The Pistons Part One

How are pistons made?

Well it’s time to start working on the pistons. The pistons will be made out of aluminum which is a good material choice for pistons, because it’s lightweight which is especially good for aircrafts, and aluminum is very thermally conductive, so it cools down fairly quickly. And the block is also made of aluminum so the heat will spread into the block. The other material choice would be steel, which is used in many, if not most cars. aluminum pistons are rising in popularity though, you might find them in new cars. Cast iron pistons are heavy though, which can be a problem for aircrafts, because they need to be as light as possible.

There are three common types of piston manufacturing method. The first method is forging, which involves either a hammer or a press, at a very intense pressure, and it drops a “forging die” onto a chunk of iron/aluminum, pressing it into shape. Forged pistons are very strong, but heavier due to compressed material. The second option is casting, which as you probably know is pouring the molten metal into a mold. In these two methods usually you would machine them further to get them into the final shape. The third method which is not very common would be simply machining the whole piston out of a block of metal, using CNC machining and lathe working. I use the second one, casting. The reason for that is because cast aluminum is very light, lighter than if you were to forge them. Another reason is that its a lot less costly for me, i already have an aluminum foundry, however getting the tools required for forging is not very easy, because you would need a huge drop hammer or press requiring up to thousands of tons of pressure. Obviously that’s very expensive equipment. And i don’t have a CNC mill so i cant use the third option, so for me the simplest option is casting.

After casting the pistons, i will move the pistons over to a lathe. My mold will be slightly larger than the final product, both due to heat expansion of the metal (which is more of a problem when you cast iron, if i’m correct) and because the casting will never be a perfect cylinder shape. I’ll simply take the cast piston, and use a lathe to machine it down to the correct diameter, which in my case is about 88.5 millimeters, 1.5 millimeters under the cylinder bore. That’s simply so that i can fit the piston rings. After i’ve done that i’ll make the groves for the piston rings. This is done on a lathe as well. After that the only thing left for me to do is to drill the hole for the wrist pin. I could simply cast the hole but that adds complexity and the hole would never be perfect so i’m better off simply drilling it manually. The last thing i do which is optional because its not on all pistons, is to drill holes from the oil ring grove and into the piston’s cavity, this is the return path of the oil.

And that pretty much sums it up, so lets get started shall we!

First thing i did was make a wooden model of the piston, slightly larger than the final product and i explained why in the text above. Here is a picture of it:

IMG_5494[1]

This is the wooden model i’ve made of the piston. I just need to file it ever so slightly so it’s perfectly symmetrical. The red marker lines you see is just what i used to measure out where to cut. Below is an image of the very much unfinished piston cavity:

IMG_5499[1]

 

The picture is slightly blurry, but what you may notice is that the bottom of it is not flat like i want it to be. This is a problem for me because i cant use my router, it doesn’t go deep enough into the piston. But the solution i’ve come up with is to buy these polishing stones you can buy in a hardware stores. You can put them in a hand drill, and use them to polish surfaces, and its a very handy thing to have.That should take care of my problem quite nicely. After i’ve done that it’s off to the foundry with it. I need to make 8 of these. I cant cast it just yet because i don’t have any molding sand at the moment, but i’ll buy some fairly soon so i should be able to get going with these babies!

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