With FAA rules (see FAR 61), you can take all written tests any time before start of flight training.
This applies to PPL, CPL and IR.
Passing grade is 70% or better.
Written exams remain valid for 2 years.
Should you not complete the flight check within 2 years, you have to complete the written again.
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There is even a FAA licence (certificate) you can obtain without an oral/practical exam.
The FAA Ground Instructor Certificate.
You can take the written Basic Ground Instructor.
Then take the Instrument Ground Instructor.
With the above, you can be issued a Ground Instructor Certificate.
It permits you to give required FAA classroom instruction.
Included, you can be a simulator/ground training device instructor and train pilots for the IR...
No medical certificate required.
Ground Instructor Certificates are permanent, no expiration dates.
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If you hold a Ground Instructor Certificate, you are NOT required to pass the Fundamentals of Instruction again, which is part of the the written CFI Flight Instructor exam.
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Happy contrails