Is it idiomatic to say "Oh, this Lego is of very poor quality. This red brick doesn't tightly fit in the yellow one"?

Question

Do we have a common or general verb to express a Lego brick tightly fits in another brick in such a way that it won't easily fall apart after we put it together, for example, "Oh, this Lego is of very poor quality. This red brick doesn't tightly fit in the yellow one"?

We often say "I can't fit in the children's playhouse"

But for Lego bricks, I am not sure if "something tightly fits in something else" is the right structure that can be used in this case? Some people suggest to use the noun "flush fitting" or the verb "to snap", but I am not so sure.

There are some faulty Lego bricks which are a bit loose, we can put a Lego house together with the faulty bricks but when we hold the house, it may fall apart (i.e, some of the bricks may fall out of the others) (note: I am not sure "fall apart" is the right word?).

Answer 1

First of all, "flush fitting" refers to two items that go together, leaving a smooth surface, without a step from one to the other. That doesn't fit the situation you're talking about.

When you say two pieces "snap" together, it means that when you try to connect them, they don't move at first, but when the force applied to them increases beyond a threshold, they suddenly move into place, and a sound may be produced, called a "snap". When parts "snap" together, a similarly strong force is needed to separate them. If you say

This brick snaps into the other one.

that suggests that it won't be loose.

If you say "This brick won't snap into the other one." that may mean it won't go in at all, or that it goes in, but is still loose.

The verb phrase "fall apart" fits the situation you describe just right.

In the phrase near the start of you post, you use "doesn't tightly fit". While that is grammatical, a more natural expression would be "doesn't fit tight."

Answer 2

The red brick does not have a high "clutch power".

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We often say "I can't fit in the children's playhouse."

When we say this we mean that we can't fit inside the playhouse. For LEGO pieces, you would say

This red brick does not fit tightly with the yellow one. (Only parts of a piece go inside another one, not the whole piece)

About "flush fitting" and "snaps", I agree with what Jack said. It could mean that the "studs" on the red brick does not go into the yellow brick (because the "pipes" on the bottom of the yellow one are too big), or that they are too loose to "snap" together. So, "snap" could mean either too tight to fit or too loose to fit, and so it does not work for your case.

In your case, the LEGO bricks are coming off, or they are falling apart because they are a bit loose. In this case, the technically correct terminology is "clutch power".

... 'clutch power' which is "the ability of its bricks to snap together tightly while also being easy to separate, thereby readily allowing for de- and reconstruction" — How much usage can a LEGO piece take before it loses its 'clutch power'? in Bricks SE.

The key is "clutch power," which is the grip that holds one piece to another. — Lego's Market 'Clutch Power' in The Washington Post.

You could say

This red brick has lost its clutch power. Or This red brick does not have a high clutch power.

If you think your child might not understand that, or if you want to use words that are not technical, you could simply say

These bricks don't stick together firmly, they keep falling apart all the time.

Here is something from the LEGO website:

Have you ever wondered how LEGO® bricks stick together and not fall apart? ...

... bricks have a much higher “clutch power” when there are evenly spaced tubes on the inside of each brick. The tubes on the bottom interlock with the studs on top of other bricks. The studs get neatly wedged in between the tubes and the sides of every brick making them stick together firmly.