expiration of written exam endorsement?

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I know the exam results for a written exam expire after 24 months, but does the instructor endorsement to take a written exam ever expire? I can't seem to find anything that says either way, so a reference to where to find that answer would be helpful as well. Exam is private pilot airplane, if that matters.

 

[dmspilot]

Not normally, but I believe the instructor can specify an expiration date as part of the endorsement. However, you do not need the endorsement anyway because you could instead use your expired test report as authorization to take the test.

Source:
http://www.faa.gov/training_testing/testing/media/testing_matrix.pdf
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