Aluminum vs. cast iron

[Guest Stllrng.]

I have an old, pre-1916 type four cylinder, the kind with the 800 pound flywheel and no counterwieghts on the crank. I runs just fine, but I am going to redo the engine. Without getting too far into thermodynamic theory and space rocket applications, I was wondering how much of an improvement aluminum pistons would be over the iron pistons ( the pistons and rods balanced of course). I am just not sure how much difference the reduction of wieght would make on a low speed engine like this. If I was to balance the clutch, crankshaft and flywheel, and pistons and rods, would the aluminum pistons make any noticeable difference in vibration? I am not concerened with how fast it rev's up, remember it's not a 351 Cleveland. Thanks.

[Guest]

What do you want this engine to do? Aluminum pistons may be a good idea for a number of reasons. 1-It will give you longer bearing life (are you using new babbitt or reusing old babbit)2- Iron pistons are fine, but now they are almost 100 years old! Even though the piston looks fine now, it might decide to break in the next couple of hundred miles causing havoc and damage to the rest of the engine. If you are just building a car to drive in local parades and around town, the original pistons are probably just fine. If you plan on doing some serious touring, I would go with new aluminum pistons. (as well as new babbitt, etc.) Remember, however, aluminum has a greater expansion rate than cast iron. So, if you go with aluminum pistons, give yourself plenty of bore clearance. When I rebuilt the engine on my '13 Overland, I allowed about six thou. clearance between the aluminum pistons and the cylinder. A good rule of thumb for aluminum pistons is a thou to thou and a half per inch of piston diameter. Good luck!

[Peter J.Heizmann]

Stllrng.<P>I know little about pistons, especially if aluminum is better than iron, and vice-versa.<P>If it helps, the comparison weight range is 1 pound of aluminum would be 3 pounds of iron.<P>As for the soundness of your existing Fe pistons, you could take them to a metallurgical lab and have them "Magnafluxed" - blue dye penetrant. It would be a lot cheaper than getting an X-Ray.<P>As for vibration, the iron would have better damping properties than aluminum, which is just my opinion.<P>Best regards, Peter.

[ply33]

I think the key here is that you said "it runs just fine".<P>The advantage of aluminum pistons would be reduction in recipicating mass. This would allow you to reduce the size of the flywheel for the same smoothness in the engine. The net effect of this weight reduction would be an engine that changes RPM easier. That is it will feel peppier on acceleration. You might be able to get a little higher RPM out of it too, but that is probably pushing things.<P>However, since "it runs just fine" the way it is means that you are satisfied with its current performance. That is a strong argument for leaving the cast iron pistons in.<P>As far as being worn out.... Once a part has been cycled a few million times, if it were going to fail due to fatigue it would have already. (This is assuming that the service it is seeing is unchanged. Upping the max RPM, running it out of oil, etc. all void that argument.) Barring too much wear (a different thing than fatigue cycling) your parts are probably fine. That said, it would still be a good and prudent exercise to check all the parts while they are out using the best techniques you can.

[1913Moline]

I wouldn't automatically assume that aluminum pistons will be quieter in your engine. What is lacking in most of the replacement aluminum pistons is a cast in steel strut for expansion control. The ones I have used (from California), just have a "t-Slot" for expansion control. This forces you to fit the piston with more clearance in the bore. This can easily lead to piston slap upon cold starting.<P>The old cast iron pistons you have could probably be fit at .002" clearance on a 3.5" bore. They are heavy as h... but they run quiet. You will be at .004" skirt clearance on the aluminum replacement pistons.<P>I am NOT advocating re-using the old pistons as your bores will need resizing. Spend some time researching to see if you can find a good quality aluminum truck or industrial engine piston which matches your bore, wrist pin diameter and pin to dome dimensions. It will take some hunting, but it is worth the effort.<P>Good luck,<P>Greg<P>

[Guest]

I like to put them back the way they were .When I ws a kid,there was a car with cat-iron pistons that would run on and on...little or no trouble.One of their competitors had aluminum pistons.Besides having poor resale value,it blew rods regularly.

[Guest Stllrng.]

Well thank you for the help guys, I guess as usual, I should have given a bit more information. I have a four cylinder White with the aluminum crankcase and cast iron cylinders. Like I said it runs just fine, but it is getting tired and the valve guides are really bad. What I also have is a second engine that was completely rebuilt way back, but never sold. This engine has all new valves and pistons, rods re-babbited etc. I am planning to install the crank and complete top end onto my original crankcase. I was going to balance the components before I put it back together. That is why I was wondering if it would be worth the expense of aluminum pistons to reduce the vibration. Thanks again for your help and I welcome any further suggestions.

[Guest]

I also have a 13 Overland with alum pistons.As posted above keep the skirt to cylinder clearence about .001/inch of bore,not any less."Experts" have told me that the old cast iron piston were better,if that were the case why do all modern engines have alum pistons?

[D Binger]

Nothing wrong with Aluminum pistons. I have them in a 1914 Buick and it runs great, just ask anyone who was on the tour in Ft. Collins, Colo last year. I also have them in a 1922 Buick. <BR>I have driven Trainguy's Overland and there is nothing wrong with that car and the aluminum pistons.<P>Dan

[Peter J.Heizmann]

TRAINGUY:<P>As posted earlier, I know little about pistons, however, my best "guesstimate" as to why modern cars use aluminum pistons is alloys have been improved, not to mention, "weight reduction".<P>Have no idea how much a cast iron piston weighs as I sit here. Let's pull a figure using a 6-cylinder car and the "cast iron" piston weighs 6 pounds each, and, the aluminum piston weighs 2 pounds, just for discussion:<P>--A known: The ratio is 1 pound of Aluminum equates to 3 pounds of cast iron.<P>Cast iron: 36 pounds<BR>Aluminum : 12 pounds<P>24 pounds is quite a difference in the board room.<P>Car manufacturer's (not to mention the "bean counters") obviously love the 24 pound weight reduction.<P>Obviously, aluminum is better for weight reduction that all of the modern cars are based upon from a cost standpoint and fuel consumption.<BR> <BR>--Characteristics / capabilities:<P> Iron: Better distortion resistance.<BR> Better wear resistance.<BR> Better vibration damping.<BR> <BR> Negatives: Weight compared to Al.<P> Aluminum:<P> Alloys today can compete with Fe (in<BR> certain atmospheres, such as <BR> pistons, valve bodies, pressure<BR> vessels, etc. ZA (zinc/aluminum) <BR> practically has the same weight as<BR> Fe.<P> Almag (aluminum & magnesium) has<BR> similar wear characteristics as<BR> Fe.<P>--Misc:<P> Fe (class 30-35) is poured at 2650 degrees on an average.<P> Al is poured at 900 degrees on an average.<P>Just my 2 cents. If aluminum works, fine. If cast iron has always worked, fine.<P>Sort of boils down to what a lot of topics we discuss over time generate: If it suits you and it works fine, go for it. However, I personally am not one to venture into the unknown only to ruin what was fine in the first place concerning "cars only".<P>Regards, Peter J. <BR> <p>[ 04-02-2002: Message edited by: Peter J Heizmann ]

[ply33]

Received the April 2002 issue of <span style="font-style: italic">Skinned Knuckles</span> today. A nice article about bearing loads and bearing failures. At high RPM a big part of bearing load is centripetal forces due to the big end of the rod and the reciprocating mass of the piston and rod. Weight reduction there does not save a whole lot on the weight of the car but greatly reduces bearing loading. An engine manufacturer would certainly look at the higher cost of aluminum pistons as being worth it for those gains. As long, of course, as the technical challenges of using aluminum could be overcome. At low RPM bearing loads are more a function of the pressure in the power stroke. There are also some comments in the article on why thin shell bearings may be less susceptible to fatigue failure than thick cast babbitt bearings. An interesting read.

[Guest leadfoot]

For my money, SK is still the best hobby magazine available if you're interested in what makes old cars tick and how to keep them running.