Poly 318 Crankshafts and Transmission Options

(applicable to 277, 301, 303, 313, 318, and 326 engines)

Introduction

Invariably, a poly A-block needs a crankshaft and transmission. The poly A-block went through a design change that needs to be considered when selecting both the year of engine and the type of transmission for a vehicle. This article discusses the different variables in details.

Poly Crankshafts

Contrary to some misinformation online, all A-blocks from 1956 – 1966 are internally balanced just like the LA 273, 318, and forged-crank 340. The LA360, Magnum 360, and cast-crank 340 are the only externally balanced Mopar small blocks.

The Gen I Hemi (e.g. 331, 354, 390) and Hemi-block poly (e.g. 270, 315) crankshafts do not interchange with A-blocks.

1956 – 1961 A-block crankshafts have a 1/2″ longer rear flange and 8 bolts to mate to cast iron PowerFlite and TorqueFlite transmissions (Figures 1a – 1c). A 1962 onward aluminum TorqueFlite and Magnum NV3500 or NV4500 will not bolt up to a 1956 – 1961 crankshaft without an adapter, which I details in a section below.

1962 – 1966 A-block 318 and 313 use the same crankshaft forging as an LA 273/318/340 and has 6 bolts. A great benefit of the poly 318 is that all factory crankshafts are forged, whereas factory LA crankshafts were forged up to 1972 and then cast from 1973 onward. Note that you can physically bolt a 1962 onward crankshaft into the 1956 – 1961 block, but the crankshaft rear flange will be 1/2″ too short and there is no adapter readily available to make the 1962 onward shorter crankshaft mate to pre-1962 iron PowerFlite and TorqueFlite transmissions.

Balancing the Rotating Assembly during Parts Interchange

It is acceptable to swap rotating assembly parts that weigh within plus or minus 1 gram of each other without requiring rebalancing the rotating assembly, keeping in mind that the rods should be weighed using a jig to measure the weight of each end. If components fall outside plus or minus 1 gram, they need to be addressed to balance the assembly, which may also include removing or adding weight to the crankshaft. Swapping in any other crankshaft (LA 273, 318, 340, or aftermarket) into an A-block requires a machinist checking and likely balancing the rotating assembly since A-block piston weight differs from the LA pistons. When swapping connecting rods, keep in mind that Mopar changed the design from the earlier A-block lightweight rods to a heavier forging. Neglecting to properly balance a rotating assembly will create vibrations, rob power, limit the rpm range, and may lead to engine and transmission parts failure.

Poly Engine Blocks

In relationship to the two crankshaft designs, the 1956 – 1961 A-block block is different from the 1962 – 1966 block (Figures 2a – 2b). The 1956 – 1961 blocks share the same bellhousing bolt pattern and alignment pin configuration as the Gen I hemi and hemi-block polys. While the 1956 – 1961 A-block has a similar bellhousing bolt pattern to the LA, the alignment pins are in different locations.

1962 – 1966 A-block blocks share the same bellhousing configuration as LA engines.

Crankshaft Pilot Bearing and Converter Snout Machining

We now must deal with the torque converter snout or pilot bearing machining. When swapping transmissions and/or crankshafts, attention must be paid to the crankshaft rear flange machining. All 1962 – 1967 A-block and LA crank flanges were machined with a 1-3/4″ diameter hole to accept the TorqueFlite 904/727 small-diameter converter snout (Figure 3a) with a smaller hole inside for the standard-transmission pilot bearing. Standard-transmission crankshafts had the pilot-bearing hole machined deeper and with a more accurate bore to accept the pilot bearing (Figure 3b). Figure 3a shows an automatic crank’s bore that I confirmed with a caliper is 1/8″ shallower than the standard crank in Figure 3b to where the pilot bearing would protrude and bind on the input shaft if I attempted to run this crankshaft with a standard transmission. Keep in mind that many automatic cars ended up with crankshafts machined for standard transmissions since the factory used what parts they had on hand.

Figure 4 details the machining requirements if an automatic crankshaft needs to be machined to accept a pilot bearing. An alternative is to use a Magnum pilot bearing SKFB287 or 4338876 (Figure 5) that presses into the torque converter snout well, but input shaft clearance should be checked and the crank drilled out if clearance is needed. Some people recommend cutting off the input shaft, but I would not because is an unnecessary workaround that will make using the input shaft in a non-Magnum pilot bearing weaker. If possible, stay away from the needle bearing version FC69907 (Figure 6) because they are known to fail and chew up the input shaft on performance applications.

In 1968, Mopar changed the TorqueFlite from 19 to 24 input splines on the 727, 18 to 27 input splines on the 904, and a larger diameter converter snout on both. Consequently, 1968 onward LA crankshaft flanges are machined to accept a larger pilot bearing (Figure 7), although the cranks retain the 1-3/4″ diameter hole for the converter snout. To make matters worse for the pre-1968 transmissions, the small pilot bearings are getting hard to source new, and torque converter stall selection for 19-spline input shafts is extremely limited and expensive compared to the 1968 and later 24-spline shafts. One fix particularly useful for those running cable-operated TorqueFlites is to install a 1968 or later 24-spline non-lockup input shaft in the 1962 – 1967 TorqueFlite to accept the wider range of later torque converters; review my TorqueFlite technical article to see what parts are needed for the conversion since there was another design change that impacts the conversion. For those interested in an aftermarket LA crankshaft, they are machined for the larger pilot bearing.

Transmission Adapters

Quality Engineered Components is the only company that I know of that makes an adapter to fit 1962 onward automatic TorqueFlite 727 and 904 transmissions to a 1956 – 1961 A-block. For using 1962 onward Mopar standard transmissions on 1956 – 1961 A-blocks, early hemi specialists such as Quality Engineered Components, Hot Heads, and TR Waters make adapters for early hemis that will work on A-blocks since the crank flange and bellhousing bolt pattern are the same. There is no adapter readily available to mate a pre-1962 iron PowerFlite or TorqueFlite transmission to a 1962 – 1966 A-block.

Flexplates

All 1962 onward A-block and LA 273/318/forged-crank 340 flexplates interchange and are neutral balanced. LA360 and cast-crank 340 flex plates may be weighted to balance the rotating assembly, although some were neutral balanced when the converter was weighted. If using an LA360 or cast-crank 340 felxplate, it should be confirmed to be neutral balanced.

NV3500 and NV4500 5-Speed Transmissions

The New Venture Gear NV3500 and heavy duty NV3500HD 5-speed manual transmissions are becoming popular swaps into mild LA engines and can be used with 1962 – 1966 A-blocks. They have an integral bellhousing that physically aligns with and bolts up to 1962 onward A/LA blocks except for the lower left bolt hole, which will not impact the bellhousing’s integrity. The NV3500 can be found in 1993 – 2001 Dodge Ram pickups with Magnum 318s, and the NV3500HD can be found in 1994 – 2004 Dodge Dakotas with Magnum 318s. GM also used the NV3500, but I focus on the Mopar version since they bolt to the A-block. The NV3500 and NV3500HD are well suited for most mild and stock street applications due to their gear ratio starting with a 4.01:1 first to a 2.32:1 second. Keep in mind that they are hydraulic clutch systems, so plan on using the flywheel, clutch, fork, slave cylinder, throw-out bearing, Denso starter, etc. The Magnum 318 is internally balanced just like the A-block, so the flywheel is neutral balanced and will not cause balance issues with the A-block.

The NV4500 is another option for A-blocks that will see towing and/or off roading. It has a separate bellhousing. The Mopar NV4500 usually came behind externally balanced Magnum 360s, so the flywheel is weighted and will not work on internally balanced A-blocks, so a Magnum 318 flywheel should be used. The NV4500 has a very low first gear of 5.61 that jumps to a second gear of 3.44. This combination will not play nicely with performance builds interested in acceleration. Of note, the 1993 – 1994 Chevrolet and GMC NV4500 has a torque-crushing first gear of 6.34:1 for those interested, but the bellhousing and input shaft length differ from the Mopar version and will need addressing. The Mopar NV4500 can be found in 1994 – 2004 Dakota, 1994 – 2001 Ram 1500, and 1994 – 1995 Ram 2500 Light Duty. As with the NV3500, the clutch is hydraulic. Plan on using the bellhousing, clutch, fork, slave cylinder, throwout bearing, etc.

To summarize my input:
  • 1956 – 1961 automatic A-blocks require a pre-1962 cast-iron PowerFlite/TorqueFlite.
  • Running a 1962 onward TorqueFlite 727 or 904 transmission on a 1956 – 1961 A-block requires an adapter kit.
  • No adapter exists to mate a pre-1962 iron PowerFlite or TorqueFlite transmission to a 1962 – 1966 A-block.
  • 1962 – 1966 A318s use the same crankshaft forging as LA 273, 318, and forged 340, although balanced for the A318 rotating assembly.
  • 1962 – 1966 A318s share the same bellhousing as all LAs.
  • 1962 – 1966 A318s will directly accept all 1962 onward TorqueFlite 727 and 904 transmissions.
  • Crankshafts used on factory automatic vehicles may not have the pilot bearing hole properly machined and may require machining if converting the to a standard transmission.
  • The standard transmission pilot bearing O.D. increased in 1968 and should be confirmed on the crank.
  • One can install a 1968 onward 24-spline input shaft in a 1962 – 1967 TorqueFlite to use the wider variety of later converters, but specific parts must be changed in the transmission as detailed in my technical article.
  • All internally balanced 1962 onward A-block and LA 273/318/foreged-crank 340 flexplates are neutral balanced and interchange. Some cast-crank 340 and LA360 flexplates are balanced for the rotating assembly and should not be used unless confirmed to be neutral balanced.
  • The Dodge Magnum 318 NV3500 and NV3500HD and the Dodge Magnum 360 NV4500 5-speed standard transmissions can be used on A-blocks when using the Magnum clutch system and Magnum 318 neutral-balanced flywheel.

Figures

Poly 318 Crankshaft
Figure 1a: 1956 – 1961 Crank
Poly 318 Crankshaft
Figure 1b: 1956 – 1961 Crank
Poly 318 Crankshaft
Figure 1c: 1962 – 1966 Crank

Poly 318 Bellhousing
Figure 2a: 1956 – 1961 Block
Poly 318 Bellhousing
Figure 2b: 1962 – 1966 Block
Poly 318 Crankshaft
Figure 3a: Auto Crank
Poly 318 Crankshaft
Figure 3b: Manual Crank
Mopar A-block poly machining schematic for crank
Figure 4: Pilot Bearing Machining

Mopar A-block poly pilot bearing 4338876
Figure 5: Magnum 433887 or SKFB287 Pilot Bearing

Mopar A-block poly pilot bearing FC69907
Figure 6: Magnum FC69907 Pilot Bearing
LA Poly Crankshaft
Figure 7: 1968 Onward LA Crank Flange w/ Large Pilot Holes

powerflite
1959 Iron PowerFlite

iron torqueflite
1957 Iron TorqueFlite
NV3500
Dodge Magnum NV3500

TorqueFlite
1962+ TorqueFlites
727 and 904
1962+ 727/904 Dimensions

727 and 904 gaskets
1962+ 727 and 904 Oil Pans