The cylinder block is the largest and heaviest single piece in the engine assembly. It is the center of everything that you want to do to the engine and the foundation for any engine project you have in mind. The cylinder heads, camshafts and intake manifolds/induction system tend to get a lot more attention but there are a lot of interesting aspects to the block.

The bellhousing bolt-pattern is machined into the rear face of the block and there are two basic Mopar® patterns – the A-engine (small block) and the big block (B-383, RB-440 and 426 HEMI® Gen II). This bolt pattern affects the manual transmission bell housing or the automatic transmission's case. The typical production block can be over-bored about .060". This worked well until the thin wall castings came along in about 1972. Then the over-bores were limited to about .020"/.030". This over-bore limitation is related to the resulting bore wall thickness. In the early 1970s in the new Pro Stock class, the 426 HEMI (Gen II in today's lingo) engineers found that the engines that went fast had thicker cylinder bore walls in the major thrust direction (toward right side of bore as installed in the car).

Sonic testing was found to be a useful, non-destructive tool in determining how thick the bore walls were and where that material was located. This began as a selection process, which means that you have two or four blocks and you want to select the best one. This evolved into measuring the bore walls for thickness. For this process to work properly, the sonic test must be performed BEFORE the block is overbored. For example, let's assume that you want to have .180" for your minimum bore wall thickness and you measure the current thickness at .200". That means that you have .020" per side of extra bore wall material so you could over-bore the block .040" from its current size. On the other hand, if you measured the block at .180" currently and wanted to over-bore .060", then you know that the resulting bore wall will be .150", which may not be acceptable.

A somewhat related issue to cylinder bore wall thickness is the engine's bore centers. This is designed in by the factory engineers and is one of the major features that define any specific engine. The bore center directly or indirectly affects the interchangeability of almost every part on the engine. There are two main bore centers: 4.46" used on the A-engine (small blocks including the Magnum® family) and 4.80" used on the big blocks (B-383, RB-440, and 426 HEMI). The bore center is the distance from the center of one cylinder bore to the center of the adjacent cylinder. For example, the distance from the center of cylinder number 3 to the center of cylinder number 1 or number 5. Let's assume that the normal minimum water jacket thickness, between the cylinder bores is .150" and that the minimum thickness between the cylinder bores is .125". Then we can calculate the maximum bore allowed on a given bore center. For example, using a small block bore center of 4.46", the two minimum bore walls calculate to .250" (you like .180" in the major thrust direction but they can be thinner, maybe .125" in the front-to-rear direction—less loads) and then add in the water jacket (.125") for a total of .375", which means that the biggest bore possible with a water-jacket is 4.085”.

So what happens if you want to use a 4.21" bore in a similar block? Adding the minimum thickness bore walls to the desired bore diameter (.250" + 4.21") yields 4.46". This means that the bore walls will touch. In a casting, they will actually join and these blocks are called siamesed-bore blocks. This is what allows for the much bigger bores used on many performance engines today. By definition, on a siamesed-bore block, there is no water between the cylinders.

Caution: the above paragraphs are a theoretical discussion of how to use sonic testing numbers in general. If you desire specific over-bore numbers and milling specifications, refer to the Mopar Performance catalog by specific block part numbers.

The water flow through the block is a very important aspect of the engine but it is usually not considered by the engine builder. It is not easy to get inside the water jacket of the block. Remember that the material for bigger cylinder bores and thicker bore walls comes at the expense of the water jacket. Note: factory engineers for NASCAR today flow the water jacket like an intake port—on the computer, but not something you can do at home.

Production blocks have server limitations on the actual bore size allowed by the block itself. Performance blocks are sold with a machined bore size which is usually listed in the catalog or spec sheet. For example, the siamesed-bore block for the 426 HEMI P5155072 has a finished bore of 4.25", the stock size. The 4.50" version is P5153862. The 440 wedge version of the siamesed-bore block with 4.50" finished bores is P5153860. Many engine builders want to hone the block themselves and if the block is finish-bored, then this isn't allowed. This is the reason that some blocks are listed as rough bore, like the 4.24" rough-bore HEMI P5153942 or the 4.49" rough-bore 440 wedge P5153863. These rough bore versions leave about .010" for your engine builder to prep the bores to his liking.

The block's deck height is defined as the distance from the centerline of the crankshaft to the top of the block's deck surface. There are two main heights—9.60" for the A-engine small blocks, and 10.72" for the 440 and 426 HEMI . Note: the 383-400 is 9.98" and the Magnum family is 9.58". The production blocks generally would allow a .050"-.060" mill off the deck surface. Performance and racing applications wanted more flexibility so many of these blocks were made with thicker decks so more material could be machined off. This hurt the water jacket by making it shorter. Many of these blocks had .750" thick decks so you could mill off .250"-.300". And then there is the special 9.025" short deck small block P4876381AB , which is based on the R3 castings. As a general statement, the shorter deck engines are fine for smaller displacement versions but they will limit the large displacement engines, so they aren't as popular today. Usually the deck height or milling number is listed in the catalog or spec sheet for each specific part number.

All of the blocks discussed above are cast iron blocks. There are also lightweight, aluminum blocks for the big block engine like the 4.50" version of the 426 HEMI P5153864 and the 4.50" bore version of the 440 wedge P5153478. There are rough-bore (4.49") versions of each.

The block typically mounts into the car by the bellhousing/transmission and the motor mounts machined or cast into the side of the block. The 440 wedge and 318/340 wedge engines have ears cast into the side while the 426 HEMI uses a small three-bolt pad machined into the side of the block. The Magnum truck engine used a large three-bolt pad (most Magnum engines are dual-machined, ears and bolts). The obvious exceptions are race-only blocks like the aluminum A8 small block (sprint car) P5007909AB and the aluminum A4 drag inline 4-cylinder for P5 heads (P5007467). These blocks are designed to be mounted by front and rear motorplates and have no motor mounts on the side.

There is an interesting situation today relative to the fairly new 340 replacement blocks (cast iron). In 1970, Chrysler built the 340 T/A performance package with a special 340 T/A block. This block was derived from the 340 and 360 production blocks. In the mid-1970s many of these T/A blocks were used in NASCAR racing until actual special race blocks became available. Now there is a 340 (or 340 T/A) replacement block P5007552AB (water-jacket) that has evolved from performance hardware by performance engineers based on customer demand. This 340 replacement block has the 340 T/A casting number plus an M cast into the side for identification. They have four-bolt mains, 318-340 crank journal sizes and are rough-bore to 3.900”, which would allow bores from around 3.91" (standard 318 size) to 4.080" (.040" over 340). I'd recommend planning on a minimum bore of a .020" over 318 – 3.93". There is also a siamesed-bore version of the replacement block P5153478AB.

The 5.2L and 5.9L Magnum small blocks are similar to the A or LA engine families. However, one unique aspect is that the 5.2L (318) Magnum block P5153579 and the 5.9L (360) Magnum block P5153452 oil the heads up through the pushrods, not through the block. Many race engine builders like to use valve trains that oil the valve gear through the pushrods so this becomes an issue for all performance blocks, including 340s, 440s and the 426 HEMI. Check before purchasing!

The 340 replacement block has four-bolt mains while the 440 and 426 HEMI performance blocks have cross-bolted mains. The 440 and 426 HEMI have a skirted-block design, and therefore they can have a cross-bolted design, while the 318-340-360 blocks do not have a skirt so they use 4-bolt main caps. In both cases, typically only the center three mains are four-bolt or cross-bolted.

The racing family of small blocks is generally called R3 blocks and these specific blocks feature a six-bolt attaching pattern for the heads since the race cylinder heads like the W8 or W9 have six-bolt heads. This pattern is designed as the standard 318-340-360 four-bolt pattern with a bolt added up and one down per cylinder to make six. Generally you can use a four-bolt head on a six-bolt block but don't try to add the extra bolts to a non-R3 block. Note: if you use a six-bolt block/head package, be sure to use a six-bolt head gasket.