No, actually use a $20 bill!

Why did this mechanic put a twenty dollar bill inside an engine?! 💸🔧
If your car engine has a loud, rapid "ticking" sound while it runs, your valves might be out of adjustment! Inside the cylinder head, a metal rocker arm pushes down on the valve to let air and fuel into the engine. If the gap between those two metal pieces is too loose, they will violently slap against each other, creating that loud ticking noise! 
To fix it, mechanics have to adjust the "valve lash." Normally, they use a precision steel tool called a feeler gauge to measure the exact gap required. However, in a pinch, an old-school mechanic trick is to use a crisp US dollar bill! US currency is printed at a very specific thickness (roughly 0.0043 inches). By sliding the bill between the rocker arm and the valve, they can dial in the perfect tension so the engine runs silently!