Is it better to stock repair parts or invest in complete component replacement? We get a variation of this question quite often during spare parts outfitting. The question can apply to a wide range of equipment on the boat including engine-mounted raw water pumps, seawater pumps, toilet components, and bilge pumps.
Repair/service kits for most OEM and aftermarket raw water pumps are available with kit names like Minor, Major, Service, or Rebuild. For example, a typical Sherwood Minor Kit includes a seal assembly, o-ring/gasket, impeller, key, and retaining ring. A Major Kit includes the parts in the Minor Kit plus bushings, bearings, cam, and wear plate. The basic Impeller Kit usually includes just the impeller and cover o-ring/gasket.
Pricing is highly variable depending on the manufacturer and the specific pump. As a general guide, a Minor Kit can be 25% of the cost to replace the pump, and a Major Kit 75%. The incremental cost to carry a replacement pump compared to a Major Kit is negligible. Instead of rebuilding the pump while the associated engine is out-of-commission you just replace the whole pump. This eliminates the chance that the one part that is failing in the pump is the one not included in the rebuild kit. For offshore or extensive cruising we generally recommend a couple Impeller Kits and a complete pump (wet end). For coastal cruisers a spare impeller kit is usually sufficient.
The same analysis can be applied to A/C seawater pumps, hydraulic cooling pumps, toilet systems, and many other components. In the long run, having replacement components may save time and money and get you cruising sooner.