Mopar Small-Blocks. Larry Shepard
family is a basic 90-degree V-8. This angle is best for dynamic balance considerations and also makes for a very rigid block.
Once the block is disassembled or the new block is obtained, it is usually mounted to the engine stand. It is typically mounted upside down or crank-side up because that section is where you are going to start.
The bare block, no plugs, is the first step in preparation for sending the block to the machine shop. In this common position, the block is upside down and the passenger’s side is toward the left, which can be confusing. If this were a Chrysler race block (cast-iron versions), the large “X” or “R” would be located on the passenger-side face of the block, which is also the passenger-side face as installed in the car. It is on the front of the number-2 cylinder, above the dipstick holes and the core plug.
Both A-engine and Magnum blocks share the same 10-bolt head bolt pattern, or 4 bolts around each cylinder, but some of the race blocks add 2 more (for a total of 6) per cylinder, and this is covered later in this chapter.
Cylinder numbering begins with the driver-side front cylinder; number-2 is the passenger-side front cylinder. The A-engine has a production deck height of 9.60 inches; the Magnum group uses 9.585 inches. Small-block race blocks have deck heights as low as 9.0 inches. Both groups of small-blocks share the 4.46-inch cylinder bore centers and the 6.125-inch camshaft centerline height. The LA-engine block is lighter than the original A-engine block and weighs about 160 pounds. The newer 1973–1974 thinwall cast versions are about 4 to 8 pounds lighter, which carries over to the Magnum blocks.
Although there are four different cylinder bore sizes, Chrysler made two basic blocks. One is the 273, 318, and 5.2L block; the other is the 340, 360, and 5.9L block. Most A-engines use a motor-mount design that attaches to the sides of the block close to the front, and the mount ears and bolts are parallel to the cam centerline. The 273 and 318 share the same arrangement.
However, the 340 and 360 share the same driver’s side configuration, but the passenger’s side has the three-bolt pattern reversed.
The Magnum engine, originally used in trucks, had a three-bolt pattern on each side, and these mounts bolted into the side of the block itself. This bolt pattern is located in the center of the block starting just above the pan rail. In addition, these Magnum blocks had the ears cast in and machined for the earlier system.
Both groups of blocks have three core plugs per side; the 318 group has them wide-spaced, and the 340–360 group has each core plug aligned with the center of the water jacket between two cylinders.
The easiest way to identify a small-block Mopar is to compare the casting numbers. Each casting is unique. In other words, you can’t machine out the 273 bores to make a 318. You also can’t make a 340 out of a 318.
These rules don’t apply to 340 and 360 blocks. You could probably bore out the 360 block to the 340 stock bore size of 4.04 inches, but the 360 has a large main bearing diameter crank. The large main bearing diameter makes the long-stroke 360 (3.58 inches) much stiffer. In addition, the 340 oil pan’s front and rear sealing surfaces are the same as the 318’s; the 360’s are smaller.
The 1970 340 T/A is the trick block for racing applications because it has thick bulkheads on the number-2, -3, and -4 main bearing bulkheads. The added thickness allows for vertical four-bolt main caps to be added to these center mains. None of the Magnum blocks have this thick bulkhead. In the mid-1970s, Chrysler offered a four-bolt main race block known as the X-block (it had a large “X” on the passenger-side front wall of the casting). In the mid-1990s, Mopar Performance and Chrysler revised this race block and offered the R1 through R4 race blocks. These were generally called the R-blocks and all featured four-bolt mains.
When you are selecting a block for your particular build, remember that for all five of my performance packages, the standard 340/360 or Magnum block can be used as long as the bore size is limited to .020/.030 overbore for the last two packages. An R-block is not required as long as the blocks are limited in overbore. If a big-bore engine is desired, an R-block is required, and the siamesed-bore version allows the largest bores. The 340 resto block is a version of the R-block. If your 340 or 360 block is already overbored too far, sleeve the bore back to 4.020 inches (360) or 4.04 inches (340).
The 340 T/A block is very difficult to find; it’s even more difficult to find someone willing to give one up for a performance project. The 340 resto block, by Mopar Performance, is more readily available. It uses the 340 T/A casting number and the 340 displacement number after it, followed by an “M,” all the way to the right after “340.”
The 318/5.2L and the 360/5.9L are the two groups of small-blocks. You can identify them easily by quickly looking at the core plugs located in the sides of the blocks. There are three in each side. In the 340 and 360/5.9L group, the core plugs line up with the three center bulkheads, and if the plugs are removed, you can see the water-jacket gap between adjacent cylinders.
On the 318 and 5.2L blocks, the core plugs are spaced farther apart and don’t line up with the bulkheads, except the center one is close to the number-3 main. The R-block will support the most power, then the X-blocks and then the T/A and then all stock blocks. Magnum blocks are slightly stronger than the standard A-engine blocks. The stock blocks with 0 to .020/.030 overbore have done over 550 with no problem (good bolts and good main caps) but the upper limit is unknown. Most members of the 700-hp club use some version of the race block. The race blocks make more horsepower.
Around 2000, Mopar Performance started offering a 340 restoration block that not only had the thick bulkheads of the 1970 T/A block, but also had actual four-bolt mains on the center three mains. It used the same casting number as the original 340 T/A with a 340-M added, but the siamesed-bore block is no longer offered. This stout non-siamesed bore block can support at least 1,000 hp, but availability is limited. The R3 race block is an excellent block for 700-hp street builds, but these can be expensive and difficult to obtain so other choices are offered.
The large round hole with a plug in the center is the cam’s rear plug. The threaded square-drive plugs on either side of the cam plug are the oil galley plugs. These allow the oil galleys to be drilled and then sealed. The main oil feed galley is the one on the right.
The 318 Magnum block is tipped over to the driver’s side so the passenger-side deck is horizontal. The oil galley plugs are in the middle toward the bottom. The distributor bore is at the left and the china wall runs up toward the right from the large distributor bore. The rear intake gasket seal is just ahead of this surface. At the upper end of this machined surface is another oil hole typically used for oil pressure senders. It