The majority of the costs related to key parts were found in the group where the parts had high price and high but stable demand. Although the result was surprising, the root cause behind it was simple. As these materials are high movers, one stock-out in a single part generates a lot of missed sales lines and thus high costs as well. Also the stock-out costs in these cases are high as the key parts are often critical for the equipment’s operation and there are no alternative sources for the customers to purchase. In this perspective, the stock-out costs in commercial parts are on the other hand more acceptable. Although parts that had sporadic demand had the lowest service performance results, they did not cause high stock-out costs. This relates to the fact that customers are stocking these materials and their inventory control policies generate large order batches but low ordering frequencies. Although this is causing performance challenges to upstream of the distribution chain, the related stock-out costs are relatively low.