suitable Al-Si alloys for pistons.

[Owen_N]

I have heard eutectic -12% si, is good, but google searches give 4032, which is 21% .
21 % Si will have a lot of free silicon in it - hypereutectic.
Should I look for a plain eutectic casting alloy in the LM series? Will I have problems with eutectic and expansion in a hot-running engine?
There is no point in having copper in it- this needs heat treatment.
I will come back with some number equivalents.
<edit>
Lm6 is plain 12% Si the alloy number is 4514 . I will search for this.

 

[lee webster]

I will be watching this thread with interest. I am in the process of sanding smooth the 3D print to cast a piston in aluminium.
Lee

 

[mu38&Bg#]

Only those building high performance ringless engines use special alloys. For a ringed design, select a common piston alloy and bore clearance to match.

 

[kf2qd]

Find an engine rebuild shop and see if they would sell you some pistons for scrap price.

 

[fltenwheeler]

ALCOA Lo-Ex Lynite Aluminum Piston alloys was used in Wall Model Engine casting kits.
Tim Meyer

 

[fltenwheeler]

I have been looking at MATWEB for more information.

I have decided that I will use 319.0 aluminum for the case, head and other aluminum parts. It has a very small expansion coefficient compared with the other alloys mentioned. It has good weldability for repair. Machinability is rated at 50%.

Typical applications for sand casting 319.0 are a wide variety of structural castings for engine parts, engine crankcases, gas and oil pans, and general commercial applications.

For the piston I have decided on A390 aluminum. It has a very small expansion coefficient compared with the other alloys mentioned and a little less than 319.0. The one draw back is the machinability is only 10% do to the high Silicon content of 16-18 %. It is a common alloy for sand cast pistons.

Tim Meyer

 

[Jasonb]

The CBN inserts are said to work well on the high Si content alloys allowing for very fine cuts to be taken when fitting ringless pistons. As Diesel Pilot says these alloys are really only needed for high performance racing engines for tings like F3D pylon racing where you may change piston or engine after each race

Does the 319.0 need any heat treatment after casting, some of the Alloys in the other thread come in T6 temper when bought in bar form

 

[Bazzer]

I don't think Owen is searching for the really high silicone content alu but a very popular type is called RSA444 and it is made in Holland by a company called RSP, they are easy to deal with.

Their 30% Si material RSA444 is superb material to work with, I use PCD tips and a good starting point for tips is this company.

PCD Inserts Associated Production Tools (shop-apt.co.uk)

Until you have tried these tips you will not believe how good they are, to be honest they take a small hobby lathe to a whole new level in terms of what can be achieved, the tips are so good they effectively take out machine 'spring back' in tiny cuts which are required when fitting pistons to bores, particularly tapered bores.

I wind the cut on by having the topslide angled at 1 degree and use that to get tiny cuts, it is possible to get a couple of micron cuts this way. On tapered bores where you desire the piston to stop/jam at a certain point in the cylinder then these tiny cuts are used to increment the piston up the bore and it feels very repeatable but the process does rely on the tool actually cutting !!

I more recently have used Garant PCD inserts (265837) these are twice the price of the APT inserts but are tougher.

Be warned these PCD tips are addictive, you will start using them on more and more jobs if you are not careful.

In the photo's you can see the turning operations in an EMCO Compact 5 lathe, an MT2 blank end arbour from Arc Euro trade is machined to pick up on the piston skirt and there is a drawbar that looks like a dummy con rod to pull the piston onto the arbour/mandrel.

The underside of the piston is machined on a Mazak 530 so one extreme to another 'V' the EMCO. To be honest and with care the piston underside could be machined with a manual mill but it just would not have the dainty detail that these have.

Jason is right the high performance glowplug engines can be very consumptive of parts, maybe more like 45 minutes running for a piston across say 30 runs and connecting rod's lifed at 40 runs (they are good for around 80 to 100 runs but failure is catastrophic).

The final picture of the piston sitting upside down is pre OD machining, that finish is roughed down on the Mazak mill to about 0.3mm oversize.

B.

 

[Owen_N]

Find an engine rebuild shop and see if they would sell you some pistons for scrap price.

I would think about this. Crucible, furnace and all the gear add up a bit in cost.

A plain eutectic 12% could be a little on the soft side, but I don't think heat-treatable si-cu alloys are used a lot with pistons, in casting.

The initial heat soak is at a very high temperature. Possibly stress relief can be done at a lower temperature, but precipitation hardening
needs full solution treatment to start with, I think.

The usual piston alloy has 21% Si and very little else. I think 30% silicon is really overdoing it. Wouldn't that be hard to machine? HSS tools would go blunt very quickly.

 

[Bazzer]

Owen

Could I suggest that you take a look at the following website which has masses of model engine tests.

Model Engine Tests (sceptreflight.com)

I have quickly scanned through a few engine tests on that site and material specifications are mentioned for a lot of components, although I did not find something specific to ringed pistons but it is probably there somewhere.

I agree 30% Si is too much for your application but it is not hard to machine even carbide tools give a great finish.

B.

 

[Bazzer]

Owen

I have talked to one of the UK's real small engine experts on piston material today and 2618 is quite favoured for making ringed pistons, in fact the well know English company called Laser who make 4 stroke glow plug engines use 2618.

Just machine the piston from solid, that is what a lot of really good model engine manufacturers do, saves the hassle of casting the pistons with all of the variables that entails.

Also check out the link below.

2618 vs. 4032 Piston Material Differences (jepistons.com)

 

[Bentwings]

Owen

I have talked to one of the UK's real small engine experts on piston material today and 2618 is quite favoured for making ringed pistons, in fact the well know English company called Laser who make 4 stroke glow plug engines use 2618.

Just machine the piston from solid, that is what a lot of really good model engine manufacturers do, saves the hassle of casting the pistons with all of the variables that entails.

Also check out the link below.

2618 vs. 4032 Piston Material Differences (jepistons.com)

most definitely machine from solid. I suppose if you have multi axis lathe or mill you could cam turn the piston diameter like automotive pistons but remember they have many decades of making pistons by cam grinding . Our race pistons were all coated. Early ones just hard anodized but now there a many coatings available. You could check the internet. Unfortunately I’d think coating could gt expensive. Of late it’s been hard to get metal treaters to just black or color anodize anything but a paler load of parts. I YHINK just settle for a low expansion alloy and be done with it . Something you can get bar stock more or less over the counter. I’d forget any home made casting and heat treating

 

[KennyMcCormick315]

hypereutectic pistons have been getting my half-ton truck down the road for 37 years now. If they're good enough for a 4.9L I6 with 350,000+ on the clock they're good enough for anything we're likely to build as a model.

 

[Bentwings]

hypereutectic pistons have been getting my half-ton truck down the road for 37 years now. If they're good enough for a 4.9L I6 with 350,000+ on the clock they're good enough for anything we're likely to build as a model.

i agree. I’ve built many auto engines with these and can’t say I’ve seen a piston failure not caused by something else. I had a supercharged Streetrod I drove as a daily driver for 10 years with no engine issues it still had good if not better compression when I finally sold it I even drove in the winter more as a challenge. .