A technician needs to replace a cracked crankcase half on a horizontally opposed reciprocating engine. What is a critical airworthiness consideration regarding crankcase halves?
- ACrankcase halves are manufactured exclusively from magnesium alloy and never require line-boring to match
- BEither crankcase half can be freely interchanged with a half from another engine of the same model without concern
- CThe crankcase halves are machined and line-bored together as a matched set and must be serviced or replaced as a matched pair, not individually interchanged
- DOnly the upper crankcase half carries the main bearing loads, so the lower half can be replaced without matching
Show answer & explanationAnswer & explanation
Correct answer: C. The crankcase halves are machined and line-bored together as a matched set and must be serviced or replaced as a matched pair, not individually interchanged
Horizontally opposed engine crankcases are typically split into two halves that are align-bored together after being bolted together as a matched set, so the main bearing bores, cam bores, and mating surfaces fit precisely only with that specific mating half. Because of this, crankcase halves are not interchangeable individually and manufacturers require replacement or repair as a matched set.
Why the other options are wrong
- A. Crankcases are commonly made from cast aluminum or magnesium alloys, but they do require precision line-boring as a matched set.
- B. Interchanging with a half from a different engine would misalign bearing bores since each pair is uniquely line-bored together.
- D. Both crankcase halves typically share main bearing support; it isn't limited to only the upper half.
Crankcase Halves (Matched Set)
The two crankcase halves of a horizontally opposed engine are bolted together and line-bored as a single unit, so their main bearing and cam bores align only with each other and cannot be individually interchanged.
- Line-bored together after assembly
- Must be replaced/repaired as a matched pair
- Individual interchange can cause bearing misalignment
Memory trick: Crankcase halves are like two puzzle pieces cut together—swap only one and nothing fits.