If the part number has gone, do not order by the outside appearance of the spring. Start with the machine and the surfaces the seal fits against.
Measure the shaft or sleeve diameter at the seal location. Record the seal chamber bore, depth and the axial space available. Add the pump make and model, any drawing or old part number, and a clear photograph of how the seal was fitted before removal.
On a component seal, photograph the rotating head and stationary seat separately with a ruler in view. Measure the bore of the rotating head, the seat outside diameter and seat thickness. Record the installed or working axial height, also called the setting or compressed length, and the spring free length. Note how the seat fits the gland recess and any drive features such as pins, slots or drive keys. Record the spring arrangement and whether the seat is held by a rubber cup or an O ring. For a cartridge seal, keep the whole assembly and its setting parts together.
A worn ring or damaged face may no longer give its original size. Cross check against the shaft, chamber and undamaged hardware. Treat a worn head bore as indicative only. The shaft or sleeve metal dimension is authoritative. Do not force a catalogue match when the fitted length, seat or drive arrangement differs. A part that goes over the shaft is not necessarily a part that will seal in the chamber.
Dimensions tell us what can fit. The fluid, temperature, pressure, speed and any solids tell us what should be fitted. Send both sets of information before asking for a replacement.
Related: Seal Selector Tool and What should I send to identify an unmarked or obsolete seal?.