Metal Working: Designing and Making Pistons

Learn the details of pouring, feeding and the solidification of iron and aluminum piston castings so you can do it yourself.


| December 2012



Alluminum's long freezing range alloy

Iron pistons are easily cast by using small gates on the top edge of the casting and no risers. Aluminum pistons, because of solidification shrinkage, are more difficult to cast. Pouring temperature and the placement of gates and risers are very important. 

Illustration Courtesy Chastain Publishing

Design and make pistons and rings for new or old engines with this must-have, heavily-illustrated guide for both beginner and experienced metal workers.  Making Pistons for Experimental and Restoration Engines (Chastain Publishing, 2004) by Stephen Chastain instructs on how to make the tools and jigs you need to quickly produce top quality replacements in your own back yard and home shop. Learn all about making pistons with this excerpt taken from chapter four, “Casting and Feeding.” 

You can purchase this book from the Farm Collector store: Making Pistons.

Making pistons: pouring, feeding and solidification of piston castings 

Pistons may be made of cast iron or aluminum. Iron pistons are easily cast by using small gates on the top edge of the casting and no risers. Aluminum pistons, because of solidification shrinkage, are more difficult to cast. Pouring temperature and the placement of gates and risers are very important.

Aluminum castings freeze by three different methods. In pure aluminum, shrinkage occurs as a deep pipe or at the centerline of the casting. Solidification of alloy #295, 94% aluminum, 5% copper, 1% silicon begins at the wall but progresses quickly to the center of the casting.  Fine grains form randomly in the center of the casting and freezing continues in a mushy state. The center of the casting may be as much as 85% solid before a completely solid skin forms on the surface. As a network of solid grains form, feed metal is unable to flow through the constricted passages and microshrinkage occurs around the dendrites. The riser height drops and distributed microshrinkage forms throughout the riser and casting.

Chills are used to force the metal to freeze quickly from one end before the network of grains forms, constricting the flow of feed metal. Chills also increase the mechanical properties by reducing the segregation of gas and impurities at the grain boundaries.

Many pistons are cast from alloy F 132, or  #332 (they are equivalent alloys). Alloy #332, silicon 9.5%, copper 3% solidifies with some gross shrinkage and some distributed microshrinkage.