Inside Dart’s New Big M Pro Big-Block Chevy Engine Block
Dart Machinery’s Dick Maskin has been in the depths of professional drag racing engine building for decades; most notably in the hotly contested ranks of Pro Stock, where he’s been involved in many championship teams. We say that to let you know, in case you didn’t already, that he’s a legit racer, and he knows what racers want. His company, therefore, caters to the hardcore racer, but is also careful to offer pretty much anything the hardcore street crowd needs to.
For years now, Dart has offered the Big M engine blocks for the big-block Chevy – cast-iron blocks with features unmatched in any stock factory block that allows you to stick in longer-stroke crankshafts and bigger-bore pistons for big cubic-inches – plus extra iron where it counts for strength, better oiling systems designed for more intensive use, and just overall more convenient features that allow an engine builder to do it his way. Then, a few years ago, Dart released its Race Series Tall-Deck Big-Block Chevy iron block. This one was far more hardcore than the Big M, offering spread bore spacing options (4.840 to 4.900 inches), a .600-inch raised cam core, spread oil pan rails (DRCE style), the same priority main oiling system as the Big M, and a very high 10.600- to 11.100-inch deck heights that allows the use of long rods with a big stroker crank. It would accept a crank stroke of 5.500-inch in order to build a real monster motor.
Big M Pro – Short Deck, Big Bore And Weighs Less
That block has been successful among racers, but it has its issue as well. Dart’s Jack McInnis told us, “We launched that tall-deck race series block a couple of years ago to have a further racing upgrade over a Big M-type block and what we ran into, although it has a great deal of flexibility, it’s very heavy. It’s a big hunk of iron. A lot of people were put off by that, so in talking to engine builders and racers, and looking at what their needs were, we came out with another casting that actually goes in both directions. It ranges from a short-deck, big-bore, raised-cam block, which I think is the sweet spot really. So you can have a big-bore, short-stroke engine with a big cam core, with the pushrod angles you want.”
In other words, it’s the best of both worlds for many engine builders. The Big M Pro can be had with a short 9.600-inch deck height (9.800 also), and the cam bore has been raised .600-inch, which allows you to use a bigger, stronger cam core, which is necessary with today’s extreme valve spring pressures and rpm.
We launched that tall-deck race series block a couple of years ago to have a further racing upgrade over a Big M-type block and what we ran into, although it has a great deal of flexibility, it’s very heavy. It’s a big hunk of iron.
This block allows bores up to 4.625-inch and a huge stroke, like the tall-deck Race Series block, of 5.500 inches, so if you take it all the way out you can end up with 731 cubic inches (that’s 12.0 liters to your goony import friends). But as McInnis says, “Of course, with the short deck that would produce truly awful rod angles, so 4.500-inch should really be considered the largest practical stroke for this deck height. Most builders using this block are favoring the big bore, short stroke concept.” This new block was aimed at the short-stroke crowd that loves rpm. Big bores will allow big valves to flow enough air to support big power, and the short stroke on the crank allows that rpm without all the rotating weight.
Carrying Over Dart’s Priority Main Oiling System
The oiling system is a carry-over from the Big M block, with true priority main oiling that directs oil to the main bearings before the lifters for extra reliability at high rpm. A stepped main oil gallery (9/16″ to 1/2″ to 7/16″) ensures uniform oil supply for all five main bearings, and two oil crossovers in the valley above the numbers two and four main webs to produce much better oil distribution to the cam and lifters.
4-Bolt Mains Hold The Crank In Place
On the bottom side, Dart makes its own steel 4-bolt main caps in house. The three center caps have splayed outer bolts that anchor them to more meat of the block, the stronger part, and the front and rear caps have vertical bolts so standard oil pans will clear. The rear cap uses a traditional two-piece seal. Another feature of the new Big M Pro that was a carry-over was the extra four head stud bosses located in the lifter valley. That provides more clamping load for the cylinder head to prevent blown head gaskets with either high compression ratios or when you really lean on the squeeze.
Finishing Options Available
You can get the blocks with a variety of options. They are optionally available fully finish prepped and if you order your block this way you can specify a final lifter spec and a final hone spec. Your block will be finish-machined to your specs, final prepped and washed, pipe plugs and freeze plugs will be installed, and then it will be bagged and boxed. So when you get it home, just wash it, blow it off, and you’re ready to start assembling. The extra beef in the Big M Pro has of course added some cost to it. Where the Big M is $2,587.74, the Pro version comes in a littler higher at $3,456.41.
Big Cubes, Less Weight. What’s to Loose?
The Big M Pro block has all the most requested features from a race series casting with all the added weight. It ranges from a short-deck, big-bore, raised-cam block, which allows you to have a big-bore, short-stroke engine with a big cam core, with the pushrod angles you want. With the ability to go up to 731 cubic inches, it is a great block for still a very affordable price, especially where racing classes are strict about running an iron block. We know here DragZine, we can’t wait until we can find an excuse to make a use out of a Big M Pro block on a future engine build!
- 9.600″ and 9.800″ deck heights.
- Raised cam location +.600″ clears stroker crank shafts.
- Oil pan rails are spread .750″.
- Accepts crankshaft strokes up to 5.500″for large displacement applications.
- Four valley head stud bosses prevent head gasket failures with high compression ratios and/or nitrous oxide. Slotted bosses allow the use of studs instead of difficult to install bolts.
- True priority main oiling directs oil to the main bearings before the lifters for extra reliability at high rpm. Stepped main oil gallery (9/16″ to 1/2″ to 7/16″) ensures uniform oil supply for all five main bearings.
- Oil crossover located at the front of the block delivers maximum oil volume to the main bearings and ensures reliable lubrication for the lifters and pushrods on both cylinder banks.
- Steel 4 bolt main bearing caps are manufactured in-house by Dart to ensure quality and compatibility with the block. Three center caps have splayed outer bolts that anchor the caps to the strongest part of the casting, front and rear caps have vertical bolts for oil pan clearance.
- Rear main bearing cap uses a standard two piece crankshaft seal without adapters or crank modifications.
- Uses stock components, with provisions for block mounted oil filter, mechanical fuel pump and stock water pump.
- Coated cam bearings, freeze plugs and dowel pins are included.