Not thin walled. Outer diameter 26.5mm, about the size of a standard impact socket. Works with original BMW wheels but will not fit my new OZ wheels. Otherwise looks good but I'll either have to buy another head or try to remove the plastic cover which defeats the purpose. Update - after removing the plastic cover it turned out to be 24mm which fitted my impeller. Luckily I don't have to buy another socket, but the product still doesn't serve its purpose.