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ENGINEERING RESEARCH

Before you order: use a BOM diff to re-quote only the changed lines

When a BOM changes from rev A to rev B, compare the two revisions, find added, removed and changed lines, and send only that delta to purchasing.

2026-10-28Updated 2026-10-284 min read
Concept illustration of engineering references and component drawings
Concept illustration of engineering research and references.
Concept illustration linking a process diagram, connection graph and equipment assembly
Concept illustration: process diagram → connections → equipment assembly.

The problem: a whole BOM re-quoted for a few changed lines

A design revision often changes only a few lines in a bill of materials. If the new revision goes to purchasing as a full BOM, every line can end up being re-quoted. Suppliers get questions about parts that did not change. Buyers have to work out which lines matter. The lines that actually changed are mixed in with all the others, and they are the ones most likely to be missed.

A BOM diff turns “here is rev B” into “here is what changed since rev A.”

Step 1: Export both revisions in the same structure

Export revision A and revision B from your CAD, PDM or spreadsheet as flat tables with the same columns. At least include:

  • Part number, which is the key used to match lines
  • Quantity
  • Description
  • Material

Clean up the part numbers before you compare. Extra spaces, letter case, or a letter O typed as a zero can make one part look like two.

Step 2: Compare A and B line by line

Match the lines by part number and sort each one into a group:

Status Meaning What to do with it
Added The part number is only in rev B New quote needed
Removed The part number is only in rev A Cancel or reduce open orders; check stock
Changed The part is in both revisions, but the quantity, material or description differs Re-quote if the quantity or material changed; check description edits
Unchanged All compared fields match Keep the existing quote or order

Look at the quantity change (B minus A) as well as the new quantity. A supplier needs to know whether it is quoting 10 more parts or a new total of 50.

A part that shows up as both removed and added is often a renumbered part or a typing difference. Check it before you send it out.

Step 3: Send only the delta

Build the purchasing package from the Added and Changed lines, plus the Removed lines that affect open orders. For each line, include:

  • the part number, description and material
  • the old quantity, the new quantity and the change
  • the revision reference (A to B) and the drawing or ECO number behind the change

Unchanged lines stay out of the package unless the supplier needs them for context.

Step 4: Review before it goes out

  1. Check the diff against the change record. Every Added, Removed and Changed line should trace back to the drawing change or ECO. A line that does not trace back is either an unrecorded change or an export error.
  2. Check material changes with the engineer who owns the part. A new material can mean a different supplier or a different lead time.
  3. Check substitutions by hand. A part swapped for an equivalent shows up as one removal and one addition. A diff does not approve substitutions.
  4. Check that nothing outside the table changed. A flat BOM comparison does not compare geometry or assembly hierarchy.
  5. Record what was sent, to whom, and against which revision.

What teams get from this

  • Purchasing works on a short, explicit list instead of a full BOM.
  • Suppliers are asked only about what changed.
  • Engineering and purchasing see the same view of the revision.
  • Removed parts are noticed before they are ordered.

Tools you can use

MST has two free browser tools for this workflow:

  • BOM Compare loads a “Before” and an “After” BOM as UTF-8 CSV or TSV files (up to 4 MB each), or as pasted text. It matches parts by part number, marks each one Added, Removed, Changed or Unchanged, shows the quantity change, and exports the comparison as CSV. For parts in both BOMs, it compares quantity, description and material.
  • Drawing to BOM reads the parts table printed on a PNG, JPEG or PDF drawing and returns editable rows that you check and correct before exporting to Excel or CSV. This is useful when one revision exists only as a drawing.

Both tools say what they do not do. BOM Compare compares flat tables only, not CAD geometry or hierarchy. Drawing to BOM output is OCR and must be checked row by row.

How to cite this article

MST Engineering Intelligence. “Before you order: use a BOM diff to re-quote only the changed lines.” MST Engineering Intelligence. Updated 2026-10-28. https://mst-us.ai/bom-diff-requote-changed-lines/

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