Poly 318 Crankshafts and Transmissions

Invariably, an A-block needs a transmission. There are multiple things to consider when choosing a transmission, which include block, bellhousing, crankshaft, and torque converter that I discuss here. I include details about the changes from early and later Poly A-blocks.

Crankshafts

First, all A-blocks 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 small blocks.

1956 – 1961 A-block crankshafts have a 1/2″ longer rear flange and 8 bolts to mate to cast iron PowerFlite/TorqueFlite transmissions (see Figures 1a and 1b below). A 1962 onward aluminum TorqueFlite and Magnum NV3500/4500 will not bolt up to a 1956 – 1961 crankshaft without an adapter (see details below on adapter setups).

1962 – 1966 A318 uses the same crankshaft as an LA 273/318/340 with 6 bolts, although balanced for the A318 rotating assembly. A great benefit of the A318 is that all factory crankshafts are forged, whereas factory LA crankshafts were cast from 1973 onward. One can physically bolt a 1962 onward crankshaft into the 1956 – 1961 block, but there is no adapter available to make the 1962 onward shorter crankshaft flange with 6 bolts work with pre-1962 transmissions.

When swapping A318 rotating parts, it is acceptable to swap parts that are within +- 1 gram of each other without requiring rebalancing, keeping in mind that the rods should be weighed using a stand to measure the weight of each end. If components fall outside +- 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) requires a machinist checking and possibly balancing the rotating assembly. When swapping connecting rods, keep in mind that Mopar changed the design from the earlier A318 lightweight to a heavier forging. Neglecting to properly balance a rotating assembly will create vibrations, rob power, limit the rpm range, and may lead to parts failure.

Blocks

In coordination with the two crankshaft designs, the 1956 – 1961 block is different from the 1962 – 1966 block (see Figures 2a and 2b below). The 1956 – 1961 blocks share the same bellhousing bolt pattern and alignment pin configuration as the early hemi/hemi-block polys. The 1956 – 1961 A-block has a similar bellhousing bolt pattern to the LA, but the alignment pins are in different locations. 1962 – 1966 A318 blocks share the same bellhousing configuration as LAs.

Crank Flange Machining

We now must deal with the torque converter/pilot bearing crankshaft machining. All 1962 – 1967 A and LA crank flanges were machined with a 1-3/4″ diameter hole to accept the TorqueFlite 904/727 small-diameter converter snout (see Figure 3a below) 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 (see Figure 3b below). 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 below 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 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 (see Figure 3c below), although they 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 find, 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, but 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 727/904 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.

Flexplates

All 1962 onward A/LA 273/318/forged-crank 340 flex plates interchange. LA360 and cast-crank 340 flex plates can be weighted to balance the rotating assembly, although some were neutral balanced when the converter was weighted. If using a cast-crank 340 or 360 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 rigidity. 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, 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 A-block crank flange may require machining to accept the pilot bearing, as I discuss above under “Crankshafts.”

The NV4500 is another option but better suited 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 requiring a Magnum 318 flywheel. 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., and plan on using a Magnum 318 flywheel.

To summarize my input:
  • 1956 – 1961 automatic A-blocks require a pre-1962 cast-iron PowerFlite/TorqueFlite.
  • Running a 1962 onward transmission/bellhousing on a pre-1962 A-block requires an available adapter kit.
  • 1962 – 1966 A318s use the same crankshaft as LA 273, 318, and forged 340 although balanced for the A318 rotating assembly.
  • 1962 – 1966 A318s share the exact same bellhousing as all LAs.
  • 1962 – 1966 A318s will directly accept all 1962 onward TorqueFlites.
  • The standard transmission pilot bearing O.D. increased in 1968 and should be confirmed on one’s 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/LA 273/318/340 flexplates 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 are an option for A blocks.

Figures

Mopar A-block poly early crank
Figure 1a: 1956 – 1961 Crank (photo J. Lotspeich)

Mopar A-block poly and LA later crank
Figure 1b: 1962 – 1966 Crank (photo J. Lotspeich)

Mopar A-block poly early bellhousing flange
Figure 2a: 1956 – 1961 Block (photo J. Lotspeich)

Mopar A-block poly and LA later bellhousing flange
Figure 2b: 1962 – 1966 Block (photo J. Lotspeich)

Mopar A-block poly and LA automatic crankshaft
Figure 3a: Auto Crank (photo J. Lotspeich)

Mopar A-block poly and LA manual crankshaft
Figure 3b: Manual Crank (photo J. Lotspeich)

Mopar LA late crankshaft
Figure 3c: 1968 Onward LA Crank Flange w/ Large Pilot and Converter Snout Holes

Mopar A-block poly machining schematic for crank
Figure 4: Pilot Bearing Machining (drawing Brewers Performance)

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

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