More questions on pistons

[HarryJ]

Over the years I have bought out several auto parts houses and dealerships of their old stock, in doing so I've acquired a few sets of cast iron pistons. These fit Chevy, Olds, and Pontiac. The question is; does anyone rebuild engines utilizing cast iron pistons? I always wondered if rebuilding an engine originally fitted with heavy cast iron pistons using light weight aluminum units would cause it to be out of balance? Also, when was the last year cast iron pistons were used in a passenger car?

[Guest Hal Davis (MODEL A HAL)]

I had cast iron pistons put back in my Model A.

[HarryJ]

Hal, Didn't Model "A" Fords originally come equiped with aluminum pistons? I would think that the use of cast iron would throw the engine out of balance.

[Matt Harwood]

Actually, in some cases, a cast iron piston would be preferable to an aluminum one. There's been talk in performance circles lately about a return to cast iron pistons. They can be made to be as light as aluminum pistons in heavy-duty applications (thick aluminum vs. thinner cast iron). The cast iron is also more dimensionally stable as it heats up, giving a more consistent fit and, believe it or not, reducing cold-start emissions. They are also a lot tougher and will withstand a lot of abuse that would damage an aluminum piston (nitrous, supercharging, etc.). Because iron doesn't transmit heat as well as aluminum, they may also increase performance by keeping more heat in the combustion chamber. Of course, nobody wants to be the first to put cast iron pistons in their Top Fuel dragster, but I bet we'll start seeing some high-tech cast iron type pistons in diesels before long.

But in an old car application, I'll wager that a lot of cast-iron pistons are NORS (New Old Replacement Stock) parts from the WWII era. For example, I know that the small-series 1942 Buicks used cast-iron pistons from the factory for this very reason (1941 and 1946 Buicks used aluminum pistons). Since people were keeping older cars on the road longer during the war, replacement pistons were still necessary. But with aluminum being in short supply, I bet a lot of cast iron replacement pistons were manufactured back then. That might explain the stockpiles in auto parts stores.

Balancing should not be a problem--any engine is going to need some "mallory metal" added to the crank for proper balance anyway. The few extra grams from a cast iron piston won't pose a problem for a competent shop.

Just a few thoughts...

[Guest De Soto Frank]

Just to add my thoughts to Matt's...

De Soto also had a cast-iron piston option during late '42 production and for service replacements during the war.

Any set of pistons should be individually weighed, the lightest one being used as the bench-mark, then the other pistons carefully lightened to match.

Then (ideally), the rods, crank, and flywheel /clutch assembly should also be balanced; might not be critical for a trailer queen, but for an old car that will tour or be driven, careful balancing can only help improve performance and legnthen engine life.

(And cast-iron and aluminum pistons should never be mixed in the same engine... )

I think for a time, Chevy used cast-iron pistons too...in the early days of their valve-in-head six, it earned the nickname "the cast-iron wonder"...

[Peter J.Heizmann]

Frank and Matt...good thread and info.

For what it is worth, the ratio is 3 pounds of iron = 1 pound of aluminum. (Green sand castings).

Regards,

Peter J. <img src="http://www.aaca.org/ubbthreads/images/graemlins/grin.gif" alt="" />

[Guffin]

Most engines before 1930 had no counter weights on the crankshaft and using aluminum instead of cast iron should not change the balance. Even on later engines there is generally no problem changing to aluminum pistons from iron. This is because the counterweights on the cranc is to balance out the cranc itself and part of the connecting rod, not the pistons. The piston is balanced by the neighboring piston 180 degrees away. The counterweights minimize the bending of the crankshaft by the centrifugal forces.

V engines may be more tricky.

Jan

[Peter J.Heizmann]

Jan, that, too is interesting.

I posted the weight ratio with the thought that using both cast iron and aluminum together may not be a good idea for balance. Either iron, or, aluminum, not both in the same engine.

Again, for what it is worth.

Regards,

Peter J. <img src="http://www.aaca.org/ubbthreads/images/graemlins/grin.gif" alt="" />