Add space below equations? \belowdisplayskip is not working?

Add space below equations? \belowdisplayskip is not working?

I use the following code in my LaTex document to specify the space before
and after the align-environment (code is before the \begin{document}):
\makeatletter
\g@addto@macro\normalsize{%
\setlength\abovedisplayskip{0.0cm}
\setlength\belowdisplayskip{0.8cm}
\setlength\abovedisplayshortskip{0.0cm}
\setlength\belowdisplayshortskip{0.8cm}
}
\makeatother
Unfortunately this is not changing anything?
If I increase the numbers to
\makeatletter
\g@addto@macro\normalsize{%
\setlength\abovedisplayskip{1.0cm}
\setlength\belowdisplayskip{1.5cm}
\setlength\abovedisplayshortskip{1.0cm}
\setlength\belowdisplayshortskip{1.5cm}
}
\makeatother
I get the error message
! Improper \halign inside $$'s.
<recently read> \halign
l.138 \end{align}