Learn the details of pouring, feeding and the solidification of iron and aluminum piston castings so you can do it yourself.
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.
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.”
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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.
Because long and short freezing range alloys solidify differently, no one set of specific guidelines can be given for the placement of all risers. General riser dimensions are given but should be modified to suit the particular job at hand. For the small foundryman, selection of proper risers is still a trial and error affair.
Guidelines that generally represent the short freezing range or skin forming alloys have been generated by years of experience in steel casting. In these alloys, shrinkage occurs as riser piping, gross shrinkage at hot spots and centerline shrinkage in uniform sections. For this situation, use hot risers gated directly from the runner when possible
Many aluminum alloys are not skin forming but freeze in a mushy or pasty state with dispersed micro-shrinkage. These alloys behave differently than short freezing range alloys. Heavy risering may not significantly improve thesituation and may make it worse. Good feeding is better produced by steep temperature gradients towards the riser. This is accomplished by proper placement of chills and insulating boards. In some situations, micro-porosity is not a problem and the foundry seeks to distribute the porosity as widely as possible throughout the casting. This is accomplished by making it solidify as uniformly as possible. When section thickness is mixed, gating into thin sections, the placement of chills and dead risers on the heavy sections helps reduce the sink marks or depressions. The dead risers still must remain liquid longer than the casting so they should be insulated or topped with hot metal.
Piston castings have two areas where the increased section thickness may cause hot spots and internal shrinkage, at the pin bosses and at the point where the gates join the casting. Heavy risering does not appear to help the situation. Generally, I prefer using chills to encourage directional solidification however; this complicates the molding for a short run part. Gating and risering at the pin boss has never produced a sound casting due the large increased section thickness. I have obtained the best results by gating into the thin sections located 90o from the pin bosses. The highest number of good castings results from using a combination of small gates and low pouring temperature.
In order to prevent hot spots from forming where the gates join the casting, the gates must be thin, similar to those used for plate castings. The maximum gate thickness is approximately .6 the plate thickness. The risers shown on the drawings are not intended to feed the casting, but to feed the gates so that they do not draw metal from the casting wall. Long, thin, tapered gates are somewhat difficult to make. A second and simpler scheme is to use very short risers and gate into the thin top section of the (inverted) casting. I recommend starting with this scheme.
Modern pistons are most likely cast from alloy #332 or #336, both of which are permanent mold alloys. They have a high silicon content making them very fluid. The solidification range of #332 is from 1080 to 970oF, and the solidification range of #336 is from 1050 to 1000oF. When using these alloys, the best sand cast pistons are made when the pouring temperature is approximately 100 to 120o F above the solidification temperature. Gross shrinkage is seen in piston 1 (see the image gallery). It was poured at 1350oF. Piston 2 is the same mold poured at 1200oF.
Scrap pistons may be melted for casting alloy if you first pour ingots. This removes the dirt, oil and water from the alloy that causes gas defects.
This excerpt has been reprinted with permission from Making Pistons: For Experimental and Restorative Engines, published by Chastain Publishing, 2004. Buy this book from our store: Making Pistons.