it would be impossible to produce the temperature gradient across the piston which is present in operating conditions, and is the reason why a piston would be produced with a complex envelope shape. The difference in temperature between the skirts/pin bores and the crown can be 150 degrees or more. In order for 'hot' machining to produce the right shape, you would need to apply this gradient and maintain it continuously for the entire machining operation.

I also suspect that at scale, operating a machine in a cloud of oil that's being maintained at 300 deg C is not cheaper/safer/more efficient than having a dedicated machine which does the operation at room temp.

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