The most common complaint from sewing machine users is faulty tensions. As a mechanic, it is essential that you remember how the parts of a typical tension assembly work together. Cleaning the tension discs is and every service situation, but replacing tension parts can be tricky. Suggestion: take digital photos of the assembly before you start. When you have the parts disassembled, take another pic. This way even if you forget, you have a photographic map for reassembly.