Metal stitching or metalocking is a method of repairing cracks in cast metals without the need of welding. Cold repairs on all cracked or broken components made from cast iron cast steel or aluminium. Breakages usually occur because of overloading, accidents, equipment misuse or flaws in the casting.

Castings which have developed cracks or which have broken from being overstressed can be strengthened in the fractured area by inlaying pre-formed, high tensile strength, alloy steel bars referred to as "locks" or "metal locks".  Locks are inserted into accurately prepared slots, which are perpendicular to and located across the crack. Due to the serrated design of the lock, the casting sections on both sides of the crack are locked together. Since the locks traverse the crack, the serrated edges of the locks are placed in shear when loads are applied which act perpendicular to the crack.

Locks are cut to any desired length from preformed bar stock that is, overall, about as high as it is wide. Lock sizes used are 1/4" to 5/16" and may be stacked.

The first step in the installation of a lock is to make a slot for it that is positioned perpendicular to and across the crack. The major portion of the metal is removed by using a drill bit size that corresponds to the lock size; for example, a 5/16" bit is used for a 5/16" lock. With the use of a drilling guide, the first hole is drilled so the centerline of the hole and the crack coincide. A temporary pin is used to hold the guide in position while the far hole on one side is drilled and also pinned. Then the remaining holes are drilled, after which the pins and drilling guide are removed.

Drilling depth must be limited to not more than 90% of the casting’s thickness in order to provide a base for the lock. Drilling depth is determined by the lock depth needed to provide the reinforcement required to match or exceed the original casting strength in the crack area.

Drill holes are normally balanced across the crack to provide equal strength on each side of the crack. The minimum number of holes drilled is 5, increasing to 7, generally alternating.

After the holes have been drilled, a web of metal remains between each hole. The webs are partially chiseled out using a pneumatic powered chisel and the metal chips are removed by hand or drilling, as necessary. Now, the slot is ready for insertion of the lock.

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