In-house, a revision change is a conversation. Outsourced, it is a transaction — and treating it as a conversation is how mixed-revision stock happens.
When engineering and production share a building, revisions propagate by proximity: someone walks over, the change is understood, the next build reflects it. Move production out of the building and that mechanism disappears. What is left is whatever your change process actually says on paper.
A workable process has four parts. Changes are proposed in writing with a reason. They are quoted, because a change costs something even when the part gets cheaper. They are released with an effectivity — a serial number or a date — so both sides know which units carry it. And the old revision is formally closed, so it cannot quietly reappear from a shelf of pre-built subassemblies.
The failure mode is not a supplier ignoring a change. It is a change arriving informally — an email, a marked-up PDF, a comment in a call — and being implemented on some units but not others. Six months later, two units that share a part number behave differently and nobody can say which is correct.
We build only against a released revision, and we say so even when it is inconvenient. If a change is urgent, we quote it fast and release it fast. What we do not do is absorb it silently, because a silent change is a debt that comes due at the worst possible moment, usually in your customer's plant.
— GANI Manufacturing engineering team