Толщина стенок разная.

Because Russian normally omits the verb to be in the present tense.

So:

• Толщина стенок разная. = The thickness of the walls is different.

But in the past or future, a form of быть appears:

• Толщина стенок была разной. = The thickness of the walls was different.
• Толщина стенок будет разной. = The thickness of the walls will be different.

This missing present-tense is is completely normal in Russian.

Стенок is genitive plural.

It is genitive because толщина usually takes the genitive to mean the thickness of something:

• толщина стенок = thickness of the walls
• compare:
  • длина кабеля = length of the cable
  • цвет двери = color of the door

So the structure is:

• толщина
  • genitive noun

That is why you get стенок here.

Here стенок is from стенка, not стена.

That matters because:

• стена usually means a wall in the sense of a room, building, or large wall
• стенка can mean:
  • a small wall
  • a side panel / wall of an object
  • in technical language, the wall of a container, pipe, vessel, casing, etc.

So in technical or descriptive contexts, стенка is very common for the wall of an object.

That means:

• толщина стен would more likely suggest thickness of walls in the building sense
• толщина стенок often suggests wall thickness of some object or structure

Also, стенка here is not necessarily diminutive. In technical Russian, it is often just the normal word.

Because разная agrees with толщина, not with стенок.

The main noun of the sentence is толщина:

• толщина = feminine singular

So the adjective must also be feminine singular:

• разная

In other words:

• Толщина стенок разная. = The thickness is different.

It does not mean that the walls themselves are different. If you wanted to say that, you would say:

• Стенки разные. = The walls are different.

Because Russian is talking about the property called thickness as a single idea.

So:

• толщина стенок = wall thickness / the thickness of the walls

Even though there are multiple walls, the noun толщина stays singular because it names the characteristic itself.

English does something similar in technical language:

• wall thickness
• door width
• pipe diameter

If you wanted to emphasize several separate thickness values, Russian could use a plural noun, but that is not the normal choice here.

Here разная means not the same.

Depending on context, natural English translations could be:

• different
• varied
• uneven
• varies

For example, it might mean:

• different walls have different thicknesses
• different parts of the same object have different thickness
• the thickness is not uniform

So разная does not just mean different from something else; it can also mean different from one another or non-uniform.

Because with разный, modern Russian normally uses the full form in this kind of sentence:

• Толщина стенок разная.

A short form like разна exists historically, but in modern Russian it sounds rare, bookish, or archaic.

So for a learner, the important point is:

• разная is the normal, natural form here

Yes. Russian word order is flexible, though different orders shift the emphasis.

The neutral order here is:

• Толщина стенок разная.

Other possible versions include:

• У стенок разная толщина.
  This emphasizes the walls a bit more.
• Стенки разной толщины.
  This is a different structure meaning the walls are of different thicknesses.
• Толщина стенок различается.
  More formal/technical: the wall thickness varies.

So the original sentence is natural, but not the only possible wording.

It is a complete sentence, but it has a compact, technical/descriptive style.

Russian often uses short, dense statements like this in:

• technical writing
• product descriptions
• engineering contexts
• reports

So Толщина стенок разная. is perfectly grammatical, but it also sounds like something you might read in a specification or observation.

In more expanded everyday wording, someone might say:

• У стенок разная толщина.
• Толщина стенок в разных местах отличается.

You know from context, because Russian has no articles like a or the.

So стенок by itself does not tell you whether English should use:

• walls
• the walls
• sometimes even a technical phrase like wall thickness

For example:

• in a general technical statement, English may prefer wall thickness
• if the walls were already mentioned, English may prefer the walls

So the Russian sentence leaves definiteness unstated, and the translator supplies it from context.