Sewline Needle Threader Cartridge Replacement - SL-CARTREPLACEMENT. Sewline Quick-thru Replacement Cartridge - If used with needles larger than appropriate or with the needle incorrectly inserted, the fine thread guide pin can become damaged or broken. Knowing this may happen, Sewline have designed the Threader so that just the mechanism needs to be replaced. This is a Replacement Cartridge and it is easy to slip out the broken one and slide in the new.