Mopar: Before You BuySome important things to know about Mopar motors.
There are essentially two different water outlet bolt patterns used by all Chrysler engines from 1924-2003. One is the older large size with a center to center bolt pattern of 3.25 inches. This pattern was used on all v8 engines from 1951 to 1978 and all flathead 4, 6 & 8 inline engines back to 1924. The thermostat diameter is 2.49 inches for this size housing. The other water outlet bolt pattern is 2.875 inches. This pattern was used on all slant 6 engines and the newer small block engines made after 1978. The thermostat diameter is 2.125 inches for this size housing. All flathead 4, 6, 8 inline, Slant 6 and V8s made before 1979 use 3/8 inch bolts. V8 Small block engines 1979 and newer use 5/16 inch or metric size bolts.
Things get more complicated. Even though the 3.25 pattern is the same for all v8s from 1951-1978 many of the 1950s water outlets have bypasses built into the housing. This is also true of most of the flathead 6 & 8 inline engines. If you plan to change to a different water outlet on these engines it is important to know what the bypass did and is changing to a non bypass water outlet going to have an adverse effect. Bypasses are used on these engines mostly to keep from having hot spots in the cooling jacket as the engine comes up to temperature because the bypass recirculates the water in the engine. They are also used were the winter temperatures are very cold so the heater warms up faster. In addition some of the thermostats were constructed to work in conjunction with the water outlet’s bypass and use a different size Thermostat. Most of these thermostats are 2.55 inches in diameter. Some of the small block v8 aftermarket manifolds have both the 2.875 and the 3.25 bolt patterns which allow you to use either size water outlet. Also not all water outlets use the same size radiator hose. The 3.25 bolt pattern is also the same as Chevy.