The break-in or mechanical run-in is a standard procedure for a brand new engine, like those on the Titanic.
I think (and someone more familiar could confirm) that "run in" is the British term. It means the same thing as the Americanism "break in". To break something in is to take something brand new and use it in such a way that it reaches a higher capacity for performance. You might break in a new pair of leather boots so they fit you better and don't cause blisters when you walk, or the engines on a brand new ship.
I found "run in" in the Oxford dictionaries, confirming my theory that it's a Britishism used almost exactly the same way as the Americanism "break in" with respect to engines and mechanical things. The Brits also use "break in" the way Americans do for other (non-mechanical) things.
Yes - You say that, but [some refutation or caveat] is idiomatically commonplace for many native speakers, in response to an assertion they disagree with.
It's nearly always used in a context where although the speaker doesn't fully agree with the preceding assertion, he recognizes that there's some truth in it.
In fact, you'll often hear You could say that, but..., which more explicitly acknowledges that the assertion is at least "credible", even if it's not completely accurate. Note that in this usage, you = one, anybody, not necessarily the person being addressed (i.e. - what it amounts to is something like It wouldn't be ridiculous for you/someone to say that, but it's not entirely true).
Also note that if the personal pronoun is stressed (You say that, but...), the implied meaning shifts significantly. This form almost always occurs in contexts where the speaker completely disagrees; he intends to dismiss your opinion on the grounds that you're ignorant or biased, and/or to cite other sources more authoritative than you to refute what you said.
Best Answer
"Good thing he has you!" is slightly less formal and more colloquial than "It's a good thing he has you." Both are grammatical and equivalent in meaning. "It's a good thing that he has you" is grammatical as well, but sounds a bit stilted; it's a bit too correct, and not the sort of thing one would expect to hear from a native speaker.