There are two distinct parts to this question. Firstly, the “well known rule”.

To quote from the Part 66 Manual of Standards, paragraph 66.A.25 (ea), a candidate applying for a licence “must have…passed the examinations for the modules mentioned…during the 10-year period before the date of the application.” (Reference: Part 66 MOS, Pages 15 to 16)

Simply put, if you apply for any licence outcome, and any of the relevant exams (as outlined in the Part 66 MOS on Page 27) were completed outside of the 10-year period, they cannot count towards your licence and will have to be retaken.

Now, the second, “not-so-well known rule”.

To quote again from the Part 66 Manual of Standards, Appendix II, paragraph 1.12, “The period mentioned in paragraph 66.A.25 (ea) for the passing of a module examination does not apply [emphasis added] if the examination has been passed by a candidate in relation to another category, or subcategory, of aircraft engineer licence held by the candidate.”. (Reference: Part 66 MOS, Page 88)

In short, if an exam has already been applied to a licence outcome, it does not expire. Consider the following two scenarios as further explanation.

Scenario 1

12 years ago, John passed CASA module exams, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 15, 16, 17 all at the B1 level. He has never held, nor applied for, a licence of any category. Now he wants to apply for a B1.1 category licence. 

Outcome 1

John will have to re-sit every exam applicable to the B1.1 category, since all of his exams have exceeded the 10-year period, and none have been applied towards a licence. (Reference: Part 66 Page 15 to 16)

Scenario 2

12 years ago, David passed all his required CASA module exams for a B1.2 licence outcome, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 16 and 17 all at the B1 level. 6 years ago, he applied for, and received, a B1.2 category licence. Now he wants to apply for a B1.1 category licence. 

Outcome 2

Because David used his Part 66 ‘Module’ results to achieve a licence outcome (his B1.2 licence), he can now apply the condition stated in the Part 66 MOS Part 3, Appendix II, para 1.12. Any ‘common’ modules that are required for both B1.1 and B1.2 licences, that contain the same subjects and knowledge level, are credited towards his modular requirement for his B1.1 licence (modules 1, 2, 3, 4, 6, 7, 8, 9, 10, 17). (Reference: Part 66 MOS, Page 88)

The only exams which he will need to resit are those that may contain subject differences, or knowledge level differences shown between the B1.2 and B1.1 module requirements (in this case, modules 5 and 11) (Reference: Part 66 MOS, Pages 28 to 87).