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We hone the block and determine bottom-end clearances.
by Mike Mavrigian
photos by author
With our new pistons in hand, it was now time to hone the cylinders to size.
As you may recall from our previous article, our target is to build an all-motor 632 cubic-inch big block featuring 15:1 compression, enabling us to produce approximately 1,200 HP in the mid-7,000 RPM range.
Now that our pistons have arrived, we were able to hone the cylinders to final size. We’ve also measured critical clearances for bearings, crank-to-block, rod-to-block and rod-to-cam.
In the third installment of this multi-part series, we’ll paint the block, balance the rotating assembly and begin our test assembly for final fitting.
PISTONS
As mentioned in Part 1, we enlisted Diamond to make our custom pistons. Well, the little buggers arrived, and they’re gorgeous. Did I say little? At a measured 4.5937″, they’re ‘ huge. If these billet beauties were adult human females, I’d ask them out on dates. Yeah, they’re that pretty. If we had room in the magazine, I’d shoot two-page spread photo of these parts.
Okay, maybe that was a bit over the top, but you can’t blame a guy for getting excited. When I think about how these puppies are going to squeeze against the Dart Big Chief 2 chambers, I can’t help getting all giddy with anticipation.
The pistons began as high-density forgings (DRP76X), and were CNC machined to the finished state. The custom dome features a dish, along with valve reliefs for both intake and exhaust valves. The intake valve depth is 0.270″, while the exhaust valve depth is 0.064″. Total dome volume is -21.1cc. Combined with a compression height of 1.120″, 0.005″ deck clearance, our Victor MLS gaskets and the 56cc combustion chambers, this should give us a tidy 15:1 compression ratio.
Features of these pistons include the use of their P/N 9310 piston pins (0.990″D x 2.930″L x 0.165″ wall), gas porting along the dome perimeter (these ports will aid in ring seating and eliminate flutter), and a serious weight-savings via their internal mill “Maxi-Lite” machining.
Because of the extremely compact ring groove layout, the pin bore encroaches through the oil ring groove. In order to provide a “floor” for the oil ring package, Diamond included a pack of its oil ring support rails that features a 0.188″ wall. These will provide bottom support for the oil ring in the pin bore passage areas. The pins are full-floaters, and are designed to be secured via Spirolox retaining rings (0.990″ x 0.043″).
Diamond noted that the installed wall clearance for these pistons should be 0.0070″. It specifies that piston diameter must be measured 90 degrees to the pin bore, at a distance of 0.700″ down from the oil land.
CRANK FIT
We first verify-measured our main bearing bores, which mic’d out at 2.9375″.
We mock-installed the steel Lunati 4.750″ stroker crank (P/N BS-421 MN) using our Clevite Tri-Armor main bearings #MS-829 HXK, tightening the main cap bolts to a value of 100 ft-lbs (with moly on threads and bolt underheads). Installed main bearing diameters measured at 2.7535″. Our crank main journals mic’d at 2.7495″, providing us with main bearing clearance at 0.004″. According to Scott Gressman at Gressman Powersports, this build requires bearing clearances on the “loose” side. Crank endplay measured 0.008″.
One obvious concern, given the 4.750″ stroke of this crank, was counterweight-to-block clearance. Happily, we had gobs of clearance, with the tightest spots at about a healthy 1/4″. This required us to do nothing but grin in terms of counterweight clearance. The boys at Dart obviously gave some thought to boneheads like us using a long stroke and machined the block accordingly.
BEARING & CYLINDER DIMENSIONS
MAIN BORE BEARING DIAMETER — 2.753″
MAIN JOURNAL DIAMETER — 2.749″
MAIN BEARING CLEARANCE —0.004″
ROD BEARING DIAMETER —2.20325″
ROD JOURNAL DIAMETER —2.200″
ROD BEARING CLEARANCE — 0.00325″
(rod bolt stretch should be —0.005-0.0055″)
PISTON DIAMETER —4.59375″
FINISHED BORE DIAMETER —4.60075″
PISTON-TO-WALL CLEARANCE —0.007″
CRANKSHAFT ENDPLAY —0.008″
CONNECTING ROD SIDEPLAY —0.0020″
ROD FIT
Using our Clevite Tri-Armor rod bearings, part #CB-743 HXK, we test-assembled all of our Lunati Pro Mod steel rods (with pistons in place), tightening the ARP 200 7/16″ rod bolts to a stretch value of 0.005″ to 0.0055″ (70 ft-lbs with moly if torqued). With our installed rod bearing diameter at 2.20325″ and our rod journals at 2.200″, we obtained oil clearance at 0.00325″. Rod sideplay was measured at 0.0020″ at all rod journal locations.
While checking for rod big-end to block clearance, we noted adequate clearance with the tightest at about 0.050″, but we’ll likely kiss the block in a few spots just for safety’s sake.
When we mocked-up a piston/rod assembly onto the crank and rotated, we noticed a portion of the rod big end encroaching into the cam tunnel. The radius hump on the upper portion of the big end needs to be reduced slightly (maybe 0.020″ or so) to ensure camshaft clearance. Considering the 4.750″ stroke, this didn’t come as a big surprise. We’ll relieve this small hump (the hump opposite from the bearing tang side) and re-balance the rods. You’ll see this in the next issue.
DIAMOND CUSTOM PISTON DATA
Finished bore size —4.600″
Stroke — 4.750″
Rod length —6.700″
Block deck height —10.200″
Piston to deck —0.005″
Compression height —1.120″
Cyl. bore length —6.350″
Piston type —conical dish
Piston material — 2618
Valve depth —INT 0.270″
—EXH 0.064″
Effective dome volume — -21.1 cc
Piston weight —542 grams
Required bore clearance —0.007″
(measured 90 degrees to the pin hole, at 0.700″ below oil land)
Engine type —BBC
Application —Drag race
Desired comp. ratio —15.00:1
Cylinder head —Dart Big Chief 2
Chamber volume —56.0 cc
Comp. gasket —0.045″
Gasket bore dia. —4.640″
Gasket volume —12.47 cc
Top land —0.285″
VALVE POCKET INFO
Int. depth —0.270″
Int. valve dia —2.470″
Exh. depth —0.064″
Exh. valve dia — 1.800″
Int. angle —11 deg.
ROD INFO
Lunati steel I-beam Pro Mod P/N LB01
Rod length —6.700″
Small end width — 1.133″
Thickness above pin —0.210″
Material steel
SKIRT SHAPE window type full roundDart CNC machined the block so all critical bore locations and sizes were already correct and to-size. Only the bores remained to be honed for proper piston clearance.
Dart CNC machined the block so all critical bore locations and sizes were already correct and to-size. Only the bores remained to be honed for proper piston clearance.
The initial bore diameters, as delivered from Dart, measured 4.590″.
Our billet pistons from Diamond were custom-made specifically for our combination of the 10.2″ deck block, 6.700″ rods, Big Chief 2 11-degree heads and 4.750″ stroker crank.
Diamond’s pistons are things of beauty, in addition to having the inherent quality that Diamond is known for. Our dome volume is -21.1cc. In combination with our block height, rod length and combustion chamber size, we should realize 15:1 compression ratio.
Our custom piston spec sheet. We carefully went through each item with Diamond’s Eric Simone, who was extremely helpful and knowledgeable.
Tags: Add new tag, CLEARANCE DIMENSIONS, CNC, CONNECTING ROD DATA, CUSTOM PISTON DATA, CYLINDER HONING, DART, DIAMOND CUSTOM PISTONS, LUNATI CONNECTING RODS, LUNATI STROKER CRANK, MAHLE CLEVITE BEARINGS, SHORT BLOCK CLEARANCES
