At AirCorps Library and AirCorps Aviation we geek out and obsess about part number methodology. Some WWII producers had no apparent numbering system, while others established organizational systems that unleash a wealth of knowledge simply by understanding their unique systems. North American Aviation scaled up production in under 100 days for the P-51 Mustang and also built the B-25, AT-6, manufacturing numerous variants of each during WWII. The breadth of work required an incredible amount of organization and planning to accommodate the design changes, revisions, drawing updates, aircraft models and variants. As you may have guessed, the process for giving each part a number was very intentional, and allows those who have a firm grasp on the system today, to glean an incredible amount of information at a glance.
North American part numbers are always located in the title block at the bottom right hand corner of the drawing. This is a standard placement amongst WWII manufacturers that is still widely adopted by draftsmen today. The part number is also printed several other places on a drawing, on the outside margin, in another box off to the left of the main listing, and in the notes that denote updates and revisions. Here at AirCorps Library, the fact that the number is printed multiple times is incredibly useful while processing the drawings. In instances when we are assigning part numbers to drawings and are not 100% sure of the part number in the title block due to the microfilm being cloudy, damaged, or torn, we can look elsewhere and are generally able to determine the number from another location. We like to think our percentage is pretty good, with only an average of about 2 drawings in 2000 that we cannot decipher. In the drawing below you can see an example of the different areas on a drawing that a part number can be found.
Numbers are assigned to North American Aviation Drawings in accordance with the Bill of Materials Chart. Part numbers start with a two or three digit number followed by a hyphen. These initial numbers indicate what is called the “North American Model Designation”. While each number corresponds to a specific model North American produced, parts frequently cross over between variants of one aircraft and even between aircraft models. Regardless of aircraft, the part number is kept the same from the model it was originally designed for. For example: a 102- and 104- prefix (like the drawing above) indicates that this specific part was first designed for a P-51B, while a 77- or 78- indicates a part made for the AT-6A. For an in-depth list of part number prefixes click here.
While the model designation is important, it’s what happens after the hyphen that things get more specific. The two numbers immediately following the hyphen indicate the function of the part. For example, a -58000 number indicates a hydraulic part, and a -54000 number means the part is used in an electrical system. The system continues, sometimes being specific enough to indicate the material from which the part is to be made! The identity of a part is determined by the functional group for which it was designed, regardless of its location in the airplane or its method of attachment. At Aircorps Library we use this same system to categorize our drawings under each model. The image below is taken from the introduction of a T-28B-C model technical manual and shows in detail the complexities of the system.
It is always exciting to find explanatory information like this chart, which can benefit not only our members, but also our restoration, fabrication, and reverse engineering teams here at AirCorps! If you are interested in having a large-scale print out of this chart, or any other drawing in our Library collection, you can add a drawing to your Library cart by going to the drawing and clicking on the shopping cart in the upper right hand corner of the screen.