how do hyper pistons work?

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how do hyper pistons work?

Post by modok » Wed Jun 13, 2018 10:11 pm

Even though I've done this for many years, and maybe even think I know what I'm doing, I realize....I don't really know how these pistons work.
What started it is, I have a motorcycle piston and iron cylinder, air cooled....just over 3.5 bore, I keep waffling about the clearance to give it.

Specification is .002, but It's kinda sketchy so I don't trust it. I was going to go with .003...but, not sure.
So, just for no particular reason, I put the piston and cylinder in the toaster and heat them up to boiling.
The piston measures .010 larger, the cylinder only .005 larger.
This is perfectly in-line with the material properties, as I thought.

But, if that's true, then it means when your engine is hot, the pistons are .005+ press fit? what??
Really?/

When I started this stuff, pistons were ROUND skirts and springy, or had anti-expansion ribs, or were racing pistons that needed TONS of clearance.

This tight fitting hyper-eutectic designs, and/or rigid slipper skirt pistons slowly became the norm now.

All of a sudden I realize I don't really understand how ti works!!! :shock:
If you told me the pistons in your chevy had .005 press fit, I would not think it would turn.
But if you shut down an engine hot, it must be that way.

Help me understand this
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Re: how do hyper pistons work?

Post by modok » Wed Jun 13, 2018 10:21 pm

Perhaps to clarify, I realize that the answer in may cases, is the cylinder is pushed out of round and conforms to the cam grind shape of the skirt. But then I wonder, what happens when you put this is a far more rigid cylinder.....
Does that mean,..... cylinder thickness is a determining factor in piston clearance?
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Re: how do hyper pistons work?

Post by zums » Wed Jun 13, 2018 10:36 pm

modok wrote:
Wed Jun 13, 2018 10:11 pm
Even though I've done this for many years, and maybe even think I know what I'm doing, I realize....I don't really know how these pistons work.
What started it is, I have a motorcycle piston and iron cylinder, air cooled....just over 3.5 bore, I keep waffling about the clearance to give it.

Specification is .002, but It's kinda sketchy so I don't trust it. I was going to go with .003...but, not sure.
So, just for no particular reason, I put the piston and cylinder in the toaster and heat them up to boiling.
The piston measures .010 larger, the cylinder only .005 larger.
This is perfectly in-line with the material properties, as I thought.

But, if that's true, then it means when your engine is hot, the pistons are .005+ press fit? what??
Really?/

When I started this stuff, pistons were ROUND skirts and springy, or had anti-expansion ribs, or were racing pistons that needed TONS of clearance.

This tight fitting hyper-eutectic designs, and/or rigid slipper skirt pistons slowly became the norm now.

All of a sudden I realize I don't really understand how ti works!!! :shock:
If you told me the pistons in your chevy had .005 press fit, I would not think it would turn.
But if you shut down an engine hot, it must be that way.

Help me understand this
You cant do that man, clearances arent measured that way, when i got my first set of aluminum rods { a loong time ago} , i put them in the oven to see how much bigger the bore would grow @220*, i almost freaked out, it sure wasnt 2.225 anymore, if it makes you feel better i have a 15 yr old 406 out there still running with hypers @ .0015, .0007 at the bottom of the skirt, you must have alot of time on your hands
Tom

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Re: how do hyper pistons work?

Post by midnightbluS10 » Wed Jun 13, 2018 10:41 pm

If the piston grows .010 and the cylinder grows .005 then it leaves a gain of .005 for the entire diameter of the piston, or .0025 per side. Wouldn't that leave .0005" clearance? Or am I thinking about this wrong?
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Re: how do hyper pistons work?

Post by modok » Wed Jun 13, 2018 10:55 pm

Yes, .0025 per side, so my piston would have .003 press fit at boiling, but i betcha it runs hotter than that.

the .005 press fit is what you'd get with a 4" bore and hyper piston that normally runs say .0015 clearance.
I have to put everything in terms the v-8 guys understand :P Like a second language. It least it isn't metrique
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Re: how do hyper pistons work?

Post by cgarb » Wed Jun 13, 2018 11:07 pm

Its also the piston skirt...not like a solid chunk of bar stock that would lodge tight. I would bet the piston expands differently inside the bore and not in open space.

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Re: how do hyper pistons work?

Post by user-23911 » Wed Jun 13, 2018 11:26 pm

modok wrote:
Wed Jun 13, 2018 10:11 pm

The piston measures .010 larger,

The piston isn't going to grow that much.

How hot did it get?
Did your measuring stick grow too?


A piston grows more at the crown than at the skirt because the crown gets hotter.
The skirt and the cylinder should be at nearly the same temperature.
Last edited by user-23911 on Wed Jun 13, 2018 11:30 pm, edited 1 time in total.

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Re: how do hyper pistons work?

Post by modok » Wed Jun 13, 2018 11:27 pm

That would be easy to test, back in the toaster?
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Re: how do hyper pistons work?

Post by modok » Wed Jun 13, 2018 11:27 pm

joe 90 wrote:
Wed Jun 13, 2018 11:26 pm
modok wrote:
Wed Jun 13, 2018 10:11 pm

The piston measures .010 larger,

The piston isn't going to grow that much.

How hot did it get?
Did your measuring stick grow too?
Try it yourself. Boil water.
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Re: how do hyper pistons work?

Post by Frankshaft » Thu Jun 14, 2018 10:01 am

zums wrote:
Wed Jun 13, 2018 10:36 pm
modok wrote:
Wed Jun 13, 2018 10:11 pm
Even though I've done this for many years, and maybe even think I know what I'm doing, I realize....I don't really know how these pistons work.
What started it is, I have a motorcycle piston and iron cylinder, air cooled....just over 3.5 bore, I keep waffling about the clearance to give it.

Specification is .002, but It's kinda sketchy so I don't trust it. I was going to go with .003...but, not sure.
So, just for no particular reason, I put the piston and cylinder in the toaster and heat them up to boiling.
The piston measures .010 larger, the cylinder only .005 larger.
This is perfectly in-line with the material properties, as I thought.

But, if that's true, then it means when your engine is hot, the pistons are .005+ press fit? what??
Really?/

When I started this stuff, pistons were ROUND skirts and springy, or had anti-expansion ribs, or were racing pistons that needed TONS of clearance.

This tight fitting hyper-eutectic designs, and/or rigid slipper skirt pistons slowly became the norm now.

All of a sudden I realize I don't really understand how ti works!!! :shock:
If you told me the pistons in your chevy had .005 press fit, I would not think it would turn.
But if you shut down an engine hot, it must be that way.

Help me understand this
You cant do that man, clearances arent measured that way, when i got my first set of aluminum rods { a loong time ago} , i put them in the oven to see how much bigger the bore would grow @220*, i almost freaked out, it sure wasnt 2.225 anymore, if it makes you feel better i have a 15 yr old 406 out there still running with hypers @ .0015, .0007 at the bottom of the skirt, you must have alot of time on your hands
Tom
Even steel will change a bunch. After finishing honing some rods once, I threw them in jet wash. Let it run maybe 1 minute max. Light rinse, said to my self, "these are still pretty hot, kinda hard to hold. Ran over the the machine, and checked size, :shock: . Wow. I don't remember exactly how much they grew, I want to say something like .005, but it was a lot. Obviously everything else changes and grows together, so it equalizes out. That's why I don't know how you would even begin quantifying hot honing a cylinder.

For modok, I don't think the whole piston is "equally" hot when running. I think the skirts stay much cooler that the top say. That's why most pistons measure smaller at the top. For this extra expansion. Also I think they transfer heat much quicker too, as they are relatively thin compared to the top.

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Re: how do hyper pistons work?

Post by Truckedup » Thu Jun 14, 2018 11:49 am

zums wrote:
Wed Jun 13, 2018 10:36 pm
modok wrote:
Wed Jun 13, 2018 10:11 pm
Even though I've done this for many years, and maybe even think I know what I'm doing, I realize....I don't really know how these pistons work.
What started it is, I have a motorcycle piston and iron cylinder, air cooled....just over 3.5 bore, I keep waffling about the clearance to give it.
What engine? I race vintage British Triumphs and do build some street engines, iron cylinders, around 3 inch bore...Normal cast piston, not low expansion, .0045 minimum. Modern forged pistons, .0055 minimum...Anything less will result in a seizure if the engine detonates..
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire

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Re: how do hyper pistons work?

Post by williamsmotowerx » Thu Jun 14, 2018 12:30 pm

Running engine, what gets hot first? Piston

Heat travels to cold.

One of the jobs of cylinder is to absorb heat from piston and give it to air ir water.

Your test had both objects same temp. Not the case in running engine

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Re: how do hyper pistons work?

Post by DCal » Thu Jun 14, 2018 1:00 pm

Don't forget the fuel, rings and the oil are all playing a huge factor, as is the air flowing around the jugs in heat removal from the piston.

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Re: how do hyper pistons work?

Post by Zmechanic » Thu Jun 14, 2018 2:37 pm

I would almost guarantee the piston will have a significant temp gradient from top to bottom. I know a lot of study has been done on this, leading to the wisdom that the top ring carries away a lot of the piston heat.

Put only the crown in hot water for a bit and see how it looks top to bottom? Might get a less frightening, but still probably not super accurate representation.

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Re: how do hyper pistons work?

Post by Morgo » Thu Jun 14, 2018 6:38 pm

Makes one wonder why new factory engines have 1 or 2 oil jets aimed at the underside of pistons..
"when uncomptent order unwilling to do unnecsessary the probablity of failure is high"

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