Block Design

Words: Larry Shepard

The engine’s cylinder block is the basic foundation for the engine. The block is the largest single part of the engine and tends to be the part that holds everything together. Foundations don’t get much glory so the block tends to be overlooked. In the past 50 years of production cars and their engines, there have been very few special blocks. In the early 1960s, even the race versions of these production engines used the production block. The two exceptions to this statement were the 1964 426 HEMI® and the 1970 340 Trans-Am. In the early 1970s thin-wall casting blocks were introduced in production, so the early blocks became more valuable and were hunted almost into extinction. In the early 1970s, specially designed racing blocks began to show up. By the late-1970s, many HP engines were going out of production. During this same time, interest in racing was increasing and the restoration market was growing off-the-charts. From the mid-‘70s to the late ‘80s, the general race engine output and performance had been going up (higher horsepower and higher RPMs) many engine builders began over-machining the basic engine parts plus these parts tended to be over 20-years-old, and the number of cycles was very high. They were being worn out!

The block is the largest single piece of the engine and therefore it is very expensive to re-tool. Once the basic engine has gone out of production, it must be re-tooled completely to make it available for all the various applications. Typically a few years of blocks are set aside by service to cover their requirements but popular performance engines tend to use this supply up very quickly. When you re-tool a block, you have the opportunity to change the design and add features that the production version may not have offered. This isn’t as easy as you may think. While it is probably much easier to design an all-out race block, like the NASCAR R5 race cast-iron block P5007427 than it is a HP replacement block, the race sanctioning body has very specific rules relating to blocks and what is allowed that can make these projects extremely difficult. I will limit this discussion to a replacement block or a dual-purpose HP application. There are a lot of things to consider such as the number of cylinders (the Viper uses 10 and there are many 4s and 6s) and the bank-angle (the angle between the two cylinder banks on a “V” engine). That means that we’ll discuss V-engines, rather than in-line engines. The typical bank-angle is 90-degrees but there are many 60-degree V-6s. The A4 in-line 4-cylinder race engine has two versions of the aluminum block – the drag-race version P5007636AB (W9 heads) is straight-up while the midget-racing version P5007466 is laid over for better weight distribution (somewhat like a Slant Six engine). These are examples of installed engine angles. For the general 90-degree V-8 replacement blocks, it is best not to change these basic specs.

After the number of cylinders (8 selected) and bank-angle (90-degree selected), perhaps the next most important specification is the block’s cylinder bore centers – the distance from the center of one cylinder to the center of the adjacent cylinder (#1 to #3 for example). There are two main numbers – 4.80" which is shared by the B (383-400), RB (413-426W-440) and 426 HEMI, and 4.46" which is shared by the A-engine family (318s, 340s & 360s) and the Magnum® family (5.2L & 5.9L). The next aspect of the block is the size of the main bearings. The RB (440) and 426 HEMI share the same main bearing size while the B-engine (383-400) uses smaller mains. The 318 and 340 share the same main size and the 360 uses a larger size. This makes the 360 race block P5153452 a unique part (only race A-engine with larger mains). The 5.2L Magnum is the same as the 318/340 and the 5.9L Magnum is the same as the 360. The other aspect of main bearings in the block is the main bearing bolts. The 1970 340 T/A block had the provision for 4-bolt mains. The replacement 340 block P5007552AB (4.04" finished bores) has 4-bolt mains on #2-3-4. The 426 HEMI uses cross-bolted mains on #2-3-4 while the 383-440 groups use 2-bolt mains. Most of the HP 440 blocks use the stiffer and stronger HEMI-style cross-bolted mains.

The next aspect of the block to consider is the deck height or height of the block, which is measured along the cylinder centerline from the crank/main bearing centers. The tallest of the blocks is the RB (413-426W-440) and the 426 HEMI at 10.72". The standard height 440 cast iron block P5153944 has a rough bore of 4.31". This allows the blocks to be finish-bored to the stock 440 bore size of 4.32". The second tallest block is the B engine (383-400) at 9.98". The standard A-engine is next at 9.60" followed by the Magnum. There are some short-deck, race A-engine blocks with potential heights in the 8.90" to 9.30" area but they make the replacement block discussion very complicated – see the latest Mopar Performance (MP) catalog for more details on these specials.

The next popular block specification is the cylinder bore size or capability. Each block has its own bore limits but in all cases it is directly related to the block’s bore centers. As you make the cylinder bores larger in diameter, the bore wall thickness gets thinner. Making the bore walls thicker to allow bigger bores makes the block’s water jacket much thinner especially between the cylinders. Siamesing the cylinder bores, which means there is no water between the adjacent cylinders, allows bigger bores and thicker walls. Why is this important? Generally, thicker bore walls makes more horsepower. More specifically, more bore wall thickness in the major thrust direction (90 degree to crank centerline) is the key. The bore wall thickness, water jacket, finished bore and valvetrain must all fit in the same space which requires compromises. The siamesed-bore 340 replacement block is P5153478AB and has a finished bore of 4.04". It can be over-bored to 4.22". The siamesed-bore cast iron 440-style block is P5153860 which has a finished-bore of 4.50". The siamesed-bore 426-style HEMI block is P5153862 (finished bore of 4.50"). Note: the cylinder bore description of rough-bored means that the bores must be finish-bored and then honed before it can be used. Tip: if you only have one block, sonic-test the bore wall thickness BEFORE you bore or over-bore the block!

Deck thickness on production blocks tends to be around .500". HP and racing blocks can be much thicker. The thicker decks on race blocks are designed for added milling capacity. If the block is milled too much, the deck will be too thin and will not support the head gasket properly—making the engine more prone to head gasket failures. The holes on the head gasket must line up with the holes on the block’s deck surface and the head. There can be holes in the deck surface that are not open in the gasket because the gasket is used to control the water flow through the engine assembly. Tip:if you plan on milling the deck surface, check the gasket to be sure that it still seals the water holes because they tend to move with deck-milling.

Next is the location of the cam—the A-engine and Magnum location is much higher than the B-RB-426 HEMI location. On the B-RB-426 HEMI engines, the cam location can limit the amount of stroke that fits in the engine (rods hit the cam lobes) which limits stroke and therefore cubic inches. Therefore, for racing, there are raised-cam versions of these blocks. Note: this raised-cam feature must be done by the manufacturer. The tappets on the B-RB-426 HEMI engines are at 45-degrees. The production A-engines and Magnums use 59-degrees. There are some versions of the A-engine race blocks, called R3s, that use 48-degree tappet angles. These 48-degree blocks require special cam blanks. Note: the revised tappet angle (for racing applications) is determined by connecting the rocker arm adjuster and the center of the cam in a straight line for the best valvetrain dynamics. See the MP catalog for more details.

The basic B-RB-426 HEMI engines have a skirted block design which adds strength and stiffness while the A engine and Magnum families put the pan rail at the main-cap parting-line. This makes the big-block family have basically a flat oil pan attaching rail while the A engine and Magnum pans have a radius on each end. The 318-340 blocks share one size radius while the 360 uses a different radius.

There are many other features of the block that have to be considered in any block design, items like head bolt holes (the A-engine and Magnum have 4 bolts around each cylinder while the B-RB-426 HEMI use 5). The top hole in the B & RB bolt pattern is a bolt while the top location on the 426 HEMI is a short stud. Oil pumps and oil pickups have to be considered, along with valvetrain and pushrod clearances. One of the truly unique features of the 426 HEMIs is the way the valvetrain oil is returned to the crankcase from the cylinder head. While all wedge-style heads return the valvetrain oil into the tappet-valley area, the 426 HEMI has two cylinder head oil returns – one on each end of the head, along the outer/lower valve cover wall. Therefore there has to be a matched boss on the end of each cylinder bank in the block to allow oil to get to the crankcase. Note: the bosses must be cast-in while the actual holes are drilled.

All of the reproduction blocks mentioned above are made of cast iron. You can also make blocks out of aluminum. You can’t just change the material because the aluminum blocks must use a cast iron sleeve in each cylinder. While they can be cast-in, they are more typically pressed in. That way they can be easily replaced or repaired which is very desirable in performance applications. There are currently 3 aluminum blocks – the 6.1L HEMI Gen III P5153897, the 440-style wedge P5153868 (finish bore 4.50") and the 426 HEMI Gen II P5153864.

More blocks and part numbers are being added to Mopar’s lineup. For additional applications, please refer to the latest Mopar Performance catalog or visit www.mopar.com. Space limits my discussion; if you have specific questions, please call the Mopar Direct Connection Tech Line at 1-888-528-HEMI (4364).