Poly 318, Polyspherical, LA, and Magnum Parts Interchange

(most parts interchangeable also applicable to 277, 301, 303, 313, and 326 A-blocks)

Introduction

I have compiled this list of polyspherical 318 parts interchange with polyspherical, LA 318 273, 340, 360, Magnum engines with notes to assist in piecing together engines. I use the A block years 1956 – 1966 understanding that 1966 parts were used through 1967 in some export cars/trucks. For parts numbers and recommendations, see the complete A-block parts list.

Blocks:

  • 1956 – 1961 A-block
    • Note: Bellhousing bolt and alignment pattern, starter location, head water jackets differ from later A-blocks.
  • 1962 – 1966 A-block
    • Note: A-block heads can be modified to work on LA blocks, but the A blocks and LA blocks are not directly interchangeable.

Camshaft: All A-block

  • Note: LA camshaft do not interchange.

Camshaft Bearings: A/LA 1956 – 1978

  • Note: 1979 onward LA/magnum camshaft bearings changed in design and do not interchange with 1956 – 1978 A/LA.

Connecting Rods: All A303/313/318 and all LA with floating pins

  • Note: In 1974, LA rods went to a heavier design.

Connecting Rod Bearings: All A/LA

  • Note: Aftermarket forged crankshafts usually require narrow bearings such as Clevite CB481HN due to a larger radius on the journals. Consult the crank manufacturer and verify during mock-up.

Crankshafts:

  • 1956 – 1961 depending on stroke
  • 1962 onward A/LA except LA360 and externally balanced cast-crank LA340
    • Note: 1962 – 1966 cranks will not interchange into 1956 – 1961 blocks since the transmission will not attach.

Cylinder heads:

  • 1956 – 1961 A-block
    • Note: 1956 – 1960 heads do not have accessory bosses drilled/tapped.
    • Note: 1962 – 1966 heads can be used on 1956 – 1961 without modification. To use 1956 – 1961 heads on 1962 – 1966 blocks requires tapping and plugging the additional water jacket in the heads to stop coolant from dumping into the lifter valley.
  • 1962 – 1966 A-block
    • Note: Contrary to garage myth, LA heads do not directly bolt to and work on poly blocks.

Distributor: All A/LA

Distributor Drive Gear: All A/LA

Exhaust Manifolds: All A-block

  • Exhaust manifolds can be used on either right or left heads.
  • A-block cars often came with one center-dump and one rear outlet.
  • A-block trucks often came with two center-dump.
  • Generator engine right manifolds had a generator bracket incorporated.

Flywheel/Flexplate:

  • 1956 – 1961 A-block 
  • 1962 onward A/LA except externally balanced LA360/LA340
    • Note: LA360 and cast-crank LA340 flexplates/flywheels can either be neutral balanced or weighted to balance the rotating assembly. Using them on the A-block requires verifying they are neutral balanced.

Fuel Pump: All A/LA

Fuel Pump Eccentric: All A/LA

Harmonic Damper: All A and internally balanced LA

  • Note: When using an LA damper with an A-block timing cover, see the notes under “Timing Cover” below and my page on harmonic dampers for more details on interchanging dampers and timing covers.
  • Note: LA360 and cast-crank LA340 are externally balanced and do not use dampers but balancers that will throw the A-block rotating assembly out of balance. See my page on harmonic dampers for more details.

Intake Manifold: All A-block

Lifters: All A-block

  • Note: LA lifters will fit but are taller than A lifters and require shorter pushrods.

Main Bearings:

  • 1956 – 1973 A-blocks and LA273/318/340 main bearings interchange and use a thrust bearing with a short vertical thrust side, such as Clevite MS540P. The overall diameter of this thrust bearing when installed is 3.374″. These bearings feature an oil groove on one half of the bearing and solid on the other. Depending on the machinist and/or machine setup, some blocks had the thrust bearing block webbing and main cap sides machined with a short rabbit to match the short thrust bearing side, while other blocks had a taller rabbit machined in.
  • 1974 onward LA318/340 blocks had the thrust bearing webbing and main cap sides machined with a taller rabbit and used a thrust bearing with a taller vertical thrust side, such as Clevite MS1344P. The overall diameter of this thrust bearing when installed is 3.53″. The bearings featured a full oil groove on both halves.
  • Before purchasing 1974 onward LA318/340 main bearings with full oil grooves to use in a 1956 – 1973 A/LA block, one must verify that the thrust bearing block webbing and main cap are machined with tall enough rabbits for the taller 1974 onward thrust bearing to fit. If the rabbit is not tall enough and the 1974 onward bearing is used, the bearing will not fully seat and will bind on the crankshaft main journal. If the use of the full oil groove 1974 onward main bearings is desired, you can either have the block/main cap rabbit machined taller, you can sand down the thrust bearing sides, or you can obtain a 1956 – 1973 thrust bearing and use the 1974 onward bearings on the other main journals.
    • Note: Aftermarket forged crankshafts usually require narrow bearings such as Clevite MS540H due to a larger radius on the journals. Consult the crank manufacturer and verify during mock-up.
    • Note: LA360 crankshaft mains are larger than the A-block and LA273/318/340 and cannot be used.

Oil Filter and Housing:

Oil pan:

  • 1956 – 1958 A-block (5 quart, Figure 1)
  • 1959 – 1961 A-block (4 quart)
  • 1962 onward A-block and LA273/318/340 (LA360 do not interchange due to main journal and timing cover changes)
    • Note: Trucks used a front-sump pan, cars a center-sump pan. Car pans will not clear the factory truck suspension (Figures 2 – 3).

Oil pump:

  • 1956 – 1961 A-block (Figure 4)
  • 1962 onward A/LA

Pistons: A-block depending on bore.

Piston Rings: A/LA with 3.91″ standard bore

Pulleys:

  • Crank: All A/LA
  • Water Pump: All A/LA
    • Note: 1957 – 1969 pumps have a short shaft and require a pulley to match (Figure 5).
    • Note: 1970 onward pumps have a long shaft and require a pulley to match (Figure 5).

Pushrods: 

  • Solid Lifters: All solid-lifter A-block
  • Hydraulic Lifters: All hydraulic-lifter A-block
    • Note: LA pushrods do not interchange.

Rocker Arms: 1956 – 1966 A-block

  • Note: Solid cam engines came with adjustable rocker arms; premium hydraulic cams came with nonadjustable rocker arms.
  • Note: Cast LA273/340 rocker arms do not interchange.

Rocker Arm Shafts: 1956 – 1966 A-block

  • Note: LA and B/RB rocker arm shafts do not interchange.

Starter Motor: See the parts article on starter motor options.

  • 1956 – 1961 auto and manual transmission: A-block or manual LA
  • 1962 – 1966 manual transmission: A-block or manual LA
  • 1962 – 1966 auto transmission: A/LA/Magnum

Timing Chain Set: All A/LA

Timing Cover: See the technical article for installation.

  • 1956 is unique
  • 1957 onward A/LA273/318/340 (LA360 do not interchange due to main journal and oil pan changes)
    • 1957 – 1961 timing covers are iron and 1962 onward are aluminum (Figure 6).
    • LA273/318/340 timing covers will physically bolt onto an A-block, but they lack one bolt hole on the left side (Figure 7). In the A-block, this boss goes through into a water jacket. To use the LA timing cover on an A-block, plug the block hole using a set screw or cutoff bolt and proper sealant (such as Permatex 80003) prior to installing the timing-cover gasket.
    • If using an A-block damper with the TDC clocked on the right with a 1970 onward LA timing cover with the molded timing pointer on the left, remark the damper TDC line with a file or timing tape with cylinder 1 at true TDC. If using a 1970 onward LA damper with a 1970 onward LA timing cover, the markers align. See the page on harmonic dampers for more details on interchanging dampers and timing covers.

Thermostat: All A/LA

Thermostat Housing: All A/LA and first-generation hemi-blocks

Valve Covers: All A-block

  • 1956 – 1958 and some 1959 have three bolt holes but will interchange with all year A-blocks (Figure 7).
  • Most A-block heads have the center bolt hole boss drilled and tapped through end of production in 1966.

Valve Springs: All A/LA with 1.70″ install height

  • Note: Follow camshaft spring-rate requirements
  • Note: Verify spring height works if using aftermarket valves.

Valve Spring Retainers: Match to desired keeper angle, stem diameter, and spring diameter. All A-block interchange.

Valve Spring Keepers: Match to stem groove, stem diameter, and retainer angle. All A-block interchange.

Water Pump: See the technical article for installation.

  • 1956 is unique
  • 1957 onward A/LA
    • Use 1957 – 1969 short-shaft pump for left-side intake (Figure 8).
    • Use 1970 onward long-shaft pump for right-side intake (Figure 8).
      • The water pump pulley must match the pump snout style for belt alignment.

Note that many of the interchangeable parts above can be found used from donor engines or new through retailers such as Rock Auto, Napa, Summit Racing, and Jegs.

Figures

Mopar A-block poly 5 quart oil pan
Figure 1: 1956 – 1958 A-block 5-quart Oil Pan
Poly 318 Oil Pan
Figure 2: 1959 – 1966 Car Oil Pan
Poly 318 Oil Pan
Figure 3: 1959 – 1966 Truck Oil Pan
Poly A-block Oil Pump
Figure 4: 1956 – 1961 A-block Oil Pump
Poly 318 water pump pulley
Figure 5: Poly A-block water pump pulleys, short-snout (left) and long-snout (right)
Poly 318 Timing Water Pump Cover
Figure 6: A-block Timing Covers
Poly 318 LA Timing Cover
Figure 7: LA 318/340 Timing Cover
Poly 318 Valve Covers
Figure 8: A-block Valve Covers
Figure 9: A-block and LA Water Pumps
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