Air Shutoff Valve
If you know a little bit about diesel engines then you've probably heard a story about one that "ran away". Take a few minutes to open up YouTube and watch a few "diesel runaway" videos.
Scary isn't it? A few things can cause a diesel engine to run away (injection pump issues, stuck fuel rail, too much ether sprayed in trying to crank it, and lastly a blown turbo inlet seal). On a Cummins 6BT the only thing I worry about is the last scenario, a blown turbo inlet seal. This would cause oil to pour into the intake and get burned by the engine. The addition of the oil from the turbo seal would cause the engine to speed up. You grab the shutdown lever and it keeps screaming higher and higher.
The only way to shutoff this run away is to cap off the turbo to starve the engine of oxygen. Good luck with that. What socket size was that hose clamp holding the rubber elbow on the turbo inlet? Where's my ladder? BOOM!
To prevent this from happening comes a little piece of hardware from wood working class. For a whopping $7 I purchased a 4 inch blast gate that people use to divert a vacuum system in a wood working shop for their equipment. Go to Tractor Supply and buy a pair of springs to pull the gate shut. Order a safety pin from Mcmaster Carr. Remove the thumbscrew on the blast gate and drill the hole larger to accept the 3/16" safety pin. Connect the springs to the blast gate body bolts. Tada! The last thing to do (not shown in these pictures) is
to connect the safety pin to a pull cable from the dash. For that I purchased a choke cable from Tractor Supply. I just have yet to run it.