Copying a part is easy. Reproducing what it does in service — including the properties the original drawing never recorded — is the actual job.
Start with the failed part, not the good one
The instinct is to measure the pristine spare. The more informative object is the one that failed. Wear patterns show where the load actually went, which is often not where the designer intended. Fretting at a mounting face indicates movement that shouldn't be there. A fatigue crack origin points to a stress concentration worth designing out. Measure the good part for dimensions; read the failed one for requirements.
Measure the fits, not just the sizes
A dimension without a tolerance is a suggestion. What matters is how the part relates to the things it touches — the interference on a press fit, the clearance on a running fit, the seal squeeze. Measuring the mating components is often more valuable than measuring the part itself, because the mating hardware defines the envelope the replacement has to live in.
Identify the material properly
Guessing from appearance is where reverse engineering most often goes wrong. Hardness testing, and chemical analysis where the application warrants it, cost little against the price of a part that fails early. Surface treatment matters as much as base material: a nitrided surface on a wear face is not optional detail, it is the reason the part lasted.
Decide deliberately between copy and improve
If a part failed prematurely, reproducing it exactly reproduces the failure on the same schedule. Once the failure mode is understood, an improvement is often trivial — a larger radius, a harder surface, a better seal compound. The right output is usually two quotations: like-for-like, and corrected, with the difference explained.
Document it so the second order is easy
The value of reverse engineering compounds only if the drawing survives. A released, revision-controlled drawing pack turns a four-week engineering exercise into a two-week repeat purchase forever after. That is the real return, and it is why we retain the pack for clients rather than treating each order as new.