New revision or new part number? Run the interchangeability test

Mon, 09 Nov 2026 · 7 min read · Written with an AI assistant

Every released change has to answer one question: same part number with a new revision letter, or a new part number? A published NASA drawing practice gives a written interchangeability test and a firm rule for how far up the BOM the new nu

Every released change ends with a small decision that the change notice rarely spells out: does the changed part keep its number and take a new revision letter, or does it get a new part number? Get it wrong one way and a bracket with moved holes sits in the same stock bin as the old one. Get it wrong the other way and new numbers ripple through assemblies, spares lists and purchase orders that never needed to change.

Most engineers answer with “form, fit, function”. Fewer can point to a written rule behind the phrase. One exists in the public domain, and it is specific enough to work from.

The decision hidden inside every ECN

A revision letter tells the reader: same item, updated document. A new part number tells stores, suppliers and field service: these are not the same, do not mix them.

The CAD file does not care. SOLIDWORKS will rebuild the bracket whichever number sits in the custom properties. The choice matters for the physical parts that already exist: stock on the shelf, work in progress at a supplier, and units in service that will one day need a spare. The question is never “did the model change?” It is “can an old part and a new part be swapped without anyone noticing?”

The written rule I will use is NASA’s KSC-GP-435 Volume I, Revision H, Engineering Drawing Practices, Aerospace and Ground Support Equipment, approved by the KSC Engineering Directorate on 11 August 2020, with a basic issue dating from 1 August 1973. It is marked as not export controlled. It also requires suppliers to assign a discrete part identification number to each configuration item and its subordinate parts in accordance with ASME Y14.100.

Treat it as one published practice, not a universal law. Your company procedure and your customer contracts govern. Y14.100 itself is paywalled, so I will not paraphrase it here.

The interchangeability test, written down

KSC-GP-435 §6.5 requires a part-number change when any of five non-interchangeable conditions applies. §6.6 says the number does not change when a new use is found for an existing part, when none of the §6.5 conditions apply, or when a commercial, vendor, subcontractor or government-furnished item is used without alteration or selection. As a checklist:

New part number if ANY is true (KSC-GP-435 §6.5):
[ ] 1 Safety, performance or durability affected so much that old items
      must be discarded
[ ] 2 Old and new not directly and completely interchangeable in
      installation and specified performance
[ ] 3 Old items limited to specific units of an end item; new items are not
[ ] 4 An item documented by another design activity is altered or selected
[ ] 5 A material, process or protective-treatment change causes 1-4

Keep the number (§6.6) if none apply, if it is only a new use,
or if a bought-out item is used unaltered.

Three hypothetical changes show how it reads in practice.

Moved holes. A motor bracket’s two mounting holes move to suit a revised base plate. The new bracket will not bolt to the old plate, and the old bracket will not bolt to the new one. Condition 2 applies: not interchangeable in installation. New part number.

Changed coating. Purchasing proposes a cheaper finish on the same bracket. Geometry is identical, so it looks like a revision. But if the new finish cannot meet the specified corrosion life, condition 5 applies through condition 2: a protective-treatment change that breaks specified performance. The test is against the specified performance, not against whether the part looks different.

A drawing correction. A misspelled note is fixed and a missing section view added. Nothing about the delivered part changes. None of the conditions apply, so it stays a revision letter. KSC-GP-435 classifies changes as Major, Minor or Administrative: Major affects function, performance or interchangeability so the acquirer must act; Minor has no such effect; Administrative is a documentation correction with no effect on the delivered item. This one is Administrative.

Condition 4 catches a quieter case. Drill an extra hole in a catalogue bearing housing and it is no longer the vendor’s item. It needs your own number.

How far up the BOM it goes

Once the bracket gets a new number, the assemblies above it may too. KSC-GP-435 §6.4 says the number changes for the non-interchangeable item, its next higher assembly and each higher assembly, but only up to and including the assembly where interchangeability is re-established. Part numbers “shall not be changed above the level where interchangeability is re-established for any reason.”

Take this hypothetical E-BOM:

EA-1000   Conveyor (end item)
  SA-2100   Drive mount subassembly
    P-3150    Motor bracket   holes moved   -> new PN
    P-3151    Base plate      holes moved   -> new PN
  SA-2200   Guard subassembly               unchanged

Bracket and plate change together, but SA-2100 still bolts into EA-1000 through the same four frame holes. Interchangeability is re-established at SA-2100. So P-3150, P-3151 and SA-2100 get new numbers. EA-1000 keeps its number.

Apply the rule too little and SA-2100 keeps its number while containing parts that do not fit the old ones. Stores now hold two physically different subassemblies under one number, and a field technician ordering a spare bracket against SA-2100 gets the wrong one. Apply it too much and EA-1000 is renumbered: the M-BOM, routings, service manuals and spares lists all churn for no physical reason. The upper bound is as important as the lower one.

Superseding cleanly, and the early-life window

When a new number replaces an old one, the two drawings should point at each other. An NDIA presentation summarising the older ASME Y14.35M edition describes the new drawing carrying “REPLACES WITH CHANGE DRAWING” with the old number and revision, the retained old drawing carrying “REPLACED WITH CHANGE BY DRAWING” with the new ones, and any post-release change recorded in the Revision History Block. That is the older edition. Check the current one before copying the wording.

Editions do differ. The same deck says revision letters “shall not exceed two characters”, while a May 2014 Y14.35 draft allows up to three alpha characters. ASME now lists the standard as Revision of Engineering Product Definition Datasets and Associated Documents, formerly Revision of Engineering Drawings and Associated Documents. Know which edition your procedure cites.

Old parts are not always scrap. Under §6.5(6), a part reworked in production or retrofitted with a kit into a later version, and completely interchangeable with everything under the later number, is re-identified to that number. The rework instruction should say so explicitly, or the reworked parts keep the old label.

There is one window where the letter alone suffices. §6.2 allows changes by drawing change-letter control without a part-number change up to the cut-off for incorporating changes on the first unit of a configuration item, provided they take effect from serial number one. After that, the part-number rules apply as well. The document’s decision tree (Figure 7) warns that this “YES” path is valid only early in the parts’ life cycle and needs rigorous control at the enterprise and affected suppliers “to ensure there are no escapes.” An escape here is a prototype-era part with the old geometry and the same number, sitting in a supplier’s bin.

What this means for PDM and CAD tooling

PDM does not make this decision for you. SOLIDWORKS PDM’s admin help is clear that a revision number is distinct from a version. A revision scheme will happily step from A to B on a part that should have become a new number.

So make the answer a required field on the change record, signed by a named approver:

Decision: revision only, or new part number.
Reason: the clause, for example “§6.5(2), mounting holes moved” or “§6.6, documentation only”.
Re-establishment level: the highest assembly that gets a new number, and why the one above it does not.
Disposition of old stock: use as is, rework to new number, or scrap.

Watch administrator access as well. §5.5 lets a system administrator change PDM data without advancing the revision only for bulk changes such as template updates or distribution statements, and those must not affect released configurations in ways touching form, fit or function.

Tooling can help with evidence. A CAD-aware check can flag that an interface feature, such as a mating face or hole pattern, moved between releases, and list the parents in the E-BOM. AI-CAD, the engineering copilot we are building for SOLIDWORKS, is still a prototype; its design has an engineer approve every decision, and this is one where the signature should sit with a person who knows what is on the shelf.

The interchangeability test is short enough to print on a card. The hard parts are applying it honestly to specified performance, stopping at the right level of the BOM, and recording who decided. If your shop uses a different rule, we would like to hear how it handles the upper bound. Follow the journal for more on change control and release.