Choice of metals to use in building a model IC engine

[awake]

Newbie here, on the learning curve. My current questions are about selection of materials when making a bar-stock build.

First, here is what I think I have gleaned from reading through a number of builds:

Aluminum seems to be the material of choice for a large percentage of most bar-stock builds - frame, head, valve block, rods, carburetor, other odds and ends.

Bronze or brass for plain bearings; oilers and sometimes gears are typically from brass.

Cast iron for the majority of cylinders, and for pistons in about half the builds I've looked through.

Steel seems to be used for crank shafts, pushrods, valves, and cams, though occasionally someone has referenced using steel for the cylinder.

Flywheels seem to be the most diverse in choice of metal, depending on what the builder has on hand.

Now come the questions:

Is there any reason one couldn't use steel more extensively in the build, e.g., for the frame, head, valve block, and rods? Obviously steel would add more weight, but all of it static weight except for the rod - would steel here be too much reciprocating mass?

What are the advantages / disadvantages in using dissimilar metals for the cylinder and head? It would seem like the aluminum head might want to expand more than the cast-iron cylinder, for example - ??

I've never seen anyone mention using steel for the piston - is this a really bad idea? (Again, it would add weight to a reciprocating part, as compared to aluminum, but no more weight than a cast-iron piston, I think?)

As an explanation for my questions: some of this is purely a matter of learning about what goes into the best-practice of a model IC engine, but some reflects the experience and materials I have on-hand. In my 10+ years of home-shop machining, I have acquired (scavenged!) quite a bit of steel from various sources and in all sizes and shapes, but relatively little aluminum, and most of that in small bits and pieces. Most of my machining experience has been in steel, with only a tiny amount in aluminum. My shop equipment has no problem handling steel, and in terms of speeds available, is arguably better suited to steel than to aluminum (Series I BP goes up to 3000+ rpm, but my Cincinnati 12-1/2 x 30 lathe tops out at 1200). Finally, if it were necessary to do any welding, I have a decent DC tig welder, but no ability to weld aluminum.

All that to say, as I have thought about trying my hand at a build, I could easily complete something like a Webster with materials on hand IF I could build most of it out of steel, including the cylinder, rod, and head. I do have a couple of small lumps of cast iron that would be sufficient for making a piston, and plenty of bronze and brass for bearings and valve cages and such. I have enough small bits and pieces of aluminum that I could make the valve box from it ... but I've also got plenty of small bits and pieces of steel that would serve. Perhaps most important, I have enough of all of the above that I could easily re-make parts if, or rather when, I mess them up.

Given that I have plenty of home-shop machining experience, but NO experience building an IC engine ... would I be setting myself up for trouble if I went this route? Half of me says, just buy the materials you will need. But the other half of me, the scrounging half, says "why not try with the materials already on hand?" (That latter half got louder when I priced a bar of cast iron from McM-C!)

Thanks for the words of wisdom to come, as well as all that I have already gleaned from reading through the posts on this forum!

--Andy

 

[Brian Rupnow]

You can use steel for mostly everything except the cylinder and piston. We use aluminum everywhere that we can because it is much easier to machine on smaller lathes and mills. Flywheels can be steel, iron, or brass. Aluminum is too light to make a good flywheel. You can make pistons from aluminum. The use of cast iron for cylinders comes from the fact that cast grey iron has a lot of graphite in it's molecular make up, so that it is somewhat self lubricating. For a low rpm engine, grey cast iron makes a beautiful piston. On high revving engines, the lighter weight of aluminum makes it quite suitable for pistons, as it is so much less mass than an iron piston.---Brian

 

[awake]

Thanks, Brian. I know the self lubricating value of cast iron, and know that it would be ideal for the cylinder ... but I have seen occasional references to using steel for the cylinder. If I use cast iron for the piston, might I be able to use steel for the cylinder?

After I wrote the earlier post, this question became even more acute, as I was disassembling my latest treasure trove of scavenged steel, aka the frame and weights of a former exercise machine - I found that one part is a 16" long section of seamless steel tubing, with an ID of .875" - seems like it is just begging to be used as the cylinder for the Webster, since it calls for a bore of .875"!

 

[Jasonb]

Contary to what Brian says steel can be used for cylinders or cylinder liners and is often the chosen metal on small aero engines where the CI piston is lapped into the liner. As you are aiming for a very fine lapped surface using steel of unknown parentage can be a waste of time as it often does not machine well. These engines will happily run to 10,000+ rpm with CI pistons.

You also need to watch what grades of aluminium you are talking about, many conrods use 2014 as it is a lot stronger and has self lubricating properties for conrods that won't need additional bearings. If you just use say 6061 the holes will stretch or it may gaul.

Balance may also change if you start to swap metals so conrod counterbalance weights may need to be altered

Aluminium conducts heat better than steel so those cylinder and head cooling fins will work better

 

[tornitore45]

Aluminum pistons, preferably 2014 0r 2024, must have CI rings
Cast Iron pistons are lapped to fit into CI or even leaded cylinders.
Brass is heavy and expensive, no good for flying, limited to small parts.
Different material fit require consideration of thermal expansion. Aluminum piston in CI or steel sleeves would seize if fit for compression when cold.
Aluminum head expansion has a different relation to the sleeve, it can slide if necessary. Not the same for the piston inside the sleeve.
As a general rule parts that need to move inside others should not be made of the same material.
Hardened steel runs better on bearings. Hard on softer but not 6061, too soft.
There are exception like CI on CI and hardened steel in ball bearing or gears.

 

[BWMSBLDR1]

Backing up 100 years aircraft engine design was focused on weight and cooling. A serious approach was the rotary engine that had radial cylinders on a crankcase that spun around fixed crankshaft. Pistons and cylinders were machined from alloy steel for maximum weight reduction. In higher performance engine design there was a short period between using cast iron and aluminum for pistons where steel was the material of choice.
So steel is a perfectly acceptable metal for use providing the design and fabrication methods utilize it's properties to the fullest. Bill in Boulder

 

[kf2qd]

I have used 4140HT for crankshafts on a .60ci 2 stroke, Continuous cast iron bar used for making the cylinder and ring. Rest of engine out of aluminum, did not use any bushings, just low nitro model airplane fuel. Reamed holes in the aluminum connecting rod. Lapped the cylinder using fine valve grinding compound on a turned steel bar that I used as a lap.

 

[GreenTwin]

One problem I have seen with steel used in steam engines is that it will easily corrode.
The piston/cylinder of a steam engine has to be flushed out thoroughly with WD40 after running on steam, else you will have a siezed engine next time you try to run it.

I have seen 100 (+) year old gray iron steam engines with some pitting on the inside of the cylinder, and on the top of the piston, but it generally is relatively minor pitting (all things considered), especially when compared to the corrosion rate of mild steel.

Cast aluminum 356 can be tempered to T6 specs, to make it much harder and less prone to gum up tooling.

There is nothing really wrong with steel engine parts to the best of my limited knowledge.

I do like how easily gray iron machines, and the saying is "it wears like iron", which is to say it does not wear out easily.

You can also buff gray iron to a mirror finish, and I guess perhaps you can do the same with steel.

Steel pieces can be silver soldered together, and so that is a big plus with built-up engines.

About the only thing I use steel for is things like piston rods and such.

You can get round steel rod with a ground surface finish, which saves a lot of time if you need an accurate shape.

Gray iron also drills and taps very nicely.

Steel is pretty easy to machine in my opinion, but it produces strings instead of chips.
The strings don't bother me, but machining gray iron with carbide tooling is just a dream.

Most of the old machinery was cast from gray iron, and I don't think that was just a random choice, but rather there are many benefits of making machinery and engines from gray iron.

.

 

[Nerd1000]

It's increasingly common for larger industrial diesels to use steel pistons. This is done mostly because these engines tend to go all-in on turbocharging, leading to monstrously high cylinder pressures that aluminium pistons struggle to withstand without being so heavily built that they're not much lighter than an equivalent steel one. Steel provides better support to the piston rings, so it can have the top ring much closer to the piston crown which helps reduce emissions. On the other hand, it's less compatible with an iron bore, so they have to coat the piston with special stuff to reduce friction and wear.

You're unlikely to see steel pistons in a spark ignition engine though. They don't run high enough pressures to benefit from them, and the higher thermal conductivity of aluminium helps to limit charge temperatures and therefore knock.

Finally, you might take note of our 'friends' over at Briggs and Stratton, who for many years have use hard chromed pistons running against aluminium liners. Clearly a hard piston can run successfully against a soft liner, if you so desire... this from the same people who brought you plastic camshafts, so your mileage may vary.

Flipping things on their heads, alloy steel liners work really well in all manner of aircraft engines. The big WW2 radials usually used a steel liner (something like 4140 as I understand it) with an aluminium cooling 'muff' shrunk over the top to provide the finning. This apparently worked well with the iron piston rings and aluminium pistons. So I'd look at grabbing a little slug of aluminium for the piston, a slightly bigger slug of grey iron for the rings, a little bit of bronze for the bearings and then make the engine from as much steel as you please otherwise.

 

[awake]

Interesting to see this old thread resurrected! As it happened, I went on from there to design a "Steel Webster," in which every part was made of steel except the piston, which was CI. Introducing ... the "Steel Webster"