Replacing a Part When the Drawing Is Gone

When an old part breaks and the drawing is long gone, the instinct is to hand someone the broken piece and ask for a copy. That is also the most common way to end up with a replacement that fits worse than the original did.

A worn part tells you its worn size, and the design size has to be worked out. A shaft that has turned in a bushing for years is smaller where it ran. A locating hole has gone oval. A face that carried the load is no longer flat. Copy those numbers and you have made a new part that is already worn. Replacing a part properly means working out what each dimension was meant to be, and then making that.

Reverse engineering is done in the shop. Between 2020 and 2025 that included work for telecom equipment makers and automotive suppliers.

Finding the design size

The part you are holding is evidence, and some of it is better evidence than the rest. Measure where the part did not wear: the shoulder beside the worn journal, the unloaded side of the hole. Then check the worn features against the part they mate with, because if the bore a shaft runs in is still good, the shaft size follows from the fit.

Old equipment also carries clues about its own units. A diameter that measures 25.37 mm on a machine built in North America decades ago was very likely one inch when it was new, and the right replacement is one inch, not 25.37 mm. Designers picked standard fits, standard threads and round numbers in whatever system they worked in. Finding that system can settle much of the drawing. On curved shapes like castings and molded housings, 3D scanning through our partner shops can give a model a starting point, but bores, threads and fits are still measured directly and set to their design size.

Repair or remake

Some general questions decide it. Is the damage in one place, or has the whole part worn? Was the part heat treated, so that welding heat would soften it? Will you need another one before the machine is retired? If the part failed because of a weakness in the original design, a faithful copy will fail the same way, and the replacement is a chance to fix that.

The same thinking applies to fixtures and jigs. They do not drift out of true on their own. Locators, bushings and clamp faces wear, or something gets dropped on them. Replacing the worn locators against the original datums can often restore a fixture without rebuilding it. We repair fixtures as well as build them.

When a mold or a tool has problems, in most cases we can fix it or recreate it without errors. The jobs vary too much to list, so bring it in and ask.

Molds are their own case. No scan or inspection tells you as much about a mold as running it, and the options are covered in injection mold repair and modification.

What to bring

• The broken part, with every piece you have
• The part it mates with, or its measurements if it cannot come off the machine
• The make, model and rough age of the machine it came from, and anything else you know about the machine or assembly
• How it failed, and how long it lasted
• The operating conditions it ran under
• Photos, and any other sample parts you have
• How many you are likely to need

What you get back

Whatever you need: a new part, a model, drawings, or all three. Often we'll also propose a different way to make the part, if it costs less or lasts longer, rather than simply remaking it as it was.

One concession before you start. If the original maker still sells the part, buy it. That will almost always cost less than reverse engineering one, and I would tell you so. Reverse engineering is for obsolete parts, for suppliers that have gone, and for parts that need to be better than the original.

If you have a part like this, bring it along with the part it fits against. The mating part often says more than the broken one.

Related services: Aluminum tooling, molding and machined parts Shop capabilities