Dart’s Latest Revisions: M2 Small- and Big-Block Chevy Castings

There’s a parable in the bible about what happens to the foolish man who builds his house on a foundation of sand and is washed away in the rain and floods. The wiser man (who obviously is also a hot-rodder) built his house on a foundation of solid rock that survives the weather.

The engine builder’s equivalent of a solid foundation is nothing less than a quality cylinder block. It’s no secret that we’re living in the golden age of horsepower. But, one of the caveats of this horsepower escalation is that a casting that was happy at 400 horsepower may be the equivalent of quicksand when subjected to loads of 700 horsepower or more.

Dart Machinery has long understood these demands and recently upgraded its cast-iron blocks to what is now called the Little M2 and the Big M2. There’s more to these blocks than just a name change.

Before we get into the individual details, there are a few overall points worth investigating. Both Dart blocks are available in either Sportsman or Pro versions. The main difference between these options is the Pro blocks are equipped with steel main caps while the Sportsman caps are ductile-iron. Both versions are available in multiple bore diameters and deck heights. Taller deck heights will accommodate longer stroke cranks for those big-inch builds.

Among the many advantages of a Dart M2 block is its priority main oiling where a separate main galley from the oil pump feeds oil directly to the main bearing and then up to the lifter galleys. Production blocks feed oil first to the lifter galleys from around the cam bearing and then down to the main bearings. There are also other options such as raised cam locations and 55mm cam bore diameters.

Constantly Improving Design

Perhaps the most important difference between all Dart blocks and a production piece is the addition of a dedicated priority-main oiling system. Chevy production blocks route oil from the oil pump through the filter and then to a galley that first lubes the lifters while pushing oil around the outside of the cam bearings before it arrives at the main bearings.

Dart directs oil from the main galley to the main bearings first, then the oil turns the corner to feed the cam and lifters. This is called priority-main lubrication. While not necessary for an engine loafing down the freeway, it is essential when pushing four-digit power out of a race engine at 8,000 rpm.

Raising the vertical position of the cam housing bore is also an option for the new M2 blocks. This creates additional clearance for connecting rods, while also shortening the distance between the lifters and the rocker arms. The shortened pushrod length makes them inherently stronger.

Dart builds all of its M2 blocks with siamesed cylinder walls to allow larger bore diameters. Plus, even with larger bore diameters like a big-block with a 4.625-inch bore, the cylinder wall will feature a minimum of 0.300-inch wall thickness. This photo indicates the relative cylinder wall thickness on the outboard thrust side painted in red.

Dart M2 blocks also feature thicker deck surfaces for increased overall strength. The deck height values stated in the charts will be delivered at roughly 0.005-inch taller than the stated value to allow room for minor machining to establish proper deck height with custom pistons.

Cylinder wall-thickness is another major feature of Dart’s M2 block lineup. Production bore spacing of 4.400-inch limits the largest bore size on a small-block Chevy to 4.185-inch. But even at this point, the Dart M2 blocks guarantee a wall thickness of 0.275-inch. Big-block foundations in either the Pro or Sportsman option deliver a 0.300-inch minimum figure at maximum bore size.

The most important wall-thickness is always at the thrust side of each bank. The piston thrust surface is the inboard side of the left (driver) side of the block with the outboard side as the major thrust surface for the right or passenger side of the block.

This main web cutaway (left) reveals the additional strength created through the use of a splayed outer main cap bolt. Angling these outer bolts into a beefier portion of the block reduces the cap load distortion on main bearings. Note the use of the steel four-bolt main caps on all five journals of the Big M2, along with clearance pan rails that can accommodate 4.75-inch stroke applications on tall deck

Better Ingredients, Better Block

Among the reasons these blocks are so good is that they are cast using an enhanced 32B iron, which offers a higher 32,000 psi tensile strength rating, combined with a Brinell hardness rating of 207 to 255 compared to grey cast iron’s 187-241. This also necessitates changes to the honing process used for cylinder-wall preparation compared to production blocks.

An additional option for those operating on the maximum power side of things, is the option of choosing compacted-graphite iron (CGI) which offers even greater strength and hardness advantages over even the enhanced 32B iron.

Now let’s get into some specifics with the Big M2 blocks. Today, a big-inch street motor starts at 540ci and escalates the displacement from there. This demands larger bore-diameters and perhaps a taller deck-height to satisfy the demand. The new Big M2 offers a variety of options listed in an accompanying chart that outlines the available bore diameters and deck heights. In fact, there are 15 new part numbers for the M2 which are a direct result of customer input.

The Big M2 Pro block also offers dowel pins for each of the main caps to maintain main cap locations under high-load situations.

 

Available Bore Diameters and Deck Heights

Small-Block M Big-Block M
Bore Diameter Deck Height Bore Diameter Deck Height
4.00 9.025 4.250 9.800
4.125 9.325 4.500 10.200
4.125 9.500 4.560 10.400
4.600 10.400

This new Big M2 also includes a beefier casting, providing for larger lifter-bore diameters and height to accommodate the longer Gen-V lifters. On raised cam blocks, the lifter valley will look slightly different as the lifter bore area will appear as a solid area, which can be machined to allow the addition of the optional Dart X lifter pattern.

The Dart X program moves the lifter locations to straighten the pushrod angles when used with a Dart Big Chief head. This option is available for both the Big M2 with steel billet caps as well as the Big M2 Sportsman. Of course, this revised lifter bore area can also accommodate larger lifter diameters, as well.

Raised-cam Big M2 blocks will also feature a revised lifter valley area which can accommodate either larger lifter bore diameters, such as .904-inch or use the Dart X lifter pattern that revises the lifter bore placement to straighten pushrod angles when used with Dart’s Big Chief heads.

Moving over to the small-block side, the Pro Little M2 version offers similar upgrades with steel caps and four bolts for all five main caps. We’ve listed the bore diameters and deck height options in the chart above,  along with a choice of either 350-style (2.45-inch) or 400-style (2.650-inch) main journal diameters. While smaller mains are very much in vogue, the larger main offers advantages when opting for longer crank strokes to increase strength by overlapping the main and rod journals.

Dart M2 Displacement Options

Small-Block Chevy Options Big Block Chevy Options
Displ. (ci) Bore Stroke Displ. (ci) Bore Stroke
383 4.030 3.750 496 4.310 4.250
420 4.155 3.875 510 4.310 4.375
427 4.125 4.000 540 4.500 4.250
434 4.155 4.000 557 4.500 4.375
440 4.185 4.000 565 4.600 4.250
572 4.560 4.500
588 4.560 4.375
598 4.600 4.500
604 4.625 4.500
632* 4.600 4.750
638* 4.625 4.750

*10.200-inch deck height

Many of the improvements to the professional level Little M2 block also apply to the Sportsman version. You also have the choice of two different deck heights, optional raised cam locations, all while maintaining most small-block Chevy small parts. The Little M2 features steel main caps on all five journals with splayed outer bolts on the middle three webs. The front and rear caps utilize vertical bolt arrangements due to space limitations.

Longer strokes on small-blocks may require additional clearancing. However, Dart offers custom machining options where they will perform this modification to your new block right in the factory. We’ve included another chart that lists all the different custom machining options.

As the future of hot street and race engines becomes increasingly more specialized, these Dart blocks offer a widely populated path of cylinder blocks that offer benefits, which even 10 years ago were only available to professional race teams. The whole idea is to achieve your horsepower goal while increasing reliability. That plan always starts with a rock-solid foundation.

Let’s talk about attention to detail. Dart’s M2 small-blocks come with a small chamfer on the Number 5 main cap bolt washer (left) that is there to clear the oil pump drive shaft collar to eliminate a potential interference problem. All of the M2 blocks come with custom AN O-ring galley plugs instead of traditional pipe plugs.

Dart Custom Cylinder Block Options

  • Custom deck height
  • Specific bore size
  • Cam bore resizing
  • Lifter bore sizing and relocating
  • Bronze lifter bushings
  • Stroker clearancing
  • Block lightening
  • Main stud kits
  • Compacted-Graphite Iron (CGI)
  • Piston oil squirters
  • Block prep

 

Article Sources

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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