You actually don't have to be referring to a habit or repeated action when you use the Simple Present with "after." You can be referring to a single event in the future, as in your example: "After the engine cools down, we can start repairing it." "After the engine cools down" is a time clause (a kind of dependent clause) expressing when an action happened or will happen. You are not allowed to use future verb tenses in a time clause ("after the engine WILL cool down" is incorrect), so the only way of expressing a single future action in a time clause is with the simple present verb tense.

As long as the first sentence is referring to a habit, it's fine. "After I watch TV, I go to bed." (This is what I do every night.) "First, I have my coffee, then I eat a donut." (This what I do every morning.) The second sentence is also fine. Hope that helps :)

It is OK.