April '19

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36 THE SHOP APRIL 2019 PRECISION ENGINE 0.020 inches, from the newly cut bottom, up 0.250 inches. Finally, the bottom nose area was radi- used into a bullet shape. With the right rear oil gallery plug removed from the block, I could view the distributor shaft's oil band location relative to the block's oil gallery, verifying that the distributor shaft's oil band was in the oil path. I carefully measured the distance from the top of the intake manifold distributor boss to the top of the oil pump inter- mediate shaft, which measured at 8.096 inches. Knowing that I wanted 0.200 to 0.250 inches of distributor-to-shaft engage- ment, I then knew that the distance from the bottom of the distributor gear's key to the underside of the distributor's slip collar should be approximately 8.346 inches. This provided me with a rough distrib- utor depth target. I was able to adjust the distributor depth via the distributor's slip collar to achieve distributor-to-intermediate shaft engage- ment of about 0.225 inches. With the dis- tributor bottomed out, I then raised the distributor (using the slip collar adjust- ment) about 0.010 inches. I then rotated the crank two full revo- lutions, removed the distributor and inspected the bronze gear for evidence of a contact pattern relative to the cam's gear. Once I was satisfied with shaft engage- ment and gear mesh, I final-tightened the distributor slip collar. Once all height adjustment was com- plete, I cleaned the distributor shaft and gear and applied a coat of Royal Purple Max Tuff engine assembly lube to the shaft, gear and engagement key before final-installing the distributor. The MSD distributor was supplied with a pair of rubber O-rings for the shaft. While some guys don't bother with the O-rings, I followed Gressman's advice by installing only the upper O-ring—the theory being that the absence of the lower O-ring allows a bit of oil to dribble down to the shaft during engine operation. PLUG WIRES The MSD 8.5mm Super Conductor spark plug wires were then routed along the bottom of the heads and cut to length. The distributor-end terminals and boots Building a 632 Mountain Motor DART 11-DEGREE BIG CHIEF II CYLINDER HEAD SPECS P/N 18500000 Material 355T6 aluminum alloy Comb. chambers 56cc (70cc also available) Intake valve dia 2.470" Intake port dim 1.815" x 2.725" w/ 3/4" radius Intake port volume FP CNC – 497cc Intake port location Raised 1.500" spread ports Exhaust valve dia 1.800" Exhaust port volume 185cc Exhaust port dim 2.020"w x 1.780"h Exhaust port location angled & raised 0.900" extended flange w/ BBC bolt pattern Intake flow 520cfm @ 0.900" lift/28" SuperFlow 1020/+20cfm on SuperFlow 600 Exhaust flow 355cfm @ 0.900" lift/28" Lifter Use 0.180" offset lifter Manifold Dart Big Chief Oval 1x4 bbl = 8" tall Milling 56cc = 0.145" (70cc = 0.080") Pistons Must use pistons for Big Chief head Retainers Titanium 10-deg Spark plug 0.750" reach, gasketed Autolite AR3932 Spring cups 1.625" – 0.060" cup Spring pockets 1.740" OD for 1.625" cup (0.030" deeper max) Springs Dart 1.625T = 330# @ 2.100" / 0.910" max Comp 948 Valve angles Intake 11 deg x 6 deg Exhaust 7 deg x 4 deg Valve length Intake 6.600" Exhaust 6.450" (for 2.150" installed height) Valve stem dia 0.3415" (11/32") (5/16" optional) Valve train Jesel or T&D 11-deg rocker system Valve guides 1/2" OD Mag-bronze Cut for 0.500" PC seals (0.002" press) Valve guide length 3.000" Valve guide clearance 0.0014" – 0.0021" (with 0.3415" dia. valve stem) Valve guide spacing widened for use on min. 4.600" bore Valve seats Copper-Berylium, 0.008" press Valve seat dim Int 2.520" x 2.00" x 0.375" Exh 2.000" x 1.560" x 0.375" Valve seat angles 55 deg Head studs ARP # 235-4312 (or Dart kit # 66120015) Titanium valves Must use lash caps Torque w/oil Head bolts = 70 ft-lb (Dart's inner head studs 3/8"-7/16" = 50 ft-lb Manifold = 35 ft-lb) Block application Dart Big M, Mark IV, Gen V, Gen VI with proper head gasket Cyl. head weight 42 lbs Image 1 Our initial distributor offered a very neat low profile, but the large diameter of the head made installation impossible due to a lack of intake plenum clearance, so I switched out to a smaller-diameter-head MSD.

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