SDRs are a mechanics’ report on maintenance problems. These are collected by the FAA and aircraft manufacturer and passed on to maintenance facilities as ADs (Airworthiness Directive) or as a maintenance suggestion. SDRs make up about 10% of all reported problems.
The most common items reported are those under the greatest stress. Exhaust valves, crankshaft weights, and valve train problems are most common. Cylinder reports relate to cracks at exhaust port area. Magneto problems relate to failed coils. Certain engine models are subject to specific reports related to inherent weakness such as case cracking in Continentals. Lycoming has engines with valve wobble problems. If ever you own an airplane, get an SDR listing for it.