Dart Machinery technician Bob Dimitrijevich installs Darton iron sleeves in cast aluminum block as a billet aluminum racing block awaits installation Aluminum block bores are honed for size and geometry before liner installation

Dart Machinery technician Bob Dimitrijevich installs Darton iron sleeves in cast aluminum block, as a billet aluminum racing block awaits installation. Aluminum block bores are honed for size and geometry before liner installation.

Flexible Honing Has Custom Engine Producer Thriving

Dart Machinery masters the "peaks and valleys" of custom engine block production with advanced flexible honing system Sunnen SV-20 honing system Honing cylinders explained Any material and hardness Advanced true-linear stroke

For a business to thrive, it must have the means and strategies to negotiate economic peaks and valleys. A custom manufacturer of engine blocks for racing and high-performance vehicles — and also replacement blocks for several major OEM’s V-8 engines — Dart Machinery’s mastery includes honing cylinder bores, to achieve production efficiency that nearly matches the mass-production programs. Advanced honing is a differentiator for maintaining Dart’s competitive advantage and achieving greater efficiencies and flexibility in processing an always-changing mix of blocks, according Dick Maskin, founder and president.

A Chevrolet performance small block is honed in the new SV-20 machine.

A new SV-20 honing system from Sunnen Products performs high-precision, highly flexible honing for a range of block designs and metallurgies in a single continuous process, with no need for stone change, he reported. The American-made SV-20 replaced a more expensive, European-built honing system that “just wasn’t a good fit for our production, not flexible enough,” said Maskin.

Maskin started Dart in 1981 in a two-car garage in Oak Park, Mich., and has grown it into a manufacturing leader in racing and high-performance engine blocks, heads, and other components. The company does its part to preserve Detroit as the “Motor City,” with a technology center and manufacturing facility in the metro area.

Today, 95 Dart employees make about 8,000 blocks and 16,000 heads per year for drag racing, circle track racing, road racing, and high-performance custom cars, as well as marine and industrial power applications.

”We produce custom in volume,” Maskin explained. Blocks are manufactured to customer order in type and material, bore spacing, cam location, bolt pattern, deck height, lifter location, oil pan bolt pattern, metric or U.S. dimensions – “just infinite variation and specialization,” he said.

Dart makes blocks in various grades of iron (including CGI) and in aluminum with iron-sleeved cylinder bores, including blocks machined from forged and heat-treated aluminum billets. The manufacturing plant operates three shifts, 6 or 7 days per week depending on production volume, with 26 large-dimension Makino CNCs churning out blocks, heads, and manifolds.

Sunnen’s two-stage diamond abrasive honing tool does roughing and finishing with 220- and 500-grit abrasive without pause for stone change. The advanced control system on the SV-20 allows Dart Machinery to achieve a wide range of finishes with these two abrasives.

Customers choose among seven different base block designs, big-block and small-block sizes, all evolved from NHRA pro-stock V-8 experience. “That’s where all the technology comes from,” said Maskin, whose own engine creations achieved drag-racing’s first 300 mph quarter-mile run and four-second Funny Car elapsed time.

Applying the same creativity to honing, Dart combines two advanced technologies – diamond honing abrasives driven with a programmable spindle, followed by profilometers to measure the variability in cylinder surface finishes – to replicate “best” block finishes and ring seal.

"Honing is all about sealing the rings to the cylinder wall," Dick Maskin stressed. "The key is valley depth: Too deep and you have too much oil in it, slowing the engine down with friction. Too little valley, and the rings can hydroplane across the valleys and you lose seal. Then, you have to run thicker oil. Unless oil can reside in the valleys, it sits on the cylinder wall and must be thick not to be torn off by the rings. We think valleys need to be in the mid to high 30 micro inches (0.000030 in.) to really seal a race engine strongly, and deeper in the mid to high 40's for a more conventional engine.

"For some blocks using today’s steel rings, we hone for peaks of 12-15 micro inches (.000012-15 in.) or smoother. When we turn the short block over manually with a wrench, the rings will knock the peaks down to 5 or less before we ever start the engine, but the valleys remain. We may not always take the peaks off during honing. If you ball-hone or sandpaper, thinking you'll accomplish this, you’ll simply reduce valley depth and the engine won’t run as good.

"In our experience, if you go smoother on honing peaks and valleys, the finish won’t last as long. Starting with a rougher finish, the block is going to wear in and be sealed up so tight it will surprise you."

Machining to Customers' Precision Specs

“Experienced racers have always had their favorite blocks, ones that runs better than others,” noted Maskin. “We’ve learned through experience that it was not the block that made the difference, but how your honing process worked for that particular block, while similar blocks might produce much less horsepower.

Dart produces about 8,000 engine blocks annually in various designs and numerous grades of cast iron and aluminum, and machined from forged billet aluminum.

“Once we understand the finish you need from honing – and we know these numbers now, thanks to profilometers,” he continued, “we can make the bad blocks good, too. We use honing to achieve the ideal Rvk (valley depth average), Rpk (peak height average), Rk (core roughness depth), and crosshatch to finish a block for a given application.”

Knowing the desired result, the honing process can be tweaked to achieve the result in any block, he said. “If you go from a 200 brinnel block of cast iron to a much tougher CGI (compacted graphite iron) block, you need to know how to achieve your finish numbers in the different materials,” said Maskin. "Finishes also must account for the type of fuel burned in the engine, which could be alcohol or natural gas."

The Sunnen SV-20 honing system gives Dart the flexibility to process any engine block efficiently, in any material and hardness. “There is no typical production run for us,” according to Maskin. “We can put a new block on the SV-20, once the program is written, and no machine can make a cylinder rounder or straighter.

“The finish is all based on programming,” he emphasized. “If you know where you want to go, you can get there.”

A ring compressor eliminates pre-compression of the rings and a square installation in the bore.

Computer control has taken much of the art out of honing, he noted. “Machines like the SV-20 produce a finish and size that previously required a very talented machinist. If you have to hone different blocks with different material content, like we do, the newer equipment is significantly better.”

An advanced true-linear stroking system with 3.36 hp (2.5 kW) servo drive keeps the honing tool concentric with the bore throughout the full stroke length to produce a consistent diameter from top to bottom of the bore. The SV-20 can hone bores with inside diameters from 0.75 to 8 inches (19-200 mm). The machine's 36 x 40-in. (915 x 1015 mm) work envelope, front-loading design, and weight capacity up to 1,500 lb (680 kg) combine for exceptional processing versatility.

Machinist Dennis Stone uses an older Sunnen SV-3 to line hone vertically and automatically.

Dart’s business also relies on good cycle times, and the high-torque 5.5-hp spindle on the new machine delivers that. “With the SV-20 and a new block we can take 6-8 thousandths out in less than a minute,” he reported. Two-stage diamond tooling – 150 and 600 grit – performs rough and finish honing without pause for stone change. This combination of stones allows Dart to get almost any surface finish, he said. Other productivity enhancements he notes are automatic size lock, dwell, and real-time bore profile display.

The PLC-controlled SV-20 uses an 8-inch, industrial color touchscreen with hand wheel to jog for fast set-up and easy operator training. The computer control ensures automatic, consistent bore-to-bore geometry and finish, without constant adjustment by the operator. The control automatically calculates the required spindle/stroker speeds, based on the operator's desired crosshatch angle, or it will display a crosshatch angle based on the operator's inputs for the spindle/stroker speeds. Variable-speed control allows a much wider range of processing options and consistent crosshatch pattern, with automatic switching from roughing to finishing at one push of a button.

Dart uses other Sunnen machines in its production processes, such as a CH-100 line hone, Maskin said, and though the SV-20 has taken over most day-to-day honing work an older machine may still be useful for some applications. "We have many Sunnen hone heads going way back, and in special situations our best machinists can do things with them we call 'cheating.'"

For more than 30 years Dart has been out in front of V-8 performance-block design and manufacturing. With its new SV-20, it is honing that edge through piston-sealing precision and manufacturing efficiencies.

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