How to Repair Cracked Cast Iron Cylinder Head

by eHow ContributorUpdated July 05, 2023
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Items you will need

  • Degrease

  • Cast Iron Welding rods

  • Slag hammer

  • Temperature sticks

  • Die grinder

  • Drill

Cast Iron Cylinder heads can be repaired with average welding skills by following a few basic principles of welding cast iron

Repairing cast iron cracks are straight forward and any one who can weld mild steel can cast iron by following a standard procedures. As an engineer I have had to repair many machines over the years this way.

The first step is to clean the metal. Cast iron is notorious for absorbing oil. Steam clean is a good idea, followed by a degreaser. Gunk degreaser and brake and carburetor cleaner type solvents work well too. If possible having the head "cooked" in a degreasing vat is the best choice. Most engine machine shops have this ability. Once the head is clean make sure it is dry for rust will set in quickly. In fact because of rust and moisture the weld repair should begin within the hour for best results.

If the crack is a stress or thermal crack it needs to be stopped from growing before a repair can be made. If the crack is visible than drill a small hole at the end of the crack. Using a small die grinder or Dremel tool dress the crack. This means grinding out a small channel along the crack. this does several things, reveals fresh metal for welding, prepares a "trough" for the weld metal to go into, and removes any carbon buildup along the crack boundaries ( We will forgo using the fancy term for that).

A lot of cylinder head cracks occur from overheating and are located in the water jacket ares or occur at the valve seats. The water jacket cracks can be hard to see sometimes. If where to grind the crack is difficult to see than using a dye penetrant test is recommended. A can of dye penetrant can be bought at any welding supply house. Spray the dye along the crack and then wipe off the area sprayed. Only the dye that soaked into the crack will remain. The next step is to apply developer, again a spray can that can be bought but a can of spray foot powder works well to. Spray an even thin coat over the area. The white powder will dry revealing a red/pink line showing where the crack is. When done inspecting clean the area good with degreaser. Again the welding shop sells a cleaner. If you might do a lot weld repairs having a set of dye penetrant cans is recommended. It will save time.

If by now the crack is clean, ground, and drilled the welding preparation can begin. First cast iron like used in cylinder heads, gray or ductile likes to be hot. This drives off moisture and relieves stress. Heating the metal to 170-200F is best. Be sure to heat long enough to allow the heat to really soak in.

Ductile Definition

Ductile refers to a material's ability to deform under tensile stress. This ability is important for automotive components because it allows them to withstand high levels of force without breaking.

A key ingredient is the selection of the correct welding electrode. A small diameter rod is best. My favorite for 25 years has been Eutectic's Eutectrode Xuper 2240. This rod is designed for oil soaked cast iron. It has good strengths and is forgiving. the exact instructions which may vary depending on the welding machine used are available form a welding supply house or . Fundamentally the electrode will be ran cold. Most cracks can be filled in one pass. After the root pass , chip the slag off and then using the chipping hammer peen the weld. Work the surface to relieve stress. If welding on an engine block this step must be performed after each pass. THIS IS THE SECRET OF WELDING CAST IRON..PEEN!. Let the part slow cool, covering with weld blankets or anything that will retard the heat. This step is important to again permit stress relieve. It actually affects the grain growth of the metal. Another reason the casting was preheated. With the Euctectic-Castolin rod the finished weld will be strong and after dressing grinding will probably color match the head nicely. If this is your first attempt practice on a scrap cylinder head first. Make sure your rods are dry, heat them in an oven to 150 for an hour if in doubt. Use while hot. One pound of electrodes is a lot. Be aware that the electrodes may cost up to $30 dollars a pound. You are paying for a professionally engineered product. There are other brands of cast iron rods. Some I have had good results with, some I haven't. Stick with major brand names not repackaged rods. A 1/4 pound of rods will repair a lot of cracks. If you think using a 7018 electrode will work, well it may look okay when done but you have just made a boat anchor. The weld will crack out the first time stressed from a little effect known as carbon migration.


Remember welding is a microscopic process that works with the molecules. A bad looking weld probably is but a good looking weld may be bad too. So follow the procedures.


Make sure to wear your personal protective gear, especially safety glasses

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