As a result of my tests, I determined that these extensions are of the highest quality .Made with vanadium, they are durable and have held up to heavy usage. The 4 inch size is convenient for tight spaces and the set of 4 pieces provides ample options for various tasks. The only drawback is that they are a bit pricey compared to other options on the market. Overall a great investment for any industrial and scientific worker in need of high-functioning extensions..