Poly A-block Engine Specifications

I have compiled these Poly 277, 301, 303, 313, 318, and 326 specifications from multiple factory manuals/brochures/bulletins, my personal observations, and filling in gaps discussing with other A-block engine builders.

Poly A-block Factory Engine Specs

USA Domestic Versions

1956 A277

Bore: 3.75”

Stroke: 3.13”

Compression: 8:1

BHP: 187 – 200 @ 4,400 rpm

Torque: 265 – 272 @ 2,400 rpm

1957 – 1958 A318 “Dual Fury V-800” Dual Four-barrel Carburetors 

Bore: 3.91”

Stroke: 3.31

Compression: 9.25:1

BHP: 290 @ 5,400 rpm

Torque: 225 – 330 @ 3,600 rpm

1959 Dodge A326 Two-barrel

Bore: 3.95”

Stroke: 3.31

Compression: 9:1

BHP: 255 @ 4,400 rpm

Torque: 350 @ 2,800 rpm

1957 – 1962 A318 Single Four Barrel (called Super-Pak from 1956 – 1959)

Bore: 3.91”

Stroke: 3.31”

Compression: 9.0:1

BHP: 250 – 260 @ 4,400 rpm

Torque: 340 – 355 @ 2,600 – 2,800 rpm

1957 – 1966 A318 Car/Truck Single Two Barrel

Bore: 3.91”

Stroke: 3.31”

Compression: 9.0:1

BHP: 225 – 230 @ 4,400 rpm

Torque: 330 – 340 @ 2,400 – 2,800 rpm

1960 – 1966 A318 Heavy Truck, Industrial, RV Single Two Barrel “Premium”

Bore: 3.91”

Stroke: 3.31”

Compression: 8.0:1

210 (hydraulic) @ 4,000 rpm

Torque: 290 @ 2,400 rpm

Poly 318 engines with the “318-3” stamped designation were considered “premium” from the factory and have exhaust valve rotators (see Figure 1 below), sodium-filled valves, stellite exhaust seats, and hydraulic lifters with non-adjustable rocker arms. These engines were rare from the factory and used in premium D400, D500, and D600 trucks, some RV applications, and some industrial applications. While they were once sought after engines by enthusiasts, current aftermarket parts availability allows one to build any A-block to surpass the factory “premium” version. There is some misinformation that the premium heads and blocks are thicker castings, but the heads and blocks are the same castings as non-premium. Also, some people claim the premium blocks had forged crank and rods while standard engines did not, but all A-blocks came from the factory with forged cranks and rods.

Canada/Export Versions

1956 – 1957 303 4-barrel Carburetor

Bore: 3.81”

Stroke: 3.13”

Compression: 9.25:1

BHP: 240 @ 4,400 rpm (270 @ 5,200 for the dual-quad HP version)

Torque: 310 @ 2,800 rpm (both PowerPak and Dual Quad HP)

1957 A301

Bore: 3.91”

Stroke: 3.13”

Compression: 8.5:1

BHP: 215 @ 4,400 rpm (2-barrel)

BHP: 235 @ 4,400 rpm (SuperPak 4-barrel)

Torque: 285 @ 2,800 rpm (2-barrel)

Torque: 305 @ 2,800 rpm (SuperPak 4-barrel)

1957 – 1967 A313 Two-barrel

Bore: 3.875”

Stroke: 3.31

Compression: 9.0:1

BHP: 225 @ 4,400 rpm

Torque: 330 @ 2,400 – 2,800 rpm

General Specs for A-Block 318 and 313

Camshaft

End Play: 0.002” – 0.006”

Bearing Clearance: 0.001” – 0.003”

Cooling System

Capacity w/out heater: 20 quarts typical

Radiator Cap Relief: 14 – 16 psi (higher with A/C)

  • Note: earlier 1950’s models require 12 psi caps, especially on questionable radiators and heater cores.

Normal Operating Temperature: 175° – 200°

Thermostat: 180° or 195° (see my cooling technical article for discussion of choosing thermostats)

Crankshaft and Connecting Rods

Main Journal Diameter: 2.4995” – 2.5005”

Main Bearing Clearance: 0.0005” – 0.0015”

Crankshaft End Play Measured at Bearing #3: 0.002” – 0.007”

Rod Journal Diameter: 2.124” – 2.125”

Rod Bearing Clearance: 0.0005” – 0.0015”

Rod Length: 6.123”

Rod Ratio: 1.85

Cylinder Heads and Valvetrain

Solid Lifter Lash (see my valve adjustment technical article for adjusting lash)

  • Intake Hot: 0.013”
  • Exhaust Hot: 0.021”
  • Intake Cold (for initial start): 0.015″
  • Exhaust Cold (for initial start): 0.021″

Hydraulic Lifter Preload: 3/4 turn after zero lash for hot, 1/2 turn for cold. (see my valve adjustment technical article for adjusting preload)

Combustion Chamber Volume: varies between 78 – 81 cc all A-blocks regardless displacement/induction/performance package

Valve Angle: 45° seat and face

Intake Valve Diameter: 1.84″

Exhaust Valve Diameter: 1.56″

Valve Stem Diameter:

  • Intake: 0.372” – 0.373”
  • Exhaust: 0.371” – 0.372”

Valve Stem Clearance:

  • Intake: 0.001” – 0.003”
  • Exhaust: 0.002” – 0.004”

Valve Spring Installed Height: 1-11/16”

Factory Valve Spring Pressure: up to 145 in. lbs. at 1-5/16”

  • Note: Always follow the camshaft’s required spring pressure.

Rocker Arm Ratio: 1.5:1 

Lifter Angle: 59°

Fueling

Fuel Pressure: 5 psi – 7 psi

  • Note: When running some Holley (e.g. 4150/4160) and Edelbrock AVS carburetors, pressure over 6.4 psi can lift the needles causing flooding.

Ignition

Firing Order: 18436572 (Figure 2)

Hot idle speed: 500 – 850 rpm depending on cam and vacuum requirements (see my Carburetor Selection and Tuning tech article for adjusting hot idle speed)

Spark Plug: 3/8” short-reach Autolite 85, Champion 63 (formerly J12Y), etc.

  • Warning: Do not install LA-type 3/4″ long-reach plugs, such as Autolite 65. They will contact the piston.

Spark Plug Gap: .035”

Points Gap: 0.017” (see my ignition timing technical article for adjusting points and setting dwell)

Dwell Angle: 26° – 28° ideal, 26° – 32° allowable

Factory Coil Resistance at 70°F – 80°F

  • Primary: 1.65 – 1.79 ohms
  • Secondary: 8,000 – 9,200 ohms
  • Balast: .665 – .735 ohms

Timing (since timing is specific to each engine, see my ignition timing tech article for discussion and instructions)

  • Initial: 10° BTDC (as a baseline for firing the engine in order to tune)
  • Total Advance w/out vacuum at 4,000 rpm: 34° – 36°
  • Total Advance with vacuum at 4,000 rpm: 42° – 52°

Oil System (Figure 3 and 4)

Hot Oil Pressure:

  • Idle: 20 – 30 psi ideal
  • 1,500 rpm and above: 50 psi – 65 psi

Capacity:

  • 1956 – 1958: 5 qt. pan, 1 qt. filter. (6 qt. total)
  • 159 – 1966: 4 qt. pan, 1 qt. filter. (5 qt. total)

Oil Viscosity:

  • Break-in: 30 weight conventional with added zinc
  • Cold and mild climates or tight engine: 10W30 or 10W40 conventional or synthetic with added zinc
  • Very hot climates or loose engine: 20W50 conventional or synthetic with added zinc
  • Note: Never use synthetic oil to seat new piston rings.
  • Note: Unless the oil is a high-zinc formula marketed as such, always run a zinc additive on flat-tappet cams with modern oil including oil designed for diesel engines. Neglecting to run additional can lead to premature cam lobe and lifter failure.

Pistons and Block

Clearance Top of Skirt: 0.0005” – 0.0015”

Ring End Gap, Compression and Oil: 0.015”

  • Note: Always follow the ring manufacturer specs.

Floating Wrist Pin Diameter: 0.9842

Cylinder Out-of-Round: No more than 0.005” before overbore, and that is very worn.

Cylinder Taper: No more than 0.020” before overbore, and that is very worn.

Lifter Bore: 0.9050” – 0.9058”

Cylinder Pressure (measured with cylinders warm and dry)

  • Stock Build: 120 – 165 cranking psi, no greater than 20 psi difference between cylinders. 110 psi minimum.
  • Performance Build: Above 160 cranking psi ideal, no greater than 20 psi difference between cylinders.
  • Note: cylinder pressure depends greatly on elevation, compression ratio, cam, testing procedure, and component wear.

FIGURES

Mopar A-block exhaust rotator
Figure 1: 318-3 Premium Exhaust Valve Rotator (photo with swapped-in adjustable rocker arm)

Mopar A-block firing order
Figure 2: Firing Order

Early Mopar A-block oiling system
Figure 3: 1956 – 1961 Oiling System (bypass valve)

Late Mopar A-block oiling system
Figure 4: 1962 – 1966 Oiling System (no bypass valve)

Mopar Plymouth Dual Fury V800 Poly 318 A-block
1957 – 1958 Dual-quad “Dual Fury V-800” Package

Mopar A-block Poly 318 Fury Super-pak V800
4 -barrel 318 “Fury Super-Pak V-800” Package

Mopar Poly A-block 318 Fury V800
2-barrel 318 “Fury V-800” Package

Mopar A-block engine sectional
A-block Sectional

Mopar A-block poly canted valves
A-block Canted Valves