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The Dart Iron Eagle 351W block is a precision-cast and raw-machined product of unparalleled quality. Dart provides a bit of extra deck height and minimal bore diameter, allowing the builder to precisely achieve desired dimensions.
By Mike Mavrigian
all photos by author
CYLINDER BORES AND BLOCK DECK MACHINING
Our block was finish-machined by Scott Gressman at Gressman Powersports (Fremont, OH…a mere 80 mile drive west of my shop).
Cylinder boring and deck surfacing was handled on Gressman’s RMC CNC machine, taking advantage of RMC’s block blueprint software for the Dart 351W Iron Eagle block.
Our Dart Iron Eagle 351W block was delivered with 4.000” rough-machined bores. Our JE pistons (P/N 232474) feature a skirt diameter of 4.1205”. JE recommends a piston to bore clearance of 0.005”, so our bores required a finished size of 4.1255”.
The Dart block features steel billet main caps, with 1/2″ diameter primary (inboard) bolts, 7/16″ splayed outboard bolts at caps 2, 3 and 4, and 3/8″ outboard bolts at caps 1 and 5. Main cap bolt tightening must remain consistent each time they are torqued, including both torque values and tightening sequence. This applies to all main cap servicing during machining, test fitting and assembly phases.
Once the main caps are removed for the first time, it’s a good idea to carefully deburr all cap mating edges in the block. This removes sharp edges left by Dart’s initial main boring operation.
Sharp edges are also deburred on the caps as well.
Scott initially rough-bored each cylinder to 4.110”, followed by a (finer-feed) finish boring to 4.121”. Each bore top edge was then chamfer-cut on CNC.
In order to achieve square decks (parallel with the crank centerline in the X-axis (front to rear) and 90-degrees to the crank centerline (inboard/outboard plane), decks were final-cut to a finished deck height of 9.4940”. Considering our crank stroke of 4.000”, our rod length of 6.200” and our piston compression distance of 1.280”, this results in out piston domes TDC located at 0.014” below deck.
Here the block is fitted with a mounting/registration bar for setup in the CNC machine.
The standardized bar is fitted with the appropriate size bushing (outer diameter of the bushing to fit the specific main bore diameter).
Here Scott Gressman lods the block into the RMC CNC machine in preparation of cylinder boring and deck surfacing.
Scott begins to calibrate the CNC for our block. RMC provides software specific to the Dart W block.
A registration bar is also inserted into the cam bores. The CNC machine’s digital probe contacts the left, right and top of this bar to determine crank-to-cam centerline.
The block is ready to be measured by the CNC’s digital probe (seen here overhead of the block).
The CNC digital probe provides the machine’s computer with extremely precise geometric information relative to bore centerlines and deck heights. This high-precision digital probe head is a costly item, at around $8000.
RMC’s CNC software provides accurate dimensional and axis-oriented data for specific blocks, including our Dart W block. The software allows you to accurately “blueprint” a block to predetermined specifications, but the operator maintains the ability to modify various settings if so desired.
The digital probe quickly locates the existing bore centerline by contacting the inboard, outboard, front and rear surfaces (90 degrees apart) of each bore.
The digital probe then checks each deck surface, at a number of points along the entire deck surface. This provides information about deck height from front to rear and side to side. This tells the operator how much material must be removed in order to establish the desired deck height, in addition to allowing decks to be corrected, if needed, relative to crankshaft centerline. In other words, decks can be accuratelt squared in both axis.
Once both decks have been probed, the display shows the deck height results. The operator can then decide how much he wishes to remove.
Once the block has been thoroughly measured and indexed, a double-cutter boring bar begins to rough-bore each cylinder. The machine automatically moves from one cylinder to the next, then flips the block to bore the opposite bank.
This bank’s cylinder have been rough bored (leaving a few thousandths for honing to final size). The CNC machine provides absolute bore diameter (and centerline) repeatability from bore to bore.
Once all cylinders have been bored, the machine tool head switches over to a mutliple-cutter surfacing head and begins to cut the decks to the programmed height. The tooling change takes place so rapidly that it’s very difficult to capture with a camera.
Once both decks have been cut, a chamfering tool cuts a neat chamfer along the top edge of each bore, eliminating the sharp edge left by the surfacer. This will ease piston and ring installation during final assembly.
This view shows the cylinder bore top-edge chamfer. Each bore top is chamfered exactly the same.
OUR FINISHED DECK HEIGHT…….9.4940”
OUR PISTON DOME AT TDC…..…- 9.4800”
PISTON BELOW DECK…………… – 0.0140”
Once all cutting procedures are finished, the block then moves from the CNC machine to the honing station for final-bore-finishing.
Deck (torque) plates are installed to the block decks prior to honing. This stresses the block to simulate cylinder head clamping. This assures a round bore in the final assembly. Here Scott torques a Goodson deck plate to 100 ft-lbs. Since bores can be pulled out of round due to the force exerted by fully-loaded head bolts, placing the block in a simulated head-installed state insures that the bores will be uniformly concentric when the heads are installed.
Honing is accomplished in steps, starting with a rough hone with 220-grit stones, followed by 400-grit stones. Finally, each cylinder is dressed with a set of plateau brushes on the honing machine. This “smoothes out” any peaks/valleys left by the honing procedure. Plateau finishing provides quicker and more efficient piston ring seating.
Fresh, clean honing oil is critical to achieving proper cylinder wall surface. This oil serves to both lubricate and cool the stones and cylinder walls during honing.
The honing stones rotate inside the bore, operating with the pressure, strokes and dwell time programmed by the operator.
The honing machine’s control panel features a display that allows the operator to monitor cylinder profile from top to bottom.
DETERMINING PISTON TO DECK RELATIONSHIP:
½ OF STROKE……………. 2.000”
ROD LENGTH……………+6.200”
PISTON CD……………… +1.280”
TOTAL……………………=9.480”
Considering our 70cc chamber Trick Flow heads and our JE pistons with –5cc flat top domes, our final compression ratio will be approximately 10.8:1 (this of course could be slightly tweaked by selecting a custom-thickness MLS head gasket to drop compression a bit further if desired).
Our lifter bores were measured and checked against our Lunati roller lifter diameters. We have a cool 0.0015” oil clearance, which should work out just fine (by the way, we’ll be sure to use an engine oil with a high zinc content to protect the lifters from scuffing in their bores).
OUR PISTONS
In this build, we’re using JE’s #232474 forged pistons.
PISTON TYPE……Flat top
VALVE RELIEFS…… – 5 cc
BORE………………4.125″
COMP. DIST. …….. 1.280″
INTAKE…………… 20 deg
EXHAUST………… 20 deg
BORE CLEARANCE… 0.005”
TOP GROOVE…..0.064” LAND 0.245”
2ND GROOVE……0.064” LAND 0.150”
OIL GROOVE……0.188” LAND 0.080”
PIN DIA. …………0.927”
PIN LENGTH……2.750”
DOUBLE SPIRAL LOCKS
WEIGHT…………490g
Our JE forged pistons set P/N 232474 included pistons sized for a 4.125″ bore, 0.927″ diameter pins, a full ring set and spiral locks.
Our pistons feature a compression distance (center of pin bore to dome) of 1.280″, which places our piston domes 0.014″ below block deck.
Our pistons feature a flat top with – 5 cc valve reliefs.
Our JE pistons weighed in at 490 grams each, just as advertised. JE’s dedication and attention to maintaining extremely precise tolerances is truly to be commended. Not only did each piston weigh-in identically, the machining (pin bores, diameter, ring grooves, valve pockets, etc.) was exactly duplicated on each slug.
CRANKSHAFT
Our crank is a forged stroker unit from Scat, P/N 4-351C-4000-6200, featuring a 4.000” stroke, 2.749” mains and 2.100” rod pins. This crank features heavy counterweights and is designed for internal balance (allowing us to use zero-balance damper and flexplate).
Our Scat forged stroker crank features a 4.000″ stroke, Cleveland-sized mains and 2.100″ rod pins. Scat did a superb job of machining this crank. Every journal measured precisely to specifications. Scott Gressman noted that this was one of the easiest cranks to balance that he’s seen in a while.
The 2.749” main journals are required to accommodate our Dart Iron Eagle block, which is set up for the Cleveland 2.749” main journals.
During test fitting, with a set of Clevite MS1010H main bearings installed, main bearing clearance was measured at a tight 0.0017”. Since I’d much prefer oil clearance in the 0.002 – 0.003” range, I swapped the bearings out for a set of Clevite MS1010HX main bearings that provide an extra 0.001” of oil clearance. This was due to the fairly tight main bore of the Dart block. Since it’s obviously easier to swap bearings than it would be to hone the main bores, I took the easy way out. With the X bearings in hand, I had two choices: use the X bearings at both top and bottom locations to achieve an extra 0.001” clearance (bringing oil clearance to 0.0027”), or use a standard bearing on the block saddle and the X bearing in the cap saddle, for an extra 0.0005” clearance (bringing final oil clearance to 0.0022”). I prefer “loose,” so I opted to use the oversize X bearing shells in both top and bottom saddles. Fortunately, MAHLE Clevite is thoughtful enough to offer a range of bearing dimensions that allow you to tune bearing clearances to your desired results.
I chose MAHLE Clevite main bearings, sized for the Cleveland mains.
The HX series main bearings provided us with an additional 0.001″ clearance to achieve a comfy main bearing oil clearance of 0.0027″.
With all main bearings in place and the crank installed, crankshaft endplay initially measured 0.006” with the standard main bearings (spec is 0.004 – 0.008”). Once I replaced the main bearings with the HX bearing set, endplay tightened up a tad at 0.0055”, still within range.
Note: when checking crankshaft endplay, don’t just lay the crank onto the upper bearings, since you’ll end up with an inaccurate endplay measurement. You must install the center main cap and thrust bearing (this is the thrust bearing location) and torque to full value (105 ft-lbs on the inboard ½” diameter cap bolts and 65 ft-lbs on the splayed 7/16” outboard bolts).
Rod bearing clearance checked out at 0.002” (spec is 0.0018 to 0.0024”). All crank rod pins measured 2.100” and rod bearing ID measured 2.102”. In order to measure rod bearing ID, I installed Clevite CB-663 HN rod bearings in the Scat rods, and placed each rod in my Gearhead Tools rod vise. I then tightened the ARP 7/16” 8740 rod bolts (with moly) to 63 ft-lbs. Using a bore gauge indexed at 2.100”, rod bearing inside diameter checked out at 0.002” clearance on each and every rod. The Scat rods checked out very consistently in every aspect, including center to center length, pin bore diameters, big end bore diameter and big/small end and total weights. Just another example of the nice work that Scat produces.
With a pair of rods installed on each crank rod pin, I checked rod sideplay at 0.018”. That’s a bit over the Ford factory spec of 0.010 to 0.015”, but no worries…better loose than tight. Actually, it’s very common for many aftermarket cranks (Scat, Eagle, etc.) to provide rod side clearance in the 0.018 to 0.020” range. Not a problem.
Rod side clearance checked out at a healthy 0.018″.
During test fitting, I found outstanding crank to block and rod to block clearance, with no need to perform any block relieving. Nice job, Dart.
TEST FITTING THE CRANK DAMPER
We’re using a crank damper from PRW, their P/N 2430203 steel race damper. This is an SFI-certified race damper with an O.D. of 6.50”.
Measuring for interference fit to our Scat crank snout, the damper bore I.D. measured 1.3745”, and the crank snout diameter measured 1.3755”, for a 0.001” interference fit. This will provide an easy draw-on installation without the need to hone the damper bore.
The PRW steel race (SFI certified) crank damper is a very pretty black powdercoated piece.
The PRW damper featured a precise 0.001″ interference fit to our Scat crank snout, with no corrective honing needed. Also, the etched timing increment marks are crisp and well defined. A very nice job.
This damper is zero-balanced (to accommodate our internally-balanced crankshaft) and features a beautiful black powdercoat finish. This damper is a precision-crafted piece of jewelry. The guys at PRW are to be commended for a very nice product.
By the way, the Scat crank snout features three in-line radiused grooves for woodruff keys. The Scat snout grooves were conveniently sized for smallblock Chevy woodruff keys.
MAIN CAP TORQUE SPECIFICATIONS
Dart specifies the following torque values for their main cap bolts…
(Note: all torque values listed are based on the use of oil on threads and bolt head undersides)
INBOARD ½”-DIA BOLTS………………………………….105 ft-lbs (w/oil)
OUTBOARD 7/16” SPLAYED BOLTS AT CAPS 2-3-4…… 65 ft-lbs (w/oil)
OUTBOARD 3/8” BOLTS AT CAPS 1 & 5…………………. 35 ft-lbs (w/oil)
(Note: with all cap bolts in place, tighten the ½”-dia bolts first, followed by the 7/16” outboard splayed bolts, followed by the 3/8” outboard bolts)
CRANKSHAFT BALANCE
OUR INDIVIDUAL WEIGHTS
PISTON……………. 490g
PISTON PIN………. 130g
PIN LOCKS……….. 4.5g
RINGS……………… 46.6g
ROD BEARINGS…. 43.0g
ROD BIG END……. 418.5g
ROD SMALL END.. 163.0g
ROD TOTAL…….…581.5g
Our MAHLE Clevite rod bearings weighed in (for our bobweight card) at 43 g.
Our Scat 6.200″ rods were very well weight-matched, with big ends at 418.5 g and small ends at 163 g.
OUR BOBWEIGHTS
ROD BIG END………..418.5 PISTON…………… 490.0
ROD BEARINGS…….+ 51.0 PIN…………………+ 130.0
TOTAL…………… = 469.5 LOCKS……………. + 4.5
TIMES RODS/THROW X2 RINGS………….…. + 46.6
TOTAL ……………. = 939.0 ROD SMALL END….163.0
OIL………………… + 4.0 TOTAL……………= 834.1
ROTATING WEIGHT …………943.0 g
RECIPROCATING FACTOR…. 834.1 g
BOBWEIGHT…….……………1777.1 g
With bobweights created and installed on the crankshaft (each bobweight is positioned centered fore/aft on each rod pin, with each pin’s bobweight oriented 90-degrees to each other), the crank was spun on Gressman’s Pro-Bal balancer. The Scat crank proved to be extremely close from the outset, with no need to add any heavy metal. 36 g was removed from the front counterweight, and a mere 14 g was removed from the rear counterweight (weight removal achieved by drilling holes on Gressman’s Bridgeport). Scott noted that this was one of the easiest-to-balance cranks he’s ever done, a real complement to Scat’s attention to detail.
Scott Gressman installs the bobweights onto our Scat crank.
Each bobweight is positioned centered on the rod pin (fore/aft). This is important for balancing accuracy.
Scott spins our crank during the initial balance check.
A minimal amount of weight was removed from the front and rear counterweights. The Scat crank balanced beautifully with no hassles.
CHECKING CRANK AND ROD BLOCK CLEARANCE
Whenever you build a stroker engine, it’s imperative to check rotating clearance of the crank counterweights and rod big end clearance relative to the block.
With the crank installed in the block, I carefully rotated the crank to check for counterweight clearance to the block. No problem. The tightest spot provided (counterweight edge to bottom of cylinder bore/web area) about 0.170”.
Next I test-installed pistons and rods on each crank rod pin and carefully rotated the crank to check for rod big end to block clearance. Once again, clearance was more than sufficient, with about 0.180” between counterweights and piston pin boss, and a more than healthy 0.200” to 0.230” clearance between rod big ends and cylinder bore bottoms. In this stroker, no block clearancing was needed at all. Very cool. Obviously, Dart had stroker combinations in mind when they designed this block.
Now that I’m satisfied that all clearances are appropriate, I’ll perform a final dressing to the block (taking care of any remaining exterior boogers, etc.), followed by a very thorough block washing, using solvents, followed by a hot soapy water wash (using rifle brushes in all oil passages), followed by a thorough rinsing in hot water, followed by a cold water rinse (the cold water rinse helps to reduce surface rust on the bare metal).
The block will then be masked and sprayed with a high-build two-part urethane primer and hand-sanded with fine grit paper. After a final cleaning, the block will then be sprayed with a basecoat color and a final application of a urethane clearcoat. Once the block has been painted (with ample curing time), final assembly can begin.
SMOOTHING/GRINDING THE BLOCK EXTERIOR
Since my intent with this build is to address a street rod/hot rod application, attention to the visual appeal is an important aspect. As part of this approach, I decided to “smooth” the block exterior to a show-like finish, eliminating any casting flashings, sharp edges or other visual imperfections. Keep in mind that, compared to a GM casting, the Dart casting is WONDERFUL. As castings go, the Dart block exterior is very, very nice to begin with, featuring none of the sloppy flashings common on GM production blocks. However, since I wanted this block to be super-smooth & tidy, I worked over the exterior surfaces with a combination of media, including Scotchbrite abrasive pads (driven by a right-angle die grinder), 3/8” mini sanding belts, bull-nosed cutting bits and rolled abrasives (tootsie rolls).
While dressing the block, I took this opportunity to test fit the front two ¼” NPT lifter galley plugs (since these need to sit flush or below the front block surface to clear the cam retainer plate). The test fit showed that they protruded out a bit, so I ran a ¼” NPT tap a few threads deeper to make sure that they clear the cam retainer plate. Also, the right side lifter galley plug protruded a bit, interfering with the distributor shaft, so I tapped a few threads deeper here as well. It’s obviously best to do this now (since I’m making a metal mess anyway), to avoid creating contaminants during final assembly.
If you’ve never dressed a block exterior before, be advised that this is a real commitment. It’s a messy, and very time consuming process.
Once all metal grinding/cutting was done, I had the block jet washed at nearby Medina Mountain Motors’ machine shop (Creston, OH) and followed this up with a thorough wash with hot soapy water and repeated rinsing (first with hot water and then with cold water to minimize surface rusting).
Once the block was absolutely clean, I lightly oiled the cylinder bores, lifter bores, main cap saddles and lifter valley.
Once the block was smoothed out, I cleaned and applied a light coat of etching primer in preparation of the buildup urethane primer.
The block was then carefully masked, sealing off the lifter valley, decks, cam bores and mains.
The entire bottom was masked to cover the mains and pan rails. The pan gasket and pan was test fitted first to verify what areas of the block pan rails would remain exposed, and masked accordingly.
Next, it was time for the base primer. I tediously masked off the block, using head gaskets as a template to make sure that any exposed deck surfaces would be painted. The distributor hole was plugged, along with the front and rear cam bores. All exterior threaded holes were also plugged using a combination of plastic plugs and clean bolts.
Once masking was complete, the block exterior was treated to two coats of high-build Valspar urethane primer. This helps to fill any remaining voids and small pits. The next day, the primed surfaces were sanded with 180-grit to even-out any surface imperfections. This was followed by another sanding with 320-grit paper to achieve a smoother surface.
Once I was happy with the surface prep, two coats of basecoat color were applied (using Valspar 1190-14 silver metallic). We waited 20 minutes between coats. Next, two coats of Valspar urethane clear was applied (again, waiting about 20 minutes between coats).
Note: the Valspar primer, basecoat and clearcoat were all 2-part mixes (each requiring an activator). All spraying was handled with an HVLP gun (high volume, low pressure spray gun that minimizes overspray).
After about 8 hours, we carefully removed all plugs and masking. After waiting an additional 6 hours, I then washed and rinsed the block to remove any sanding dust that might have entered the block.
At this point, we were ready to install the cam bearings and begin final assembly.
BLOCK PLUGS
The Dart block requires a series of threaded plugs for the oiling system.
At the front face of the block, three ¼” NPT plugs are required to seal off the two lifter galleys and the main galley. Note that the threaded holes need to be tapped a bit further in order to sink the plugs for clearance, The upper lifter galley plug on the right side (passenger side) and the lower main plug (also right side) need to seat flush with the block surface in order to clear the camshaft retainer plate. The upper left side lifter valley plug also needs to sink a bit further to clear the distributor.
At the front of the lifter valley, a 1/2″ NPT hole is featured. Inside the hole are two angled ports. The port at the right side (passenger) features a 1/8″ NPT thread a few inches down inside the passage. I plugged this with a 1/8″ NPT plug. With our front-oil-pump location for our wet sump system, this provides main oil priority, assuring that the mains will be fed first before the lifters.
I tilted the block on my Goodson engine stand to ease installation of the oil passage plug (to prevent the plug from falling off of the hex wrench).
Once the 1/8″ NPT plug was installed, a 1/2″ NPT plug was installed to seal off the top port.
The port at the rear of the lifter valley was sealed with a 1/2″ NPT plug. No restrictor was installed deep inside the right hand passage, in order to provide adequate oil flow to the hydraulic roller lifters.
The front oil passages were plugged with 1/4″ NPT plugs. Holes were tapped a few threads deeper, so that the right side (passenger) main and lifter plugs were below-flush to clear the cam retainer plate. The left side (driver) lifter passage plug was also seated a bit deeper to clear the distributor (seen here in the upper shadow area).
The rear face of the block also requires three ¼” NPT plugs to seal the rear of the lifter galleys and main galley.
This additional threading was done before final block washing, and rifle brushes were run through the passages to make sure that all loose debris was removed.
The rear upper block area (just above the bellhousing face) features a 1/8” NPT threaded hole for picking up oil pressure (for the time being, I installed a 1/8” NPT plug). Adjacent to that hole is a ½” NPT hole intended for an external oil feed. Since I wasn’t using an external feed, I plugged this hole with a ½” NPT plug.
At the front left lower side of the block (forward of the oil filter base), a –10 AN straight-thread hole is featured for plumbing a front oil inlet for an external oil pump. Again, since I’m running a standard style oil pump and no external feeds, I plugged this hole with a –10 AN straight thread plug (Earls P/N 981410ERL). This male-hex-head plug includes a sealing O-ring.
At the upper rear right side of the block is a 1/2″ NPT hole, available for plumbing an external oil feed. Since we’re running an internal wet sump arrangement, I sealed this hole with a 1/2″ NPT plug (I chose a blue anodized aluminum plug for appearance). The 1/8″ NPT hole above the 1/2″ NPT plug is available for picking up the oil pressure signal. Until the engine goes to the dyno, I plugged this with a blue aluminum 1/8″ NPT plug.
At the block’s lower left forward area, a -10 AN straight thread hole is available for plumbing an external oil feed. I plugged this with a -10 AN straight thread plug (hex headed with a sealing O-ring), Earl’s 981410ERL.
In the lifter valley are two ½” NPT holes (one front and one rear). There are two lifter feed passages under the front and rear ½” NPT crossover plug locations. The lifter feed at the front and rear of the lifter valley are threaded for an 1/8” NPT plug. The lifter feeds are located aiming toward the right (passenger) side at each front and rear location. The 1/8” NPT threaded area is located between the main oil galley and the passenger side lifter oil galley. This restricts both left and right lifter galleys. Because it restricts both sides, the orifice size in the 1/8” NPT restrictor plugs (if used) should be large enough to feed both sides. In order to restrict the lifters you need to either install restrictors at both ends, or plug one end and restrict the other. Some engine builders prefer to plug the end that if feeding the main oil galley and install a restrictor at the other end. This provides priority main oiling before feeding the lifters. I spoke with Dart, and they advised that since we’re running hydraulic roller lifters, it would be best to plug the front (where the main is being fed from our pump) and leave the rear lifter passage open (no restrictor).
All of our engine test fitting, pre-assembly and final assembly takes place in our engine-assembly “clean” room, outfitted with Lista cabninets and workbench tops. Note the computer monitor above the workbench (for convenient searching of part numbers and specs). Cleanliness is vital with any engine assembly. We actually wear hospital booties whenever entering this special room.
In the next article installment, we’ll cover camshaft, crankshaft and rod/piston installations, including file-fitting our rings.
Note: The finished engine will be on display at the upcoming Hot Rod & Restoration show, March 12 & 13, 2010 at the Indianapolis Convention Center (in the Hot Rod & Restoration/Precision Engine booth).
Tags: 351W, BALANCING, BLOCK MACHINING, block plugs, CLEARANCES, CNC, CRANKSHAFT, DART, FORD, PISTONS, priority main oiling
