I used the code http://www.texample.net/tikz/examples/circular-arrows-text/ and modified it a little bit. The text below is HI and it works fine. When I change HI to $\pi$, the typesetting does not finish. Any suggestions?
MWE
\documentclass[tikz,border=10pt]{standalone}
\usepackage{amssymb}
\usetikzlibrary{decorations.text}
\newcommand*{\mytextstyle}{\Large\bfseries\color{black!85}}
\newcommand{\arcarrow}[8]{%
% inner radius, middle radius, outer radius, start angle,
% end angle, tip protusion angle, options, text
\pgfmathsetmacro{\rin}{#1}
\pgfmathsetmacro{\rmid}{#2}
\pgfmathsetmacro{\rout}{#3}
\pgfmathsetmacro{\astart}{#4}
\pgfmathsetmacro{\aend}{#5}
\pgfmathsetmacro{\atip}{#6}
\fill[#7] (\astart:\rin) arc (\astart:\aend:\rin)
-- (\aend+\atip:\rmid)
-- (\aend:\rout) arc (\aend:\astart:\rout)
-- (\astart+\atip:\rmid) -- cycle;
\path[decoration = {text along path, text = {|\mytextstyle|#8},
text align = {align = center}, raise = -0.5ex}, decorate]
(\astart+\atip:\rmid) arc (\astart+\atip:\aend+\atip:\rmid);
}
\begin{document}
\begin{tikzpicture} \fill[even odd rule,red!30] circle (3.8) circle (3.2);
\arcarrow{3}{3.5}{4}{-30}{30}{5}{red, draw = red!50!black, very thick}{HI}
\end{tikzpicture}
\end{document}
