METAL-LOCKING - DEFINITION, TYPES OF REPAIRS, PROCEDURE

What is metal-locking?

Metal-locking (also called "metal stitching" or "cold repair") is a mechanical cold-repair technique used to repair cracks in cast-iron and steel components (engine blocks, cylinder heads, bedplates, housings, pump casings, etc.) without welding. It involves machining a series of interlocking keys (dovetail-shaped inserts) and locking pins across the crack, which are driven/ peened into prepared slots to mechanically stitch the two faces of the crack together, restoring strength and sealing. Because it is a cold process, it avoids the thermal distortion and residual stress of welding and is ideal for cast iron and for components that cannot be heated.

Types of repairs carried out by metal-locking

1. Repair of cracks in cast-iron engine blocks, cylinder heads, bedplates, and crankcases.
2. Repair of cracks in pump casings, valve bodies, gearbox housings and other castings.
3. Repair of cracks in cylinder liners, pistons and other engine components.
4. Sealing of cracks in pressure-containing parts (where the crack is not under extreme pressure/ temperature) to restore pressure tightness.
5. Repair of cracks in machinery foundations/ bedplates and structural castings.
6. Repair of cracks in heat-exchanger shells, boiler/ pressure parts (within limits) and other marine machinery.

Procedure for a metal-locking repair

1. Preparation: Clean the area around the crack thoroughly (degrease, remove paint/ scale). Determine the extent of the crack (dye-penetrant/ magnetic particle check) and mark the crack ends.
2. Drill stop-holes: Drill small holes at each end of the crack to prevent it from propagating further.
3. Machine the key slots: Using a special metal-locking machine (a portable milling/ slotting machine), cut a series of dovetail-shaped slots across the crack, perpendicular to it, at regular intervals along its length. The slots are cut so that they straddle the crack.
4. Fit the keys (stitches): Insert a dovetail key (a hardened steel insert of matching dovetail profile) into each slot, spanning the crack. The keys are driven/ peened into place so they are slightly proud of the surface.
5. Peen the keys: Peen (hammer) the keys and the surrounding metal to cold-work and expand them, locking them tightly into the slots and drawing the crack faces together. The keys are then ground/ filed flush with the surface.
6. Fit locking pins (optional/ for sealing): Along the crack line between the keys, drill and tap holes and fit threaded locking pins (studs) that are screwed in and peened, sealing the crack and providing additional strength. The pins are cut flush and peened.
7. Finish: Grind/ machine the repaired surface flush and smooth. The repair is then tested (e.g. pressure test, dye-penetrant) to confirm sealing and strength.
8. Record the repair and, where required, obtain surveyor/ Class approval.

Note: Metal-locking restores the component's strength and pressure tightness without welding, and is a permanent repair when carried out correctly. It is particularly valuable for cast iron where welding is difficult or would cause distortion.