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Flat Head Engines: Plymouth-Dodge-DeSoto-Chrysler Six and Eight

1953 flathead six engine

The L-Head motor, or what is more commonly known as a “flathead,” was the mainstay of most auto manufacturers before World War II.

In a flathead motor, the head is a flat piece of cast iron that is bolted down on top of the block. The valves and the entire valve train are in the block itself; the head is relieved to allow for compression and for valve clearance, but it contains no moving parts (except the thermostat). The head is partially hollow to allow coolant to pass through to assist in cooling the engine.

flat-head six cylinder (Chrysler)Virtually all early cars were flatheads, and the flathead stuck around for a long time, but few flathead motors had the staying power of the Chrysler Corporation’s flathead six, which began production in 1929 for DeSoto. As late as 1972 [Keith Windsor], this engine was being produced for industrial uses such as stationary pumps, arc welders, forklifts, and farm equipment such as harvesters and combines. Because of this, parts are still available on the shelves of better auto parts stores. I have purchased water pumps, fuel pumps, and tune up parts off the shelf for my 1948.

The flathead, because of its design limitations, was destined for doom as the fabulous fifties began to roar [though it may have been the best design in its early days]. The public’s desire for more power prompted Chrysler to produce the legendary Hemi in 1951. The Hemi powered Chrysler 300 in 1956 was the most powerful and fastest production car in America that year, averaging 139.9 mph at Daytona.

What made the flathead so versatile was its dependability and torque. The largest flathead six Chrysler used in its automobiles was 265.5 cubic inches that produced 218 ft/lbs of torque at 1600 RPM. So, at just off idle, this engine produced maximum torque, making it ideal for industrial use.

1956 PowerFlow Six

In comparison, the last flathead 8, produced in 1950, with 323.5 cid and 270ft/lbs @ 1600 rpm. Although the 8 was more powerful, most people found the smooth power produced by the six was sufficient (and far more economical), and so a vast majority of older Chryslers found today are equipped with sixes.

The flathead eight-cylinder engine was used in a large variety of vehicles. Because it was a straight-eight (not a V8) design, the revolutionary Airflow models had to be lengthened to accommodate it.

Chrysler Eight flat head engine

The flathead six in frcn’s 1948 Chrysler Windsor Club Coupe was a smooth, powerful, efficient powerplant. On the highway it regularly got from 17-19 mpg at average speeds of near 60 mph — in a car that weighed over 3,600 lb — with maximum speeds of over 100 mph possible.

dodge flathead sixAfter World War II, the 251.6 and 265.4 cubic inch engines used in Dodge medium and heavy duty trucks were equipped with sodium cooled exhaust valves and stellite seats. The hollow exhaust valve stems were filled with metallic sodium metal which would liquefy at operating temperature and transmit the heat up the stem and through the valve guide to the water jacket.

Because of the long stroke, heaps of torque were available at low rpm. My dad ran them in his propane delivery trucks. These old engines were dogs on power but would run forever, especially when fueled with LPG. Getting one started on a zero degree morning in January was an exercise in patience.

Larger trucks used another family of in-line flathead six cylinder engines. There was a 331 cubic inch and a mammoth 413 cubic inch lugger was available in the T series and up. There may have been another engine in the 370 CID range, but I am not sure about that. The 331 was highly regarded in trucking circles as a durable, hard working engine. The 413 was equipped with two single barrel carburetors and had a very large appetite for gasoline. It was noted for twisting the drive sprocket off the end of the camshaft.

powerflow six engineChrysler’s move into the six-cylinder, low price field was so unprecedented that Automotive Industries magazine devoted many pages of their February 4, 1933 issue to the machinery installed in the Plymouth plant, machinery which would not only produce the 6-cylinder engine but would cut Chrysler’s cost of producing those engines to a point to make it worthwhile in a low priced automobile. (What makes this truly amazing is the fact that it was done in the depths of the Depression — Walter Chrysler believed in keeping workers busy and paid.)

The big news for the year 1933, then, was the introduction of the Plymouth 6-cylinder engine. The valve-in-block engine displaced 189.8 cubic inches from a bore of 3 1/4" and a stroke of 4 1/8". With the standard compression ratio of 5.1 the engine, which retained the Silver Dome name, pumped out 70 horsepower at 3,600 rpm. With the optional aluminum "Red Head" the compression ratio jumped to 6.5 while the horsepower increased to 76.

The engine used a redesigned water pump for better cooling efficiency, aluminum alloy pistons and had a new first as it was fitted with insert bearings on the main and connecting rod bearings, as well as the first camshaft bearing. Power was transmitted from a 9" dry clutch plate through a three speed transmission with helical gears for smooth, quiet operation.

plymouth flathead four

Remember to swap enough bolts! I put a 1959 Plymouth engine into my first car, a 1948 Plymouth. There were obvious parts you had to swap, like the carburetor, water pump, crankshaft pulley, generator (wide vs. narrow belts, 6 vs. 12 volts), oil pan (to fit a different frame), and starter (6 vs. 12 volts again.) All went well until I pushed the starter. Klunk! There were different numbers of teeth on the ring gear, and the starter would not mesh. The old flywheel was needed.

The new engine proved to be a most reliable and an economical powerplant. The basic Plymouth six remained in production from its introduction in October 1932 until the cessation of 1959 model production (and it was based on the existing DeSoto Six). At that time the Valiant moved to the new Slant Six.

The old reliable flathead six kept many an old Plymouth on the road long after its original engine had been scrapped; the later engines were bolt-for-bolt swaps into the early cars (see note on right).

The engine received a fully jacketed water system in 1935 which meant moving the starter slightly outboard on the bell housing. For a later engine to be put into the ’33 or ’34 models the bell housing had to be changed, but this was unnecessary from ’35 on.

straight six engine

1953-1959 changes (Lanny Knutson)

Under the hood of the 1953 models was the same 217 cubic inch L-head six found in Plymouths since 1942. However, this year’s horsepower came at an exact 100, an increase of three, thanks to a 7.1 to 1 compression ratio (up from 7:1).

plymouth six

Introduced-mid-year, 1954, was a 110-horsepower "Powerflow Six", a 230 cubic inch unit formerly exclusive to Dodge. The appearance of the new automatic transmissions, plus the V8 fever spreading in the car-buying public, probably dictated the move to a larger engine.

flathead engine

Canadian-built 1953-54 Plymouths featured the usual variations, most notably the long-block six. Manual transmission cars came with the familiar 218 cubic inch motor. When Hy-Drive, and later, Powerflite, transmissions were ordered, a new 228 cubic inch engine (formerly exclusive to the Canadian-built, American-style Coronet) was installed.

  P-30 (Straight Six), 1957
Taxable HP 25.4
Gross Brake Horsepower 132 @ 3,600
Torque 205 @ 1,600
Bore x Stroke 3.25 x 4 5/8
Compression 8:1 (120-150 psi, max variation between cylinders: 10 psi)
Cylinder numbering
from front of engine
1-2-3-4-5-6
Firing order 1-5-3-6-2-4
Connecting rod bearings Replaceable steel backed babbit, .0005 to .0015 inch clearance
Main bearings

Replaceable steel backed babbit, .0005 to .0015 inch clearance;
2.5 inch diameter (nominal)

In 1955, Plymouth advertised the “new” PowerFlow 6 as “one of the world’s most economical engines to drive. It is smooth and quiet in operation with flashing breakaway at the traffic light... [with] lightweight aluminum pistons, a 4-in-1 aluminum carburetor, and chrome-plated top piston rings for longer engine life and better oil control.”

camshaft

In 1956, 40% of Plymouth buyers still chose the tried and true 230 cid PowerFlow Six shared with Dodge. The Plymouth version pushed out 125 hp, the Dodge 131 hp at 3600 rpm; but Plymouth buyers could buy a Power Pack for the six, which made it the same as the Dodge, with a Stromberg dual throat carburetor, special intake manifold, larger air cleaner, and a 3.9 rear-axle ratio. For 1957, the Dodge and Plymouth engine both put out 132 hp.

Heat-resistant exhaust valves were used in 1959. Horsepower was still 132 at 3600 rpm.

oil

The Last Mopar Flat Head Engines

The last use of the flat-head engines at Chrysler was in 1954 with the 264.5 cid six; at DeSoto, with the 1954 250.6 cid six. Plymouth and Dodge cars soldiered on until 1959 with the same 230 cid versions. Most Dodge trucks dropped the flathead six after 1960, when it displaced 230 cubic inches. David Youse said that the 230 continued through to 1968 for the Dodge M37 three-quarter ton military cargo truck; it was similar to the civilian version, but had a waterproof ignition and crankcase sealing provisions so it could run submerged, using intake and exhaust extensions.

PowerFlow 6 engine

In its last civilian year, the Dodge truck 230 engine made 120 horsepower at 3,600 rpm, and 202 lb-ft of torque at a low 1,600 rpm; the compression ratio was 7.9:1. Features included exhaust-valve seat inserts  and chrome-plated top piston rings.

Dodge trucks continued through to 1968 with a 250.6 cid version, used in the 1961-68 WM300 Power Wagon. Richard Davis noted that this version had replaceable hardened exhaust valve seats, full flow filtering, and full pressure oiling. It had 115 net hp at 7:1 compression until 1966, and in 1967-68 it had 125 net hp at 8:1 compression.

Technical data above taken from The Standard Catalog of Chrysler, John Lee, 1990; and from Motor’s Auto Repair Manual, 17th Edition, 1954.

Click here to see (or download) the photo

Photo of frcn’s 1948 Chrysler’s 251 cid L-Head during installation, after an overhaul.

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