Definition
Long bolts that pass completely through an aircraft engine crankcase, holding the two case halves together and providing the structural support for the crankshaft main bearings.
Plain English
Long bolts that go all the way through the engine case to clamp its two halves together and hold the crankshaft in place.
Context Anchor
Seen in aircraft engine maintenance, inspection notes, and discussions of engine case or cylinder attachment.
Derivation
Called 'through bolts' because each bolt passes all the way through the structure rather than threading into one side. The name describes the path the bolt takes.
Why Pilots Care
Correct installation and torque of through bolts prevents cylinder separation, crankcase cracking, or engine failure under the stresses of flight.
Analogy
A through bolt works like a long clamp built into the engine: it reaches through the parts and pulls them together from both sides.
Intuition Check
Do not read “through bolt” as just any bolt used on an aircraft. The key idea is that the bolt passes completely through the parts it is holding and clamps them together.
Example Sentence 1
During the engine overhaul, the mechanic torqued the through bolts to the manufacturer's specified value before installing the cylinders.
Example Sentence 2
During the annual inspection the technician checked the through bolts for stretch on the Lycoming engine.