Aluminum bar stock for pistons

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petertha

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I was having a discussion with a buddy about choices of bar stock aluminum for making (ringed) model engine pistons. Somewhere in my travels I assembled some notes of what others were using & why, but they seemed to have vaporized. His point was that you cant simply use alloy strength parameters which are referenced at ambient conditions because strength diminishes with increased temperature & differently among the alloys. Fair point. I did a quick forum search here, a few posts like below, maybe more orientated to casting material vs. bar stock.

https://www.homemodelenginemachinist.com/threads/aluminium-grade-for-pistons.33930/#post-375308
I started to Google what I thought would be straightforward Strength vs Temperature comparison of bar stock choices one typically sees referenced in model builds (at least in N-Am) 6061, 2024, 7075. I was hoping to stumble on a direct hit cross plot something like I've manually constructed in Excel from alloy specific sources, but no such luck. So take this chart as a guess for discussion purposes. Maybe you have better properties links & I will update as required. I'm suspicious of the 2024 data, I assumed better than 6061. Also recognizing that many commercial model engines are forged and from from their own alloy recipes, so even harder to comparatively evaluate. Here is as far as I got. 7075 looks the best but the question is what is the max operating temperature range where the values diverge? The top of piston will be the hottest, but it also cycling between strokes so maybe some average?

https://pdfs.semanticscholar.org/253b/e284d172b00d6d7ca21d64106a7154f94e89.pdfhttps://www.rolledalloys.ca/shared-content/technical-resources/datasheets/2024_DS_USA_English.pdfhttps://www.extreme-bolt.com/aluminum-7075-screws-bolts-nuts.html
 

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Maybe not so fast. I'm suspicious of the 2024 data. Kind of odd why this information is not more readily available.
Found this link showing 6061 vs 2024. At first glance 2024 is better vs 6061 at elevated temp in the tail region. But I don't quite understand the rationale of the strength metric. Its like they are normalizing each materials strength at room temperature (all start at 100%). Its interesting to see the curves shape I suppose, but I assume the (MPa or Ksi) strength value itself that matters?

https://www.nakka-rocketry.net/therm.html
 

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They would be 100% of the individual materials strength so all would be different values to start with, it is a comparrison of what percentage they drop at not the actual value they drop to
 
Are you going to build a Nitro burning, high speed, sustained high temp over 200c engine??
Maybe you are referring to the yellow temperature range on my chart? The same chart I mentioned take this as a guess for discussion purposes?
To answer your question, yes, built a 4-stroke engine similar to thousands of RC glow engines on the market. These all burn nitromethane in a methanol based fuel. Race or specialty engines as much as 40% nitro. I don't think the cc displacement scales or factors over other parameters, but I could be wrong. I think the combustion temperature of these fuel mixtures is quite similar across all engines, or at least the temperature required to ignite which is a minimum requirement to run. Here is a chart from Model Engine Mechanics By Gordon Cornell. You can see flame temp is orders of magnitude higher than 200C, albeit for a very brief duration.

I don't know what the upper limit is, that's really the million dollar question. Obviously there is heat dissipation in many forms. Practically speaking, I've seen seized pistons, pitted detonation pistons, no melted hole-in-the roof pistons. 6061 melts at about 580C, so there's that. 200C was a spitball number because I've measured that routinely on exhaust headers. Do you have some data to share?
 

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@petertha
Thanks for the chart !
Looking at your chart, I prefer 6061 aluminum
I can’t give you the detailed tests but being long time drag race in fuelgas nd alcohol ranks. Today most pistons are forged and get heattreated 7075 usually coated with a lolive drab colored coating much like ore seen in industrial hard anodizing it is generally very hard early pistons som some mfg use their own specialty versions early pitonscwcast with then briefly 20 24 forged with fancy coatings. Some were just hard anodized. The coated ones now hold up pretty well even when you abuse them. When we burned pistons in 392 hemi yo could just turn the around in the hole and burn the other side but gen 2 hemi has a thunder area forvthe intake valve notch if her is one that prevents that . For a while I modified evdamaged pistons for the next size up thicker rings but now it’s usually not done , just toss them in the scrap barrel $100 to $ 200 a iece 8 per run. Alcohol engine wear the coating off a little more bu hold up better my superchsde street rod had 35k miles on it when did and pistons looked new . Bores are 3-4 step Moro honed with vey fine cross hatch most modelers don’t have profile meters so the traditional 3 step is used you hone about .002” rough diethen .0015 to si and then .0005” plateau to finish. Most use stainless dyke top ring some use a hapless double top ing thin ply filled second and standard 3 piece oil rings butt gaps are smoothed but we just used a fil in-the vice and worried about how square things were there are ring filers available both hand cranked and powered just debur well. I haven’t had any reason to service model engine for a long time but most seem o be some aluminum . As hard a s they run at speed I’d guess 7075 is what in them
 
I found this link. Lots of detail on heat treatment & depiction of piston temperatures. Has similar chart comparison of 7075 vs 6061 but not 2024.
Data suggests 7075 is better at elevated temp from strength standpoint, but maybe other factors come into play.

https://www.mdpi.com/1996-1944/13/15/3441
 

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have you looked at some of the high silicon content alloys, the guys that make and run the high performance competition model engines seem to use them such as RSA444
 
Thanks Jason. I've heard of the name referenced but never considered sourcing it. I know the ring-less 2S guys get very fussy about strength & expansion rates at elevated temps too, maybe that factors. I'll do some digging to integrate RSA444 Strength/Temp data on my chart when I get home just for interest. But (assuming the same strength metric is being referenced) it appears to ~follow the 7075 trend just eyeballing the chart. TBC.

Seem like many proven model engines use one of the 3 bar stock alloys mentioned, so I'm not particularly worried. This was more for curiosity. 2024 & 7075 might be a bit more prevalent? and that list of mine is what I've managed to misplace. Also, many of those engines were gasoline fueled which, if I understand, might run a bit hotter than my methanol engine. But I want to venture into gasoline/spark next so worth pondering now.
 

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I've combed through quite a few engine review articles on Sceptre website for other reasons, which is a fantastic resource btw. I don't believe I ever encountered an alloy specified for pistons other than a generic 'high silicone' or 'high strength' aluminum alloy. Part of the secret sauce I guess, along with con rod material.

This link describes one of the veteran pylon engines (Nelson). Considered a robust workhorse for class specific racing at 7.7 BHP/CI (albeit many engines these days have higher metrics). But as touched on in the article, expansion rate (relative fit) of components at temperature factors heavily into material choice. Above my pay grade but interesting nonetheless.

http://sceptreflight.com/Model Engine Tests/Nelson QUARTER 40.html

http://sceptreflight.com/Model Engine Tests/Index.html
 
Pertha,

Thanks for posting the reference to the Model Engine Tests! I really enjoyed the ones I have read. Reminds me of the high school library where I would love to read Peter Chinn's review of model engines in Model Airplane News. I found the review of the supercharged OS engine particularly interesting.
 
You're welcome. I saw that OS engine run in competition but I was always more of a YS man back in the day. Still have a scar to prove it haha

Well RSA-444 appears to have a shallower degradation profile, but they reference Ultimate Tensile vs others were listed as Yield Strength no not quite apples to apples. Still looking for better 2024 data, I know I've seen it somewhere.

1687826352842.png
 
everyone I know of in Bay Area (BAEM Club), including me, uses 2024 for pistons (heat resistant), and 7075 (high strength) for conrods, and 6061 (cheap!) for everything else. every once in a while someone will use 7075 for heads as well, since then you don't really need inserts for valve guides or valve seats in this case, and if you can afford it it really machines much nicer than 6061. BTY, the 7075 conrods are usually run directly on the crankshaft without any additional bearing material, that's how good and hard (no tendency to gall) 7075 is !!!
 
Thanks Peter, I think that's good advice. And seems to corroborate with the data/chart assuming its correct. What I hadn't appreciated is how 7075 necks down like that at elevated temperature, so learned a few things. Now, is it truly operating in that high temp band, who knows. Some commercial glow engines are pretty thin & swiss cheesed by comparison, although they likely have their own alloy recipe & are forged. Maybe better than bar stock, but doubtfully Kryptonite

1687933645294.png
1687933692889.png
 
Thanks Jason. yes I have that Jim post bookmarked. Fantastic wealth of information. And probably where I heard the (RSA-444 T6) piston alloy mentioned. I'm quite familiar with Nelson engines (or at least used to be in a former hobby life). But running them is different than building them haha.
 
Go visit a local engine rebuild shop and see if you can buy a used diesel piston. They tend to have more meat to them, and they are high silicon.
 
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