Classic truck restoration often means fixing old parts. Keeping the vehicle original or doing an actual "matching numbers" kind of buildup, there are times when you must restore an existing casting or metal part. Cast-iron repairs, including housings and engine blocks, generally fall into the last-resort category, as these repairs can prove costly and risky.

Iron is often less expensive to produce, yet it is durable, readily machinable, and can be reasonably strong. Not all iron is the same. Cast iron differs in carbon content, ductility, yield, and elongation. Alloyed irons resist wear, fatigue, and heat cycling.

Automotive applications of cast iron fall into several categories: 1) gray cast iron, used for engine blocks and gear housings; 2) alloy cast iron (with higher-tensile strengths), used for cast cylinders, pistons, rings, and brake drums; and 3) pearlitic malleable iron castings, which differ in stiffness for various applications. Malleable iron is common to axle and differential housings, camshafts, and even crankshafts.

Similarly, ductile cast iron (popularly called "nodular iron") has strength and shock resistance. When we talk about a "nodular crankshaft" or Ford 9-inch "nodular" differential housing, these are high-tensile-strength yet ductile castings. Ductile iron means more elongation or "give" and less of a tendency to split or crack under high stresses. Since ductile iron can be flame or induction hardened, it is also popular for use with emission-era iron cylinder heads. Flame or induction hardening of exhaust valve seats allowed the use of unleaded fuels without the risk of rapid valve seat wear.

There are as Many Welding Techniques as Iron TypesAsk five specialists what makes the best cast-iron repair and you will likely get five answers! Repairing iron takes into account the ductility, composition, and granular structure of the base material. One type of casting may require a different welding technique or filler material than another. Of the irons, nodular types have a graphite and alloy content that is complex and not easy to duplicate during repairs. Gray iron, common to lighter-duty housings, requires preheat steps to prevent cracks during a repair. Malleable iron, such as that used in conventional crankshafts, is frequently welded in the automotive machine trade.

Since iron types have different metallurgy, any repair starts with identifying the iron involved. The kind of filler rod used in the welding process must be compatible with the iron. If not, even if the weld is successful, the weld area may not behave like the surrounding iron. One iron may be more ductile than another type--or the fill material may not behave properly. This can cause post-weld stress and cracks at the margins of the weld or "heat-affected zone" (HAZ).

A large problem when repairing cast iron is expansion and shrinkage. In some cases, the casting must be heated entirely to a uniform, high temperature before welding. There are procedures that place a casting inside an actual furnace. The piece heats gradually to red hot, and the welder makes the repair through a door in the furnace! Furnace welding helps prevent weld area shrinkage that causes cracks.

The greatest challenge in an iron repair is cracking. Extreme care must be taken to prevent cracking. Preheating is a requirement or recommendation with most cast-iron repairs. Forms of preheating include local heat with a rosebud torch, electrical resistance through the entire piece, cooking in a charcoal oven, or searing in a gas-fired furnace.