# Aluminum Piston Material sources?



## mu38&Bg# (Oct 6, 2009)

I've been searching for some piston material sources. This is for a working engine for model airplane so the normal aluminum grades are not so good. I've heard of people cutting down large truck pistons to make model pistons. I stopped by a truck repair shop today and the gentleman said he has a buyer that pays him 200-300USD per piston as scrap! Needless to say he didn't have any to offer, and I'm not going to pay $200. I haven't tried the scrap yard, yet. Are the any other scrap sources of >15% silicon alloy aluminum out there that might be large enough to produce a 25mm diameter piston? 4032 Aluminum would be acceptable, but it runs about 20USD/Lb. Better (>25% Si) alloys in small quantities run 10 USD per inch for 1 or 1.5" diameter.

Greg


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## RobWilson (Oct 6, 2009)

am i reading this right $200 to $300 for scrap pistons ??? you must be joking ,,,,,there (7"dia) not worth £2 to £3 over here

Regards Rob


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## bentprop (Oct 6, 2009)

$200-$300????He was having you on,Greg.obviously couldn't be bothered.
Find another place that wrecks trucks,or even cars.It doesn't necessarily have to be a piston.
I've used bits of waterpump housing that worked just as well to replace OS pistons.
Good luck on the hunt 8)


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## vlmarshall (Oct 6, 2009)

bentprop  said:
			
		

> $200-$300????He was having you on,Greg.obviously couldn't be bothered.



No kidding. For that price I could bring home an ENGINE from the local "Pick & Pull" or "You-Wrench-It" scrapyard.


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## itowbig (Oct 6, 2009)

how about car engine pistons ive got about seven left out of a small block chevy new never used.
so u could look at car engines


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## Diy89 (Oct 6, 2009)

I have a source for school bus engine pistons, usually dt466 or small cummins. I have not checked recently, but if you interested i'll have a look over there to see what is available. 

Silly question time....You would have to remelt them right? Or is the dome thick enough to cut a slug from? 

Paul


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## rake60 (Oct 6, 2009)

The top of the line Aircraft Grade Aluminum Alloy is 7075.
It has a tensile strength of 83000 psi and a yield strength
of 73000 psi. It would fit your application quite well.

I don't know what size of piston you are making so let's
be liberal. A 12" length of 1" diameter 7075 cold roll round 
stock is currently $13.03 at *Online Metals*.
It is something to consider.

Rick


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## mu38&Bg# (Oct 6, 2009)

That's what he said. These are for 12L 6 cylinder truck engines. Maybe he said per set, but that's still a lot. I'm hoping to find a hypereutectic alloy. I guess a trip to the auto scrapper is worth while.

I hope to find a piston large enough not to have to deal with melting it as I have no equipment for that.
http://www.enginehistory.org/bristol_aquila.htm

A guy on the RC forums has recast pistons before.
http://www.rcuniverse.com/forum/m_4929500/tm.htm

Standard grade aluminum is not normally used for pistons. They have greater rates of expansion and gall more easily. For display engines anything works because there is no load. I need to make a piston for an OS FS-52 that will run about 10,000 RPM at .75HP for 15 minutes or more at a time. If I can't find the right material I'll use something readily available. for proof of concept.

I'll try calling around again tomorrow or next week. Thursday, I get wisdom teeth removed.


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## doc1955 (Oct 6, 2009)

I don't believe 7075 t651 will gall it is a very tough material as the heat treat of t651 is very durable material but I'm not sure of the expansion rate right off. Ive worked with this material a lot and have never seen it gall up in any applications I've used it for.


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## Diy89 (Oct 7, 2009)

I'll stop by and see whats in the bin. I dont know if the domes are thick enough to cut a slug from or not. I'll find out and let you know.


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## rklopp (Oct 9, 2009)

You can buy 4032 aluminum alloy bar stock from McMaster-Carr. It has good wear resistance due to 11-13.5% silicon content. I just made 13 (baker's dozen!) pistons for a set of Upshur Twins using the stuff. It machines really beautifully, as it is less gummy than 6061. It does tend to wear carbide tools noticeably, such that you'd probably want to use diamond tools if you were making 1,000,000s of something.


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## Mo deller (Oct 10, 2009)

I have a large amount of aluminium but no idea what grade it is. Is there any way of identifying a half decent grade from the cheap n cheerfull stuff? 

Peter.


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## Kermit (Oct 10, 2009)

There are density differences. 2024 for example will weigh more than 6061 or 7075 because of the copper it is alloyed with.

The only way I could do it without a laboratory would be by density. weight the piece carefully and get also get the cubic volume by displacement in water. That will give you density which can be compared to a textbook with that data.

{fill a jar with water until it overflows them put something under it to catch any water. immerse the mystery piece of metal then measure the amount of water that overflows out of the container. It will be the cubic displacement volume of the piece.}

Lotta work for identifying a piece of aluminum. :
Kermit


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## Mainer (Oct 10, 2009)

Before you choose 7075, check out its strength at elevated temperatures...such as what a piston might reach. While it is very strong at room temperature, its strength advantage falls off rapidly with increasing temperature.

I'd take a look at McMaster-Carr. Their online site www.mcmaster.com has good summaries of the properties of various alloys.


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## mu38&Bg# (Oct 10, 2009)

rklopp  said:
			
		

> You can buy 4032 aluminum alloy bar stock from McMaster-Carr. It has good wear resistance due to 11-13.5% silicon content. I just made 13 (baker's dozen!) pistons for a set of Upshur Twins using the stuff. It machines really beautifully, as it is less gummy than 6061. It does tend to wear carbide tools noticeably, such that you'd probably want to use diamond tools if you were making 1,000,000s of something.



Well I'll be... Somehow, I didn't check McMaster when searching for the 4032. McMaster never showed up in google searches either. This solves my problem, thanks! I have a few other things to get from them.

I am familiar with 7075 high temp strength. 2024 is actually stronger above ~150°C.

As far as sorting out aluminum alloys it's not easy. I think there was a thread here at HMEM about it. It involved some chemical to be applied and looking for the resulting color after reaction. If you are just making parts with checking for hardness is good enough. If you chuck it up and it's gummy, it's a non hardened grade. If you are looking for specific corrosion properties or strengths, I would just buy the right material. I've used some scraps and the good stuff just machines nice. The worthless stuff is just awful to machine. The better alloys have a nice ring to them, while the soft ones sound dull.


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## rcplanebuilder (Dec 1, 2009)

Mo deller  said:
			
		

> I have a large amount of aluminium but no idea what grade it is. Is there any way of identifying a half decent grade from the cheap n cheerfull stuff?
> 
> Peter.



Hardness test. 

Also, model aircraft pistons don't need to be nearly as strong as the full scale, due to the scaling factor. 6061 will never come unglued at that scale, but 2024 is dandy, and over kill, if you have some. 7075 is strong stuff, but has corrosion issues, that are likely to show up in that application.


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## jabezkin (Dec 9, 2009)

Maybe Wiseco pistons could help so you could get the right type. The right type may not be the strongest. Cheap insurance. Ceramic coat the top, the dome surface and the exhaust port and let 'er wail!

Some aluminums are a third stronger than 7075, up to 780 MPa!( That is about 113,000 lb tensile)

7118 goes to 90,000 and 7079 might pass 90,000 also. Haven't used these for a while.


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## raym 11 (Dec 9, 2009)

*4032  * aluminum has a very low coefficient of expansion to allow for a closer fit to the cylinder wall. It seems to be readily available. If the need be I'll take a look at my records for vendor.

Ray M


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## mu38&Bg# (Dec 9, 2009)

I did end up ordering 4032 from McMaster. 4032 has a reasonable coefficient of expansion, but there are much better materials. They are just hard to get.


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## raym 11 (Dec 9, 2009)

I am getting ready to make a piston (1"od). Could you suggest some lower expansion aluminum suitable for this low RPM hit and miss?


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## 1Kenny (Dec 9, 2009)

Here is a good article about piston material that is used.

http://www.kb-silvolite.com/article.php?action=read&A_id=38

 I have used both forged and hypereutectic pistons in engines. They both have their place depending on how you are going to use the engine.

Kenny


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## raym 11 (Dec 9, 2009)

Thanks for the article 
Kenny. I need all the help I can get. 
My latest build was the VAL (#15&16 Model Engine Builder). Photo in issue #20.
It has a 1.124 bore. I used 4032 and allowed .0015 clearance using the criteria atwww.eriperformace.com. The engine turns ~9k .
I often build less demanding engines and try different materials as a test platform. Only one to destruction so far. ;D What a hobby!! :

Ray M


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## 1Kenny (Dec 9, 2009)

Here is what Alcoa says about the 4032. It has pictures of the grain too.

http://www.alcoa.com/gcfp/catalog/pdf/alcoa_deltalloy_4032.pdf

I have had a few beers with one of the engineers at KB. He has told me that he has run as little as .0005 clearance with the hypereutectic pistons with a 4" bore on an air cooled engine. To do that, the bore needs to be straight and round using vertical boring. Gravity pulls the stone down and will oblong the bore when done horizontally. The bore needs to be smooth. Almost like it is polished with so lite of a 60 degree cross hatch your mind will question if that is right.

Kenny


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## Kaleb (Mar 1, 2010)

I am using an aluminium piston in my steam engine build, and I go to a shop called Bearfast, which deals in general tooling and industrial supplies, for all my brass and aluminum. A 300mm length of approx. 12mm diameter aluminium bar can be had there for about AU$5.


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