P PT. Astra Honda Motor (AHM) is a motorcycle manufacturing company that produces models such as the Legenda, Supra, Tiger, Mega Pro, GL Max, Kirana, and Karisma. Amidst intense competition from numerous new market entrants, PT. AHM is compelled to enhance its corporate performance while simultaneously improving the quality of its products. Recently, however, the company has faced a serious challenge: a high volume of customer claims resulting from product defects. This indicates a lack of sufficient product quality, leading to customer dissatisfaction and subsequent claims. To address this, PT. AHM must re-evaluate its quality control methodology to ensure that products are manufactured with minimal—or ideally, zero—defects. This study focuses on implementing a quality control approach designed to minimize defects by utilizing the Six Sigma methodology, specifically the DMAIC (Define, Measure, Analyze, Improve, and Control) framework. In the "Define" phase, "Critical to Quality" (CTQ) parameters—identified as the root causes of defect-related claims based on historical company data from January 2003 to May 2003—were established. These include: bent valves, crankshaft run-out issues, porous cylinder heads, connecting rod clearance issues, faulty stator components, damaged crankshaft threads, torn oil seals (30x42x4.5), jammed intake tappet cap threads, and oval-shaped valve guide seats. From this list, three critical CTQs—which account for 80% of the total defect-related claims—were prioritized for immediate resolution: crankshaft run-out, connecting rod clearance issues, and damaged crankshaft threads. Following this, a measurement phase was conducted for the three identified issues, using samples collected between June 1, 2003, and June 30, 2003. The process capability for the crankshaft component was determined to be 0.9848, with a sigma level of 4.06. The next phase, Analyze, involved brainstorming sessions with managers and QCO staff to develop an Ishikawa (fishbone) diagram. Subsequently, a questionnaire based on this diagram was distributed to operators and employees working on the production floor. The results from the questionnaire were then subjected to a "5-Whys" analysis to identify the root causes of the defects—specifically, crankshaft run-out, connecting rod clearance issues, and damaged crankshaft threads. The identified root causes included inadequate scrap-clearing methods, ineffective tool-replacement procedures, a lack of rigorous application of TPA (Total Productive Maintenance/Process Assurance) methods, and unsuitable storage containers for the crankshafts. Improvements (the Improve phase) were then implemented using the "5W-2H" method. During the Control phase, a follow-up sampling was conducted from October 26, 2003, to October 31, 2003. The process capability was found to be 0.9977—an increase of 0.0129—while the sigma level reached 4.21, representing an improvement of 0.15 sigma. A test for the difference between two proportions confirmed a reduction in the proportion of defects following the implementation.