Definition
A small, precisely machined pin used to position two parts in their correct relationship to each other before they are fastened together. The pin fits into matching holes in both parts, ensuring exact alignment so the assembly goes together the same way every time.
Plain English
A short metal pin that drops into matching holes on two parts so they line up perfectly when joined. It does not hold the parts together — it just makes sure they sit in the right position before the bolts or screws go in.
Context Anchor
Seen in aircraft maintenance manuals, parts installation instructions, and component assembly procedures.
Why Pilots Care
Components installed without their alignment pins seated correctly can shift slightly, leading to uneven loads, leaks at mating surfaces, or accelerated wear. On inspection, a missing or sheared alignment pin is a sign that the assembly was not done to spec.
Analogy
Like the small plastic pegs in flat-pack furniture that drop into pre-drilled holes — they don't hold anything together, but they make sure the panels line up before you tighten the screws.
Intuition Check
Do not assume an alignment pin is the main part holding the assembly together. Its main job is to position parts correctly; other hardware usually provides the holding force.
Example Sentence 1
Before torquing the cylinder to the crankcase, the technician confirmed the alignment pin was fully seated in its hole.
Example Sentence 2
During cylinder installation, alignment pins ensured the head seated properly on the engine case.