The black cap length is much shorter than the original silver cap. That means when you place the cap
on, it doesn't sit deep enough into the pen (it almost just rests on top) and it easily falls off.
If you go to take the pen out of your pocket, if the wax is partially on the cap, or if the
wax+thread retracts quickly, there's a risk of the cap coming off.
I'm not sure why he
would make the black cap different dimensions from the original. I got a replacement thinking it was
defective but the replacement was the same. Penguin customer service was excellent, but
unfortunately this product is basically unusable due to this issue.