Thread: Make your own pistons

Thanks, Pippin, all,

Originally Posted by pippin88

Cans are made of an alloy that is good for extrusion / stretching, not good for casting. The best scrap for home casting is actual castings.

That explains why the can scrap isn't wanted. Thanks Guys.

Home made pistons should only be made out of aircraft grade billet..
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Originally Posted by BaronJ

Thanks, Pippin, all,
Originally Posted by pippin88
Cans are made of an alloy that is good for extrusion / stretching, not good for casting. The best scrap for home casting is actual castings.




That explains why the can scrap isn't wanted. Thanks Guys.

Actually, the top of the drink cans is made of a high strength Aluminium/Magnesium alloy. A different alloy from the sides/bottom of the can. That is a problem when recycling cans, because the resulting alloy mix is unsuitable for making either tops or sides of a new drink can. It is necessary to analyze and purify/correct the composition of the melt to obtain again a known alloy of predictable properties suitable for reuse.

If you have seen failed pistons or broken up pistons, you will notice how consistent they are.

The cast is realy smooth, fine grained even and clean, when it is machined the result is clean and silky.

A piston has to withstand a great deal, even in a crude old stationary engine.

The piston has to have just the right heat resistance, durability and other chariteristics. There is a great deal of difference beteen cheap pistons and high performance pistons. there are only a few reputable high performance piston makers that have the resources to make a realy nice piston.

I doubt very much that it would be possible to make a home cast piston that would be more than mildly sucessfull.

I have not gone into the process, But I suspect that good pistons are a bit more than simply pour cast......because of the volume in which they are made, they would have to be machine cast and probably under pressure

Look at the unmachined surfaces in modern pistons and there is no hint of sand grains...the surface is smooth, clean, even with well defined detail......even the chep ones.


Yes many pistons are oval at room temperature....as they heat up they become round in the bore.

I recomend shopping harder for your pistons and putting your casting and machining resources into things like manafolds and ram tubes and the like.

An old mate of mine....a trained pattern maker who did his time in a foundry, used to cast ram tubes in his back yard, from any good scrap he could lay his hands on.....back in the day, he could sell as many ram tubes as he could make.



The other think I am sure some of you will be aware of is, the issue of oil contamination when using engine and gearbox castings....aluminium absorbs the oil and that presents a couple of issues.

As far as I know mag wheels are still the prize of the scrap metal heap.

cheers

I have not gone into the process, But I suspect that good pistons are a bit more than simply pour cast......because of the volume in which they are made, they would have to be machine cast and probably under pressure

Back when I played with cars the best pistons were regarded to be made by TRW. These were forged. The company I worked for, as I have said made pistons. They were simply cast. They also made racing pistons using the same method, but a different alloy.

They could supply 12:1 compression ratio pistons for a 351 ci ford V8 and were well regarded in the field. Such a piston in that motor is under massive stress compared to your average motor. They supplied these pistons for many different motors including the Holden Grey Motor for some reason. These were pistons intended for racing.

Yes many pistons are oval at room temperature....as they heat up they become round in the bore.

That sounds like rubbish. Pistons are oval because there is no need for full diameter on the sides (where the pin ends are) Many pistons are recessed here in the casting process and many also are relieved completely here. ie Virtually no skirt. The skirt at right angles to the pin is to reduce piston pivot in the bore. To keep it straight. On the sides where the pin ends are there is no need.

Dean

There is a big difference between no need and going to the trouble of maching pistons oval by a poofteenth right at the crown and where the ring lands are.

if you look at racing motorbike pistons......they are nothing more than a crown, sufficient meat to carry a gudgeon pin and only enough skirt to stop the piston turnng turtle.
The skirts ( such that they are) are even releaved so they have minimum bore contact.

As for those US derived V8s and sixes...there where never big on fine engineering.

We used to joke about the six cylinder and v8 holden and ford guys raving about things like solid lifters, forged pistons and crossflow heads
Most of the japanes and europen cars came factory with nothing less..even the european 4 cylinder fords.

cheers

I've no idea what the practice is with modern pistons, but for many years piston skirts were machined slightly oval to stop piston slap when the engine was cold. The long part of the oval is across the axis of the gudgeon to prevent the skirt slapping back and forth in the bore at start up. The short part of the oval at the gudgeon axis is undersize to the bore so the skirt can expand as the piston reaches operating temperature.

I recall seeing pistons being oval ground or maybe even being oval turned in an worksop some years ago. The piston was rotating slowly and there were pivoted weights that flopped in and out every 90° to somehow provide the oval profile.

Originally Posted by bob ward

I've no idea what the practice is with modern pistons, but for many years piston skirts were machined slightly oval to stop piston slap when the engine was cold. The long part of the oval is across the axis of the gudgeon to prevent the skirt slapping back and forth in the bore at start up. The short part of the oval at the gudgeon axis is undersize to the bore so the skirt can expand as the piston reaches operating temperature.

I recall seeing pistons being oval ground or maybe even being oval turned in an worksop some years ago. The piston was rotating slowly and there were pivoted weights that flopped in and out every 90° to somehow provide the oval profile.

Where I worked many years ago they had several cylindrical grinders which had cams built in to grind the oval shape. They were specific piston grinders.

Dean