Why do I get this undesired vertical space between “chapter #” and Chapter Heading?

Question

In my document, one of my chapter title lasts more than one line, so I want to increase the space between its lines. To do this, I used \linespread{1.8}\selectfont in the code below, but by using that, the space between "Chapter #" and the Chapter Heading increases, too. How can I avoid this latter undesired space? By the way, I'd prefer not to use additional packages like titlesec or setspace.

\documentclass{book}
\makeatletter
\def\@makechapterhead#1{%
  \vspace*{50\p@}%
  {\parindent \z@ \raggedright \normalfont
    \ifnum \c@secnumdepth >\m@ne
      \if@mainmatter
        \huge\bfseries \@chapapp\space \thechapter
        \par\nobreak
        \vskip 20\p@
      \fi
    \fi
    \interlinepenalty\@M
    \Huge \bfseries \linespread{2}\selectfont #1\par\nobreak
    \vskip 40\p@
  }}
\makeatother
\begin{document}
\chapter{Chapter Heading which is very very very long}
Some text ...
‎\end{document}‎

Answer 1

It doesn't! In your code you have a vskip20\p@ and this remains constant. (See the below minimal, where I zeroed it.

\documentclass{book}
\makeatletter
\def\@makechapterhead#1{%
  \vspace*{50\p@}%
  {\parindent \z@ \raggedright \normalfont
    \ifnum \c@secnumdepth >\m@ne
      \if@mainmatter
        \huge\bfseries \@chapapp\space \fbox{\thechapter}
        \par\nobreak
        \vskip 0\p@
      \fi
    \fi
    \interlinepenalty\@M
    \fbox{\parbox{\textwidth}{\Huge \bfseries \linespread{1.2}\selectfont #1\par\nobreak}}
    \vskip 40\p@
  }}
\makeatother
\begin{document}
\chapter{Chapter Heading which is very very Hery long}
Some text ...
‎\end{document}‎

Also don't forget to modify the star version of the command using \@makeschapterhead