From 1935 through 1959 Chrysler passenger car flathead 6 cylinder engines developed a reputation for durability. They looked similar, but went through several changes along the way. For our purposes we will concentrate on the Plymouth and Dodge engines (PD) and the 1937 through 1954 Chrysler and DeSoto engines (CD). The PD engines are both 23.5” long, as measured at the head. 1935 through 1941 Plymouth engines had a bore of 3.125” for a displacement of 201 cubic inches. The Dodge engines throughout had a bore of 3.25” as did the Plymouth engines from 1942 though 1959 giving either 218 or 230 cubic inches. Chrysler and DeSoto engines were 25.5” long and came in 2 bore sizes: 3.375” from 1937-1941 and 3.4375” from 1942-1959. A variety of displacements were arrived at due to different strokes including 228, 236, 241,250 and 265 cubic inches. For our purposes, the bore size tells us what we need to know about the internal parts.
Each family of engines has their own gaskets, sharing only a few common parts. From 1937 to 1950 (for PD engines and through 1954 on CD engines) all engines had an external bypass from the thermostat housing to the water pump. This allows coolant to flow prior to the thermostat opening. In 1951, PD went to an internal bypass. You can see this plainly by a bump-out on the forward part of the head and a corresponding protrusion on the block. All modern reproduction head gaskets have the provision for the internal bypass.
These gaskets can be used safely on the earlier engines. You may snip off the extra gasket material on vehicles with an external bypass with no ill effects. Exhaust and intake manifolds went through changes as well. Some of these were governed by chassis changes such as location of the exhaust outlet. Others were due to the increasing displacement of the vehicles. For example, through 1941, Plymouth engines had a smaller diameter exhaust outlet than did the same year Dodge vehicles.
On the intake side, carburetion changes were made. In mid 1954, Plymouth changed from the tried and true Carter BB 1 barrel to the Carter BBS. The new carb was shorter, allowing lower hood lines, and had a built in choke. Dodge, on the other hand, went to a 2 barrel Stromberg WW carburetor. To be continued…