Hear is a great example for needing an accurate straight edge. Use it to determine the play in your timing chain. Place the straight edge against the chain at both gears. Use a machinist rule to push in the chain from the straight edge. The rule will show your measurement and you can determine if is reusuable during the teardown process.
A piece of metal machined to be precisely straight within a thousandth of an inch for the length of the tool on all sides.
A long, solid straight edge tool can be use for various purposes.