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From movie Titanic(1997), you can find a transcript here

Ismay: So you've not yet lit the last four boilers?

Smith: No, I don't see the need. We are making excellent time.

Ismay: The press knows the size of Titanic. Now I want them to marvel at her speed. We must give them something new to print! This maiden voyage of Titanic must make headlines!

Smith: Mr. Ismay, I would prefer not to push the engines until they've been properly run in.

Ismay: Of course, I'm just a passenger. I leave it to your good offices to decide what's best. But what a glorious end to your final crossing if we were to get to New York on Tuesday night and surprise them all! Make the morning papers. Retire with a bang, eh E.J.?

Ismay: [Smith nods reluctantly] Good man.

What's the meaning of run in here? From thefreedictionary, run in can be translated into two meanings:

  1. A quarrel or an argument.
  2. Printing Matter added to a text.

Of course, neither is suitable for the context. So how to explain "run in" here?

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up vote 5 down vote accepted

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.

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About being a British term, I found a definition of run-in that is close to this usage in Macmillan, sense 2: "the period of time before an important competition". – Damkerng T. Feb 4 '14 at 15:01
@DamkerngT. I've never heard that use of it. It seems like yet another separate meaning for this phrase. It's everywhere! – hairboat Feb 4 '14 at 15:02
As a Brit, I can confirm that "run in" is the usual term for engines or anything mechanical needing a period of less-than-full-power operation to allow its components to settle and bed in. No doubt a few screws, bolts and gaskets will need checking at intervals. "Break in" would be used for shoes. There is also to "knock in" a cricket bat to prepare it for use. – toandfro Feb 4 '14 at 19:45
Americans generally say break in; run in sounds very British to me. – BobRodes Feb 4 '14 at 19:57

As they are talking about engines, the logical meaning would be that the engines would have to operate (run) for some time before they become fully capable of working at their hardest.

This was a new ship, with brand new engines. Logically, the machinist would be hesitant to push the engines to the max without any good reason. After they would have been used for some time, all flaws would be know and removed, they would be "run in", and they would be more trustworthy under full steam.

An alternative expression is "break in":


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