What is the proper orifice size for a differential tester?

Thanks for your question.  The proper size of the orifice depends on the size of the engine being tested.  Specifications for the orifice are outlined in the current version of Advisory Circular 43.13-1B in paragraph 8-14 b. 
For your reference:
For an engine cylinder having less than a 5.00-inch bore; 0.040-inch orifice diameter; .250 inch long; and a 60-degree approach angle.

For an engine cylinder with 5.00 inch bore and over: 0.060 inch orifice diameter, .250 inch long, 60 degree approach angle.
 The A.C. also states the indicated failure should be at a reading of 60/80


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