I just received 4 chuck keys the small end has 3/16" pilot the rest are 1/4" it is made of cast aluminum and coated with black paint. The paint makes the shafts larger so they dont fit in chuck. I sanded the paint off mine so it wouid fit. There were no sparks and its nonmagnitic. I twisted as hard as i could and it didn't break but since it is made of aluminium Im sure it will not last long but 4 for 12 bucks is hard to beat.its a cheap lite use chuck keyUpdate.All four have broke. The heads twist off after only a few uses. They are made of cast aluminum.