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Engine

Thread repair kits for motorcycle and bicycle engine work. Helicoil-style inserts restore stripped threads in aluminium crankcases, cylinder heads, and frame bosses without drilling oversize or re-tapping to the next pitch. Kits include the tap, installation mandrel, and a pack of inserts; individual insert refills are sold separately when you've used the originals.

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Match the kit to the original thread — M5 × 0.8 and M6 × 1.0 cover most ancillary bosses (oil drain plugs, inspection covers, cable clamps), M8 × 1.25 and M9 × 1.25 suit engine mounts and torque-arm pivots, M14 × 1.25 handles spark-plug holes in four-stroke heads. Check the pitch with a gauge before ordering; motorcycles sometimes mix ISO coarse and fine threads in the same assembly, and bicycles with internal-gear hubs or mid-drive motors occasionally use metric fine where you'd expect coarse.

The tap in each kit cuts an oversize thread to accept the insert's outer coil. Wind the insert onto the mandrel, screw it into the tapped hole until the top coil sits below flush, then snap off the drive tang with a punch. The rebuilt thread matches the original fastener spec — you don't need a longer bolt or different torque value. Inserts work in magnesium and cast iron as well as aluminium, but the parent metal must be thick enough to take the oversized tap without breaking through; crankcases and head castings are usually fine, pressed-steel bicycle frames are not.

Keep a master kit in the van for the common sizes. Individual insert packs let you restock the size you've just used without buying a complete kit each time. If the stripped hole is in a high-load location — swingarm pivot, cylinder-base stud — consider whether a solid bushing or oversize helicoil (next thread size up, sleeved back down) gives you more purchase in tired alloy.