Chevy Small Block - Short vs Long Water Pump
There is absolutely no denying that the OG Gen 1 small block Chevy is one of the most “swapped” engines in the world. We’ve seen them in just about every make and model of hot rod, muscle car, kit car, and even a few exotics over the years, and rightfully so. Their sheer numbers (over 100 million produced in production and crate engine offerings) mean they are relatively inexpensive to build, and parts can be easily found in any corner of the free world. That said, there is one facet of the Gen 1 small block Chevy (and even GM’s big block engine) that can cause some head scratching amongst enthusiasts and that is using the correct water pump for the application.
There are just a couple basic things to consider when you’re purchasing a new water pump. Do you have a short or long style pump, and is it v-belt or serpentine-belt driven?
Short Water Pumps
GM used the short water pump on car applications up through the 1968 model year (and 1972 for light trucks) except for the Corvette, which used the standard short pump through 1970. The water pump is usually easy to spot when installed, as the relationship between the water pump back plate and the engine’s timing cover does not allow your hand any room between the two. While this is not definitive, it is often used as a quick “junk yard” test. Another quick visual check if the engine is complete with all accessories is where they are mounted. Short water pump applications placed the alternator on the driver’s side of the engine. If the water pump is unmounted, you can take a measurement from the pump’s gasket surface to the pulley’s mounting hub surface to confirm that it is indeed a short water pump. Place the pump on a flat surface with the shaft pointing up. Use a straight edge or a ruler placed across the pulley hub face. Small block “short” pumps measure 5-5/8-inch (5.625-inch). The Corvette used its own version of a short water pump from 1971-1982 due to engine compartment clearance issues with a special 5.795-inch height, so be aware of those as well. Big block “short” pumps measure in at 5-3/4-inch (5.750-inch). The big block pump’s mounting spacing is different as well, but obviously that can be difficult to determine with the pump off the engine, so always confirm the “height” of the pump is correct for your needs.
Long Water Pumps
Starting in 1969 GM moved to the long water pump for cars and 1973 for light trucks. The long water pump appears similar in design to the short water pump, however, it has longer cast “legs” that move the main body of the pump away from the engine. Similar to the short water pump, you can usually spot a long water pump installed on an engine by the extra-large gap between the back side of the pump and the engine’s timing chain cover. The gap is approximately two finger’s widths wide and most people can slip a good portion of their hand between the engine and the water pump as a simple test. Like the short pump visual clues, if the alternator is mounted to the passenger side of the engine it is most likely a long water pump application. Again, though, the best way to confirm is by measuring from the gasket surface to the water pump pulley hub surface. For a long pump these measurements come out to 7-inches for the small block long pump and 7-1/4-inch (7.250-inch) for the big block long water pump. Also, some of the later style aluminum water pumps with a serpentine-belt drive system used a reverse rotation pump.
One other thing to consider is the diameter of your water pump snout. Chevy used two common sizes, which were either 5/8-inch or the heavy duty 3/4-inch. Short or long, most GM’s water pumps used the 5/8-inch (.625-inch) diameter water pump shaft and pilot stub for the fixed or clutch fan from the factory. Keep in mind that mixing and matching pulleys between short and long pumps can cause misalignment issues. So, when shopping for a replacement water pump once you’ve confirmed your short or long pump application be certain to measure the shaft/pilot so you can remount your water pump pulley and fan without issue.
To be 100% sure before you order, the check is simple. Just set your original pump on a flat surface with the pulley flange facing up, take a straight edge and lay it across the pulley mount face and measure the distance between the block-side mount flange and pulley flange.
Due to the longer production run, long water pumps hold the distinction of having more accessory brackets, pulleys, and other hardware available for them, but obviously the short pump will save you roughly 1-3/8-inches (1.375-inch) of room, which in a cramped hot rod engine bay might be just what you need to have your fan clear the radiator or your water pump pulley clear your electric fan mounting. But if we’re talking a C10 pickup, early Camaro, Nova, or other muscle car, the long water pump should not be a fitment issue and will allow you a wider choice of accessory drive items to mount to it.
All applications can vary and pulley alignment can change on custom set-ups. In certain cases it may be handy to purchase water pump pulley shims or spacers to achieve the correct alignment.
Updated by Mark Houlahan
