Crash box
Crash box turns up in 2 fields: gears, automotive. Here is what it means to the people in each one.
Crash box in gears
Shop name for an unsynchronized manual gearbox, where shifts grind unless engine and road speed are matched.
A transmission without synchronizers, in which sliding gears or dog clutches engage directly, so the driver must match shaft speeds by double clutching or timing the shift. Heavy trucks, race gearboxes and many tractors use them because they are simple, strong and quick to shift once the skill is learned. A missed shift produces the grinding crash that gives the box its name.
"It's a crash box, so double clutch it, and don't force it into gear or you'll round the dogs off."
Crash box in automotive
A sacrificial structural component designed to absorb impact energy during a collision.
A crushable metal or composite structure mounted between the bumper beam and the main frame rails. It is engineered to deform predictably during low-speed impacts, absorbing kinetic energy to protect the chassis. In older mechanic slang, a crash box also refers to an unsynchronized manual transmission that requires double-clutching to avoid grinding gears.
"Cut the welds on the deformed crash box and prep the rail for the new replacement section."