A friend was saying aluminum piston do grow when heated. Actually he meant cycled, heated then cooled. If piston temperature was high enough it would grow a little.
He said manufacutres do some sort of heat treatment to stop it?
Anyone of you guys with metalurgy knowledge could comment?

Its been a while since I studied material sci in college but I'll take a stab...


All materials expand when heated and contract when cooled (with the exception of water that expands when frozen). Thus When your engine heats up all the metal parts expand. Ideally, if the entire engine was made of the same metal, it would all expand at the same rate. This of course is not true for 2 reasons:

1. Most engines are made of multiple different metals.
2. The engine does not heat evenly, ie some parts (combustion chamber, exhaust) remain hotter than others.

These two factors contribute to uneven expansion of the engines parts. This poses a problem, how do you ensure that the tolerances stay correct (not too tight, not too loose) when the engine heats up (in order to get good compression etc)? Well, there are a couple of solutions to this for the piston/cylinder fit:

1. Use a ringed piston. The ring is usually spring steel, so it can expand/contract to make up for slight changes in cylinder diameter due to heat expansion. Also, because of the ring, the piston and cylinder never come in direct contact, thus making the expansion of the piston less of an issue.

2. The tapered cylinder used in an ABC/ABN engine. Since the brass liner expands more than the aluminum piston, it is made with a taper so that its very tight at TDC when cold. As the engine heats up, the sleeve expands a bit and loosens up at the top, but not so much that compression is lost. If the liner were made straight, it would open up a gap when hot and you would loose compression.

3. Use an AAC piston/cylinder. Since the sleeve and piston are the same metal, in theory they expand at the same rate and keep compression. I'm believe that AAC sleeves have tapers as well (though probably less than ABC). The disadvantage is that its difficult/expensive to chrome plate aluminum.

As far as I know aluminium doesn't do this but cast iron definitely does. For a very good explanation of this have a look at http://www3.telus.net/dieselcombat/breakin.htm

I just so happen to make Automotive pistons for a major manufacturer, but I can still only guess.

All of our pistons have to be heat treated before machining. They are put in "age ovens" which bring the pistons up to just under 500 degrees (I think) and cooled. If they are not heat treated properly they will warp and react otherwise unpredictably when machined and when heated.

Auto pistons have ridiculously tight tolerances and require specialized CNC's to make (the pin holes for example are +2.5 microns/-1.0 micron: a micron is 1/25th of a thousandth of an inch), but I wouldn't know about these little guys. I would say that if a model engine piston wasn't made using the proper controls it could very well grow. (Ford just sent us back 16,000 pistons for this very reason.) But if it was made right, it wouldn't.

Ringed pistons absolutely do contact the cylinder walls, ever so slightly.

Really? I thought only the rings made contact with the cylinder wall. I dont work in the auto industry so I could certainly be mistaken...

Your friend may have been talking about trying to rejuvinate an old ABC engine. Years ago it was found that some ABC pistons would grow slightly when given a heat treatment, increasing their size and giving back some compression seal after they were worn. As I recall, we heated them in the oven at 350 degrees for 5-8 hours, then cooled.

Engines with heat treated pistons, OS, in particular, did not grow to any appreciable amount. I seem to remember that Super Tigres did grow.

Well, I could be too for that matter. I'm just a machinist.

We have a finish requirement on the "skirt" which helps insure proper oil retention and many pistons are graphite coated on the "skirt" to aid in assembly. If you think about it, with the rings so high up toward the crown, the force of the power stroke and the the way the piston/rod assembly is made, how could you stop it?

The distance between the piston and the cylinder wall is so small that Ford matches pistons of different sizes (grades) to cylinders. Again, were talking MICRONS here. For example, with the piston I make (Ford 3.0 litre duratec) there are 3 "grades" (sizes) of pistons. the GREATEST diameter variation from the smallest grade to the largest would be 30 microns or just over a thousandth of an inch. There is no way it WON"T touch the walls.

Brian-

Interesting comments. Maybe we need an automotive engineers opinion? . . .

It was my understanding that the lower part of the piston rides on an oil film (with a clearance in the thousandths of an inch range) and that the purpose of the oil scraper ring was to clear this oil film to give the compression rings direct metal to metal contact with the cylinder.

I always thought that there was no metal to metal contact anywhere. It is all separated by a film of oil. If you had direct metal to metal contact it would gall and sieze. I also thought that the oil ring on a four stroke was to stop excessive oil from getting into the combustion chamber and being burned (ie burning or using oil)

Alan Angus

Well, you got me on a technicality, There is the oil film, I mentioned the finish requirement for oil retention earlier. (the tool leaves tiny grooves across the skirt which hold the oil) I was thinking that it was being assumed that the piston somehow "floated" in the cylinder without going near the wall, and that is what I was addressing. Sorry.

There is always a minute amount of metal to metal contact or the engine would never wear out, and I believe that the rings are the main culprits, in an old well worn engine the wear ridge at the top of the cylinder stops where the top edge of the upper ring stops. The piston skirts apparently have enough surface area so that side thrust does not seem to wear the cylinder as much as the rings do. Good lube is everything and that is why I refuse to use any fuel without some castor in it.

Used to work in an automotive machine shop.The auto pistons are cam ground,in other words oval shaped, with the fattest part 90 degrees to the wrist pin. They are designed to expand and contract along the pin direction so they can get away with .0015 clearance on average depending on the diameter of the piston.

That's my understanding also. Its from the excess carbon nodes interacting with the surrounding iron. Doesn't happen to any great extent with steel, but I don't know for sure but the high silicon aluminum has excess silicon crystals which may react in a manner similar to the carbon crystals in cast iron. A good engine of any material will be heat treated to prevent this. It was one of the differences in the old days of cast iron pistons between the likes of Enya and Fox vs say a McCoy.

The piston skirt can wear because that is the primary part of the piston that contacts the cylinder. That skirt is there to prevent the piston from cocking inside the cylinder. If worn excessively you can have piston slap which will in turn wear the ring groves. A good overhauler will check for wear on the piston skirts, some even check the diameter at the skirt.

Well, they're not ground. They are turned with diamond tools. (and axially they are also barrel shaped)

Ed_Moorman:

Thanks for the answer and the link for the web site. That's what my friend was talking about.
When you said to heat piston to 350 degree, do you mean Farenheit or Celsius?
What about the cooling? Rapidly, by quenching on water or oil or cooling it slowly? Any "aging" after that? I mean, heating it again to a lower temperature for 20 to 30 minutes.

Some understood I was asking about expansion with temperature, sorry but this was not the question. I meant growth, piston actually increasing diameter after being cool again.

If I knew this would be true we wouild take an old low compression ABC piston/sleev set and would do the experiment.
But need to have an idea if this really happens and which temps, cycle, aging and time is needed (aproximate).
I found a tread on another forum talking about changing hardness on aluminum alloy and it seems there are some sort of phase setting with the additional chemical elements added to Al.

Sharp333

Down_under:

tankas for the web site link.

Sharp33

I think you two guy's discussing automotive piston design are in two different technical era's here. Yes, they use to be of an oval design. I've done my fair share engine rebuilding and some of those piston skirts looked like they were machined with a .030" feed rate and a 500 micron finish.

What do you mean USED TO BE? Auto pistons are oval radially and barrel shaped axially to compensate for expansion of the piston. I operate the machines that finish the O.D.'s of the pistons (as well as the the machines that finish the pin holes and the valve pockets). We have a $250,000 machine in the lab the ONLY checks the ovality and barrel shape. Now they are even starting to make Briggs and Stratton pistons like this as well, they are being made at our LaGrange, GA plant.

What was the topic of the thread again?