Repairing Cracked Cast Iron Cylinder Heads

With just average welding skills, cracked cast iron cylinder heads may have more life left in them than you thought.

If you can weld steel, you should be able to repair a cracked cast iron cylinder head following these steps:

Clean the Cylinder Head Carefully

Because cast iron tends to soil easily from oil absorption, you’ll need to take proper care that it’s clean and dry before starting the repair process.  Steam cleaning the metal, then using a degreaser is ideal for removing grease and oil.  If you can, have an engine machine shop “cook” the head in a special degreasing vat designed for this purpose.  Whichever method you use to clean it, you need to ensure it is good and dry to avoid rusting.  Due to the tendency for cast iron to rust quickly, you should dry and begin the welding repair immediately.

Stop the Crack from Getting Bigger

Before you can begin repairing, you need to make sure the crack does not get any larger.  If you can see the crack just by looking at the cylinder head,  take a drill and create a small hole at the start of the crack.  Then, from this starting hole, use a small die grinder to grind a small channel next to the crack to uncover fresh metal to help you weld and repair the part.

If you don’t see visible cracks, you may find the most cast iron cylinder heads get cracks near the water jacket areas or valve seats.  If necessary, use a dye test to expose the crack so you know where to make your channel with the grinder.  Purchase a can of dye penetrant from a car parts store or welding supply store, and spray it where you think the crack is.  Wipe of the cast iron cylinder head and all of the dye will wipe off except for what has gone into the cracked iron itself.  This will show you where you need to repair the part.

Heat the Cast Iron Cylinder Head

Heat the metal to between 170 to 200 degrees Fahrenheit.  This will drive away moisture to prevent rusting, and relieve stress from the metal.

Begin Welding to Repair the Crack

Selecting the appropriate size and type of welding electrode is important to successfully repairing cracked cast iron cylinder heads.  Most people find a very small diameter rod works best, and you should look for one that is meant for use on cast iron that has been soaked in oil.  Most cracks in cast iron cylinder heads can be filled in one go.  Once you’ve passed over the crack once, chip off the slag and then peen.  Use a chipping hammer to tap the surface and relieve stress from the iron.  It’s a process that ensures the end result is smooth.  Relieve stress by allowing the part to cool slowly.  You can cover it with blankets designed for this purpose which help pull the heat away slowly to avoid stress.

If you have never welded cast iron before, you may want to practice first.  Get a scrap piece of cast iron, or ideally, a cast iron cylinder head that is beyond repair,  to make sure you don’t damage the good one you’re attempting to repair.

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