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Big-Buck Small-Block Solution - Motown Magic

Bill Mitchell's Sensible Alternative to Big-Buck Small-Blocks.

Aug 1, 2001

Step By Step

The basis for Bill Mitchell's affordable big-inch small-block is his proprietary design "Motown" cylinder case--or engine block. The most important aspect of the Motown block is its ability to be bored far larger than there are rings to fit. A 4.175-inch ring size is the largest ring available (normally for a Chrysler) which will leave .225-inch cylinder wall thickness. This produces a 438 cubic inch engine with a 4-inch stroke and a Fel-Pro 1004 head gasket. Theoretically, if there were larger 4.200-inch rings available, you could easily go up to 443 cubic inches and still have a .190-inch cylinder wall thickness.

The Motown block does not have a raised cam like the Rocket block. Even though this limits the stroke to 4 inches, it does simplify the parts compatibility. There are no special timing chains or belt drives required, just normal small-block components. The concern is almost moot because if you can get the additional displacement by means of bore, then you can keep stroke along more traditional dimensions.

The use of four-bolt nodular iron main caps makes the Motown block strong, but the doweled main cap alignment and larger 1/2-inch main cap bolts make it practically indestructible. Billet splayed caps are available on request but are not necessary on a street car. Strength is also improved by casting the front and rear bulkheads 1-inch thick compared to GM's .100 inch (typical).

The main webbing is the same thickness as a Chevy Bowtie block, and thicker in some places where GM machines or "windows" the webbing for weight savings.

You'll note that there are just two freeze plugs instead of the normal three found on a 400 block. This is an artifact of using one less post in the sand core--the water jacket is larger so the sand core is thicker, making the block easier to pour and less prone to core movement. Here you can see the additional material "bulges" around the cylinder bores.

Late-model street cars will benefit from the inclusion of mounting pads for the hydraulic roller lifter retainer. These are only tapped when the customer requests a hydraulic roller valvetrain. Note that the lifter bores are tall enough to accommodate factory-style hydraulic roller lifters. The Motown's lifter bores are indexed to provide greater control over production tolerances.

The rotating assembly for the Bill Mitchell 427 consists of a Scat 4340 forged 4-inch stroke crank (2-piece rear main), Manley 6-inch H-beam forged rods, Wiseco 20cc dished pistons, JE rings and Clevite 77 bearings. This motor is destined for an aluminum-headed street motor to be run on ordinary pump gas. Right now, the 427 is geared for pump gas so there is only one piston available.

The 6-inch Manley rod is mated to the Wiseco piston with a 1-inch compression height. This produces a 1.50:1 rod ratio with a 4-inch stroke.

The compression height of the 427 piston is one inch even. Customers who opt for a smaller 415 short-block will get a slightly higher 1.065-inch compression height. The dished piston crown is designed to work with pump fuel for most cylinder heads.

Mitchell's block is machined at only two CNC positions. Says Bill, "If a GM block sees 200 machines on a transfer line, there's a better chance its tolerances will vary."

Cylinder head bolt holes on the Motown are blind-tapped to prevent leaks. This photo displays the extra material around each cylinder.

All '86 and newer small-blocks were made with a one-piece rear main seal, but Mitchell uses the older two-piece style. Why? Because two-piece rear main cranks are more plentiful and more reasonably priced. If you're putting one of Mitchell's 427s in your late-model car, all you have to do is order an early-style flywheel or flexplate (since the bolt patterns are different).

All of Mitchell's 427 and 415 short-block crate motors are hand assembled in his Ronkonkoma facility. Not including the balancing, this unit took close to the entire day for one skilled worker to assemble.

Let's face the obvious here: Those of us wanting to bore and stroke our way to big cubes are really at a disadvantage when it comes to finding a suitable small-block to work with. If money were no object, a trick race block would be installed in everything from the family truckster to grandpa's Rascal--and dominating the drag strip would be a blip of the throttle away. Due to the fact that a roof over your head and a little dietary fiber is absolutely necessary, however, a big-inch bullet and words like "budget" usually don't go together. Why? Because cubes cost money. In light of this astounding revelation, here are a few options, moneybags: 1. Somehow locate a good used 4-bolt 400 and buy your way to bigger displacement, 2. Hawk a vital organ for an expensive big-block (and then find some way to shoehorn it into your small-block-only ride), or 3. Take the cash for your girlfriend's ring and get the best small-block that money can buy.

Now, before you go off and do something completely insane like getting between a girl and her wedding, listen up. There are several engine blocks available that have traditionally made good power with the right parts. Let's start at the bottom.

If you were seriously jonesing for big cubes, you could build a production 400 block. Reinforce the bottom end, verify that the cylinder walls aren't paper-thin, and don't lean on it too hard, and you would better your chances of not driving over the crankshaft someday. Sounds risky, doesn't it? For those that believe in the 'ounce of prevention' theory, the next step up would involve paying a little more money, but receiving much more for your performance dollar. GM Performance Parts offers several excellent Bowtie blocks, but even they share some of the more undesirable characteristics of the regular production blocks. The upper-echelon units are sturdy enough to withstand tons of horsepower, but sometimes require exotic valvetrain components. As you can see, block selection can be a feast or famine situation.

Fortunately, someone recognized this middle ground, and the result, Bill Mitchell's Motown, means that now you can have a big-cube (Can you say 427?) small-block without a painful price tag. By drawing comparisons between GM Performance Parts' Rocket and Bowtie blocks and a production 400, you will see that Mitchell's Motown is everything you cube-hungry Neanderthals need, and maybe more.

As the owner of Bill Mitchell Hard Core Racing Products, Bill Sr. has been screwing together performance engines for many moons. Sometime during the last 35 years, he began to notice an absence of suitable small-blocks to punch out.

"With the engine business, we started to get tired of having no small-blocks to hot rod," Bill explains. "We had all of the tools for the Merlin crate engine sitting around, and when your choices are between using a stock 350 or a 400 with no wall thickness, designing the Motown became a necessity."

And design they did. Since Mitchell casts the blocks himself, he was able to create a piece that eliminated many of the inherent flaws of a production block. Starting fresh, he made the Motown with better thickness and rigidity than production units, while still ensuring that it was completely compatible with production parts.

"It's great to take out a clean piece of paper and make something however you want," Bill says. "If you do that, it also turns out the way you want."

The Challenger

The Motown's cylinder wall thickness is perhaps the most important aspect of this evolutionary small-block. Since most production GM blocks can only be bored .030- to .060-inches safely, not much material is left to work with. The cast-iron Motown plays by an entirely different set of rules, emerging from Mitchell's two CNC machines sporting an outstanding .460-inch cylinder wall thickness with a 3.990-inch bore (honed to 4.000 inches). Hollow out the Motown and there will be .250 inches of material left at the maximum bore of 4.200 inches. This enormous bore allows the Motown's 4-inch stroke to produce 427 cubic inches--without running into the problems that a factory casting would.

"If you want lots of cubic inches, you need to bore (the factory blocks)," Bill explains. "The amount of ring seal determines how much power you're going to make. If the cylinder walls get too thin, and all of the combustion goes right through those rings, you don't make any power." In the case of this 427, it can be bored until you run out of rings to fill the hole--and still have .225 inches of cylinder wall left!

The Champ

In comparison with the Motown, the GM Performance Parts' Rocket block (part Nos. 22551788, and 22551790, raised deck) offers a .275-inch cylinder wall at its maximum bore of 4.155 inches. Both the standard and raised-deck-height versions of the cast-iron Rocket have a minimum .625-inch deck thickness, while the new Motown gets by with only .600-inch. Both blocks offer blind-tapped head-bolt holes to enhance head gasket sealing and prevent coolant leakage, priority main lubrication systems that direct oil flow to the mains before the cam bearings, the choice of 350- or 400-type main bearings, and expanded water jackets to enhance cooling. And at around $2200, the Rocket appears to be just as affordable when compared with the Motown.

Stickin' It To Ya

The real problems for us change-in-the-seat cushion guys are the Rocket's raised camshaft location and other proportional deviations from regular production blocks. Non-stock oil pan rails have been widened by .800-inch (Custom pan, anyone?). Cam journals have been enlarged. Oh, almost forgot, for the best 4340 3.750- to 4.25-inch crankshafts from Callies or Crower (You will want the best if you're buying this block, pal), it will run you about $1,700. And although the cam's .390-inch rise allows a 4.125-inch crank to clear (with potential displacement increases of up to 500 cubic inches with the right stroke), you can forget about using production valvetrain parts when building this giant. Jesel offers a belt-drive system for this motor, but at a thousand bucks, it's hardly cost-effective. With prices like that, it would be difficult to justify this block if you were on a tight budget. Yes, we also salivate at the words 'belt drive', but the Rocket's cubic inch-to-dollar ratio is, to use an unavoidable pun, in the stratosphere.

The Favorite

GM offers several different options for those who are considering a Bow-Tie block for their next buildup. In the interest of keeping this an apples-to-apples comparison, we wanted the potential for large displacement, CNC-machining, and compatibility to production parts. GM's 4-inch bore cast-iron racing Bow-Tie block (part No. 24502503) offers engine builders a solid foundation for any high-performance application. This block displaces 391 cubes at its max bore of 4.155 inches, using the designed maximum stroke of 3.75 inches. It's important to note that crankshafts as large as 3.875 inches can be used in Bowtie blocks, pushing the total cubic inches to over 400, but it is not recommended if longevity is an issue. This CNC-finished block features a blueprinted front and rear case, production-height cylinder deck and oil pan surfaces, siamesed cylinders, and factory oil pan widths (using a pre-'86 oil pan). For crank retention, the Bowtie relies on a four-bolt setup with 7/16-inch main bearing cap studs splayed at 20 degrees for the intermediate main caps, and 3/8-inch straight bolts on the outer caps. Nice standard features of this block include bearing cap inner bolts that spread to permit the larger 2.65-inch crank journal, and drilled and tapped galleries for a dry-sump oiling system, if you are so inclined to go that route. As a race-prepped block that doesn't need a slew of exotic hardware to run, the Bowtie certainly offers bang for the buck--but how does it stack up to the Motown?

Again, the most important issue here is cylinder wall thickness. Yes, at max bore, the Bowtie is almost as big as the Motown. But thanks to Mitchell's beefier block, a buyer is guaranteed a quarter-inch of thickness. The Bowtie is sent out only with data from a sonic bore check, and the technicians we spoke with had no way to verify how thick the block would actually be (Although, at its 4.155-inch max bore, it is supposed to come with a minimum of .250 inches also.) This isn't as crucial when you are first buying it, but it may come into play the first time your big-bore engine needs machining. Point: Motown.

One place where the Bow-Tie appears to be stronger is in the main caps. With 8620 billet used in all five locations, a side-by-side comparison makes you wonder why the Motown is beefed up everywhere from the cylinder walls to the main webbing, but doesn't use a splayed main bolt setup. The 427 actually does offer billet splayed caps, but half-inch main cap bolts and doweled and aligned nodular iron main caps will take whatever a nasty street car serves up. Also, the Motown's bulkheads are an entire inch thicker, a testament to Mitchell's dedication to providing an extremely strong bottom end.

"It was very important that the webbing was strong," he says. "If you look at an engine head-on, draw an imaginary line through it vertically, and you take the oil pan off, not much material is left holding the two halves together." Quite an interesting thought with as much power as these two blocks will see. Thicker webbing or no, both will provide the strength needed for a street-oriented buildup.

An area where the Motown definitely one-ups the Bow-Tie is price. Both are heavy-duty blocks with the potential for 750+ horsepower, but a race-prepped Motown, which includes washing and cleaning the block, boring and plate-honing, installing cam bearings and freeze plugs, and coating it with oil, goes out the door for $2125. Barring a sale, a similarly-prepped GMPP Bow-Tie will cost the average Joe $2,700 from Scoggin-Dickey. Keep in mind, you are buying into a block that when bored, will be back to where a standard production block is in regard to strength.

The Chump

At this point, it would almost be ridiculous to compare a production 400 block to the new Motown. Thin cylinder walls were a constant problem during the 10-year life span of the 400, siamesed cylinders without larger water jackets meant that low-rpm cooling was an issue, and main webbing weakness affected the engine's durability. All things considered, this was a great option 20 years ago when building a performance motor. Thankfully, things have changed. Nowadays, the best service you can do yourself when looking at a 400 core for a performance-oriented buildup is to look past it. Accomplished engine builder and author David Vizard ( sees no advantage to using most production 400s.

"The Motown is a much better deal than the Poor Man's Bowtie (production) block," Vizard opines. "Even when they were new, they had a lot of problems with fatigue. Add in getting a dodgy block once in awhile from core shift during production, and building one gets expensive in a hurry. Sure, I've seen two 400s with great cylinder walls, but I've been building engines for 42 years!

"There was one instance where I decided to bore a 400 because I didn't have a sonic tester at the time. I couldn't accurately tell how thick the block was, but during machining, I could actually see the bores flexing when the tool went through."

Not exactly the Gibraltar-like strength you're looking for.

Knockout Punch

And now, the short-block factor. In addition to offering bare and race-prepped blocks for sale, Bill Mitchell Hard Core Racing Products offers a 427 short-block for people looking for a partially-assembled engine. Included in the $4195 package is a Scat 4340 forged 4-inch crankshaft with a 2-piece rear main, Manley 6-inch H-beam forged rods, Wiseco 20cc dished pistons, JE rings and Clevite 77 bearings. Since the 427 will be drinking premium unleaded, only one street piston was available at the time of this writing. Which happens to be one more piston type than is available through Chevy. GMPP doesn't offer a Bow-Tie short-block, but if you would like an equivalent short-block built elsewhere, you'll pay more.

"We could build you a similar short-block using the best available components," MTI's Jayson Cohen explains. "Using a Bowtie block with a Callies crank and their forged connecting rods, it would run you about $4,700. The rotating assembly could be a little more expensive than what the average guy would want to spend, but we build fast, reliable street engines and we stand behind them. As far as using the factory ECU, I wouldn't do it for a pre-1993 car--your cam selection would be very limited. But after '93 it would probably work okay, although we would recommend you use a Fel-Pro aftermarket system. A complete motor set up for fuel injection would run around $9,000 with decent aluminum heads and a hydraulic valvetrain. Keep in mind, however, that the fuel injection system and the injectors would add almost $3,000 to that."

Aaron Strope of Stropes Speed Shop also informed us that beating the Motown short-block's price would be a tall order.

"Building a replacement engine for a late-model car equipped with emissions equipment and fuel injection will be spendy, especially with a CNC block," he says. "The Bow-Tie CNC piece is full-on race, and that's going to cost around $9,000 to build the short block right. Heck, even using the non-CNC Bow-Tie would put it around five grand. I think what Mitchell's done, bridging the gap to make a big engine affordable, is great. They're real nice blocks."

Bill Mitchell is also pleased with the way his Motown turned out, and he is confident that his product offers tremendous value along with customer satisfaction.

"Since we hit all of the trade shows, we get the chance to bend the ears of people as to what their wish lists include. We know what our customers are looking for, and that coincides with what the manufacturers want, also.

"We were able to create an engine that should keep 95 percent of the customer base happy."

Another great advantage of this block is how it will go between the fenders of a late-model performance car--unlike the previously mentioned big-block.

"This block would drop right into a Camaro or Corvette. And since fuel injection is for the most part self-adjusting, your small-block would have a lot more inches and a lot more power. Mind you, taking a 350 out and replacing it with a 427 is a major difference--like putting a big-block in without the 200 extra pounds."

So you just dropped a mouse with a big-block attitude into your ride, how much more power do you think it will take?

"I have customers who tell me they're getting 850, 875 horsepower out of it running tunnel rams and nitrous. They might burn a piston once in awhile, but it won't hurt the block," Bill laughs. "I never thought we would see the day that you could build a hot rod Chevy without a Chevy block."

Neither did we.


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