How to read an RV parts diagram

What do the numbers on a parts diagram mean?

A parts diagram has two different kinds of number. The small number pointing at the drawing is the item or callout number — it only means "the ninth thing in this list" and is unique to that one document. The longer number in the table beside it is the actual part number you order. Item numbers are not interchangeable between models.

Two numbers, two jobs

An exploded diagram shows the appliance pulled apart, with a small number pointing at each component. That number is the item number, sometimes called the callout. Beside the drawing is a table: item number, part number, description.

The item number is a pointer into that one document. The part number is the thing you order. Confusing them is the most common way to end up with the wrong component, because item 9 on one parts list is a completely different part from item 9 on another.

Item 9 means two different parts

  • Aqua-Magic V, Rev E — item 9 is part 31708
  • Aqua-Magic IV Pedal Flush, Rev D — item 9 is part 33027
  • Same position in the assembly. Different appliance. Different number.

See both, with the documents

Using the item number properly

The item number is genuinely useful once you have the right document. If you can see the failed component in the drawing, the callout gives you the row in the table, and the row gives you the part number.

Where we can read an item number out of the manufacturer’s own parts list, we put it on the part page. It is the fastest way to confirm you are looking at the same component the document is describing.

How a callout number relates to a part numberA diagram callout labelled 9 points at a component in a drawing. The same number 9 appears in the parts table beside part number 31708, blade seal. The callout identifies the row; the part number is what you order.On the drawing9same numberIn the parts tableITEMPART NUMBERDESCRIPTION731705Water module931708Blade seal1031713Bolt capOrder the part number. The item number only means "row 9 of this list".
The callout number identifies a row in one specific document. Item 9 on a different appliance’s parts list is a different part entirely.

Watch the revision

Parts lists are republished, and the contents change between revisions. A number can be on one revision of a model’s list and gone from the next, which means an old diagram can point you confidently at a part the manufacturer no longer lists for your appliance.

Check the revision on the document you are reading. Where we state that a part fits, we resolve it against the newest revision we hold, and we say when an earlier revision listed something the current one does not.

A real revision change

  • Thetford 31708 appears on the Aqua-Magic Residence parts list at Rev C.
  • By Rev D it is gone.
  • We do not claim it fits a Residence, because the current document does not say so.

Look up your part

Search a part number, an appliance model, or the component that failed. We check the manufacturer’s own parts lists first.

Open the parts finder

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Last checked 2026-08-30.