Definition
A precision measuring tool used during aircraft reciprocating engine maintenance to check whether a poppet valve has stretched beyond its serviceable limits. The gauge compares the contour of the valve head and stem against a known reference shape; if the valve has elongated under heat and operating stress, it will not match the gauge and must be rejected.
Plain English
A shaped tool a mechanic places against an engine valve to see if the valve has been pulled out of shape by heat and use. If the valve doesn't fit the tool's shape, it's worn out and gets thrown away.
Context Anchor
Seen during aircraft engine inspection or maintenance, especially when checking exhaust valves for heat damage or wear.
Derivation
Stretch' here means physical elongation -- the valve has been pulled longer by repeated heating and mechanical loading. The gauge detects this stretch by comparing the valve's current shape to its original specification.
Why Pilots Care
A stretched valve is a failing valve. Catching one during inspection prevents in-flight valve failure, which can cause cylinder damage, power loss, or engine stoppage.
Analogy
It is like using a ruler to check whether a metal part has been pulled longer than it should be. If it is too long, the part may no longer be strong enough for its job.
Intuition Check
Stretch does not mean the valve is bending like rubber. Here it means the metal valve has permanently become longer from heat and stress.
Example Sentence 1
The mechanic used a valve stretch gauge on each exhaust valve before reinstalling them in the overhauled cylinders.
Example Sentence 2
A reading above the service limit on the valve stretch gauge meant the valve had to be replaced before the engine could be returned to service.