Cranks not going on fully/enough
So after a lot of work and effort I finally got my bike built, after my second ride the left hand crank arms square tapper connector literally shredded (side note, why did this happen?) Anyway after buying a cheap replacement from halfords... I put it on to find that the gap in between the bb and the cranks was 1cm+. I understand that it's supposed to have some space, but this seems unsafe to ride and I don't want to have to splash out on a 3rd crank arm. How do I fix this? am I doing something wrong? I'll attach a picture.