ALLEN KEY SIZES
A hex key, Allen wrench, Allen key or hex-head wrench is a tool used to drive hex screws and bolts, which have a hexagonal socket in the head.
UK Specification BS:2470
International Specifications: ANSI B18.3.2M; ISO2936; DIN911.
Dimensions are measured across flats (AF).
Hexagonal screws are commonly used to hold control knobs onto shafts. Allen keys, especially in the smaller sizes, are precision tools. Make sure that you have good quality allen keys, I bought a cheap set and found that the small sizes were out of tolerance and these can easily damage the hex screws. With the smaller sizes, it is easy to strip the head if you are not careful. Clean out the hex hole and make sure that the key is correctly seated before applying pressure. A quick squirt of WD40 oil will help release the thread.