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Interchangeability between the 318/360 Magnum V-8s and the millions of '91-and-earlier 318s and '92-and-earlier 360s is limited mainly by design departures in the Magnum heads, oiling system, and camshaft provisions. So, few Magnum parts (aside from the crankshaft, connecting rods, oil pump, distributor, distributor driveshaft, timing chain, sprockets, and some nuts, bolts, and gaskets) are compatible with non-Magnum versions of the same engines.
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With its inlet system and a cylinder head removed, a Magnum is distinguished by the lack of an exhaust-heat crossover passage in the remaining head's intake face, and by the odd-looking holes for attaching the intake manifold. The 5.9L Magnum's cast-aluminum pistons weigh 100 grams less than those in previous 360 versions. With 11.20cc dished domes, the pistons are also used in Mopar Performance's (MP) 360 Magnum crate motors. The retaining spider on the valley floor is dictated by the roller tappets' yoke setup. Note the lack of a head-oiling passage in this block's deck. The rear-sump oil pan is an OE truck item.
Magnum Interchangeability
As with most swaps, the practicality of exchanges between early and late A-engines depends on the availability of parts and financial considerations. As interchange hardware develops in response to market demand, Magnum/non-Magnum swaps will become simpler and more direct. Mopar Performance's (MP) Magnum crate motors, for example, offer bolt-in benefits for most Chrysler models, some of which we'll sample for ourselves when we screw a hot 360 into our Plymouth Valiant test-mobile. The tweaks and techniques MP uses to morph these motors into crate shape are the basis for the conversions covered here. This review of the main mechanical factors will help clarify which early parts work with which late pieces, and vice versa.
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The Magnum's cast-iron nodular cranks share journal sizes and spacing with pre-Magnum versions of the same (5.2L and 5.9L) V-8s.
Pre-Magnum Heads On A Magnum Block
The installation of pre-Magnum cylinder heads on a Magnum block-to take advantage of their availability, flow capacity, shaft-mount rocker arm system, and a wide selection of rocker gear and intake manifolds-involves numerous considerations, some of which have yet to be resolved completely. Since a stock Magnum head flows reasonably well, the only possible performance enhancement would be the late 360 4448308/4772576 castings (1989-1992), or one of MP's W-series race or W-2 econo heads. If oversize valves or porting are part of the plan, the 308/576 heads offer considerably more flow potential than the Magnum. But earlier production A-series heads (even the legendary J and X muscle-motor castings) can't beat a Magnum in out-of-the-box low- and mid-lift airflow. Beyond these considerations, mechanical adaptability isn't as straightforward.
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Hrdp 9809 03 O+magnum And Non Magnum Parts Exchange+steel Forge 4/13
Steel forgings are 6.123 inches long and have 3/8-inch big-end fasteners. The rods exchange directly between 5.2L and 5.9L motors and between Magnum and non-Magnum A-series V-8s.
In the first installment of our series, we discussed the differences between the lube routes in Magnum and pre-Magnum engines. To review, the oil passages in older A-blocks run from the second and fourth cam journals to each deck face and feed lube to the rocker gear and valvetrain. Many early-run Magnum blocks had oil holes drilled in the conventional fashion, but some Magnums now come through without them. In order to use early heads on a late block, the top-side oiling routes must either already be open or be opened up. Although the machine-tool-friendly gray iron blocks have sufficient structure through which to cut the holes, the required passage isn't a direct one. Lube flow follows a multiplane route, which makes drilling for oil a machine shop operation.
But even with open oil holes, there are still concerns as to whether the Magnum's pushrod angles are compatible with the provisions of a non-Magnum head. Don't forget, Magnums have roller tappets, which are taller than slider cam tappets. That affects the pushrod paths between the tappets and the rockers, as well as the clearance at the A-head's pushrod tunnels and outer port walls. The 308/576 pre-Magnums (and roller cam 318 4323302 pre-Magnums) were cast with enlarged pushrod passages to work with the '89-'92 360 (and '85-'91 318) OE roller cams, so they shouldn't cause a conflict. The tighter "tubes" in all earlier heads are another matter, however. If you try to work around this clearance problem by using a pre-Magnum-style slider cam (and its shorter tappets), you must also convert the front of the engine to contain the longer camshaft. But it is not known whether using shorter tappets in taller Magnum bores creates any clearance clash when various valve lengths and lobe and rocker combinations are employed.
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To adapt pre-Magnum A-engine intake manifolds to Magnum cylinder heads, a series of angled spacers gives the attaching bolts square surfaces to seat on. Tightening torque should be applied from the center out in gradual 12 lb-in increments: first to 72 lb-in, then to 12 lb-ft. Don't try to fit an OE MPI manifold to pre-Magnum heads by reversing this angled adapter plan-it doesn't work.
Magnum Heads On A Pre-Magnum Block
Tips on installing Magnum cylinder heads on a non-Magnum A-engine must be prefaced with the question, "why?" Sure, there's a major price difference between new castings of both types: A bare P4876256 Magnum from MP lists for $127, while the P4529269 pre-Magnum 308 counterpart is $365. But the actual costs depend on the parts you already have on hand. Performance-wise, there's little, if any, advantage to using Magnum heads-again, depending on what's already on the motor. Magnums have slightly larger intake valves (1.925 inches versus 1.880 inches) than most previous A-heads, but an out-of-the-box Magnum casting doesn't respond as dramatically to modifications as the 308 head does, particularly on the intake side.
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The unique orientation of a Magnum cylinder head's intake manifold bolt holes (right) is illustrated by this comparison with the fastener angle in a pre-Magnum casting.
Preliminary investigations have shown that a Mag casting flows effectively only up to 0.550 inch of lift, a factor that limits competition applications to stock-style assemblies. Displacing 65 cc, compared with the nominal 70 cc of the 308 open chambers, the Magnum's smaller chambers raise static compression by fractions of a point. But installing Magnum heads means using the retro rocker train rather than the A-motor's advantageous pivot shafts. The swap also requires pushrods of the correct length to fit between the Magnum rockers and non-Magnum tappets and still maintain proper geometry throughout the lift range.
In addition, the vertical intake manifold fasteners on the Magnum head make inlet castings especially sensitive to tightening torque and sequence upon assembly. And carbureted intake systems from earlier Chrysler A-engines are attached by bolts nearly perpendicular to their head face, making it difficult to adapt these systems. The selection of Magnum-compatible intake manifolds is currently limited to a pair of four-barrel M1 alloy castings from Mopar Performance: the P5249500 dual plane and the P5249501 single plane.
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Bolted to the tappet chamber floor, this sheetmetal spider's flexible legs (tabs) retain the alignment yokes that link each tappet pair. Not all A-series blocks can accept an OE spider.
One solution for an early-to-late intake switch calls for elongating the bolt holes in an early manifold and using six drilled, angled spacer blocks along the top of each flange for the appropriately longer 5/16-inch attaching fasteners to clamp on to. A somewhat cruder approach is to redrill and tap the bolt holes in the Magnum head to suit the attaching specifications of the early manifold. Although there's not much metal for new fasteners to grab, this setup will seal if all the mating face angles are correct. With the Magnum's intake port only 0.0625 inch wider and about 0.012 inch shorter than a 308's, minor matching can make two-, six-, and eight-barrel tunnel-ram Magnums practical. And rules-restricted Magnum bullring racers will have 30 years' worth of OE iron inlet castings to pick from.
Pre-Magnum Cam In A Magnum Block
To compensate for the limited selection of replacement Magnum camshafts, we strongly recommend MP's crate motor conversion plan. Last month we outlined how an early A-engine hydraulic cam fits into a Magnum block by using a deeper non-Magnum timing case cover (P5249930) and the appropriate tappets. The deeper cover makes room for the fuel pump eccentric on an old cam's longer snout, and the cover accepts the mechanical pump. But using a deep cover also means forgoing the OE Magnum serpentine accessory drivebelt setup. The Magnum timing chain and sprockets will work with an A-series cam, but a non-Magnum cam needs a longer key (Chrysler PN 106751) for its drive sprocket. Want to try a solid-lifter conversion? MP offers an extruded-aluminum, full-roller, adjustable stud-pivot rocker arm kit (PN P5249800).
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Pre-Magnum hydraulic-cam rocker gear interchanges almost completely within Chrysler's A-engine series, but rights and lefts cannot be swapped.
Maximum Magnum: The Crate Motor Version
Like both of its cross-town rivals and a whole slew of aftermarket suppliers, Mopar Performance builds prepackaged Magnum horsepower in the form of assembled crate motors. These crated eights come in two versions and make more power and torque than MP's first-generation 360 Commando and Super Commando crate motors (which we reported on in our April and October 1992 issues); and they're available nationwide.
Based on a production 5.9L Dodge Magnum truck short-block and iron OE cylinder heads, both versions feature replacement camshafts, intake manifolding, and support equipment. An MP ignition kit (including a magnetic impulse distributor, an electronic control box, and a harness) is bundled with each engine. Customers must supply a carb, linkage, fuel and exhaust plumbing, secondary cables, spark plugs, a coil, an alternator and brackets, pulleys, belt(s), and fluids. The current Magnum 380 makes 380 horsepower at 5,300 rpm and carries a sticker price of $3,995. The more sedate 300-horse Magnum 300 lists for $3,595. Maximum torque is rated at 410-plus lb-ft at 4,400 rpm, and 375 lb-ft at 4,000 rpm, respectively.
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Magnum rockers are not interchangeable with the rockers on non-Magnum versions of the A-series small-block, but intake and exhaust Magnum rockers are swappable.
In converting an OE Magnum truck V-8 to a crate motor, MP follows hot rodding horse sense and eliminates the breathing limitations of the production powerplant-the intake system, cam, and exhaust. In truck trim, Magnums are fitted with sequential multipoint electronic fuel injection (SMPI). Although SMPI improves metering to optimize driveability and gas mileage, the conservatively calibrated management system doesn't offer much of a high-performance advantage. And because a stock Magnum's bulbous "beer barrel" intake manifold is too tall to tuck under a flat passcar hood, MP engineered substitute castings for conventional carburetion.
MP's 380 comes with a single-plane, high-rise M1 manifold that, used with the recommended 750-cfm carb, clearly favors top-end output. The torque-oriented 360/300 comes with a dual-plane casting that's also suggested for service with the same size carburetor. The replacement cam in the 360/380 is an aggressive pre-Magnum, A-engine-style MP roller. The tamer 360/300 is assembled with a milder roller grind that makes it much more suited to towing duty than the 380 is. And although the truck Magnum's iron exhaust manifolds are as effective as any molded for an old A-motored musclecar, the crated eights come without them.
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A Magnum's hydraulic roller tappet is considerably longer than the conventional A-engine version. The rollers dictate the Magnum block's tall tappet bosses and correspondingly shorter pushrods.
In time, demand is bound to drive the aftermarket to meet enthusiast needs with more-practical bolt-ons. But until then, viable solutions will depend on the ingenuity of hands-on hot rodders.
Chrysler Small-Block: OE V-8 Cylinder Head Applications
Cylinder Head Designation Model Years Engine Size, Intake Type Casting Number Valve Sizes (intake/exhaust)
X 1968-1971 340, 1x4 2531894 2.02/1.60
1970 340, 3x2
2.02/1.60
J 1972 340, 1x4 3418915 1.88/1.60
1971-72 360, 1x4
1.88/1.60
308 1989-1992 360 TBI 4448308 1.88/1.60
4772576
Magnum 1992-98 318 SMPI 5302466 1.925/1.625
360 SMPI *
*Note: A "53006671" casting number has also been seen in Magnum service.
Pivot Points
When Chrysler Corporation acquired American Motors in late 1987, it absorbed some of its employees, facilities, and manufacturing techniques. So when Jeep Truck Engineering convened to convert Chrysler's small-block engines to Magnum form, AMC thinking influenced the redesign. This is most evident in the truck motors' rocker gear, where cost considerations prevailed over time-honored technology. And instead of retaining Chrysler's proven shaft-pivot rocker arm system (and the stability, durability, and precise action it delivers), engineers formulated a more cost-conscious ball-stud pivot arrangement to work the Magnum's valves.
Compared with a shaft system, a ball-stud rocker train is noisier and requires pushrod guideplates, and the arms rock with more wasted motion. The studs work better if the valves don't have to be lifted too fast or too far. (For extreme performance, endurance, and high-lift applications, the OE sled fulcrum's 5/16-inch attachments are way flimsy, and 3/8-inch replacements are recommended.) Moreover, the cylinder head flow gets choked where the studs' support structure sticks through the roofs of the intake ports. And the ball studs' oiling requirements dictated new lube routes through the Magnum's block and heads, which limit their swapability.
None of this is meant to imply that there's anything mechanically wrong with the stock Magnum rockers. They're perfectly adequate for heavy-duty truck use. Our dragstrip tests and Chrysler's warranty policies and TSB history bear this out.
Production Magnum rockers have even seen severe service in NHRA Stock classes, where Dakota trucks have been running hard for years.
It's just that hard-core Mopar guys have always been proud to point out that when competing manufacturers' engines get prepped for maximum performance, their production-style stud-mount rocker pivots are usually swapped for a shaft setup.
Because ball studs are less expensive to manufacture and assemble in volume, the bean counters ruled again, and enthusiasts took a giant step backward. Fortunately, we'll get to do our own rocker-durability check on the 380 Magnum in our Valiant test mule. -AK
Non-Magnum Block In A "Magnum" Engine Assembly
It is feasible to use a pre-Magnum cylinder case in a Magnum engine assembly. The differences between the two are molded into the Magnum's taller tappet bosses and tappet-retainer attachment provisions. The short tappet bores in an early block can be extended by overboring the bosses and press-fitting bronze bushings that are 0.400 inch taller (and honed to the stock 0.9055-inch inside diameter). But because the bushings will block oil feed to the tappets, they have to be drilled or reamed longitudinally to restore the flow. And although the unsupported upper segments of tall bushings may have a hard time handling high pushrod side loads, they could be made to work in an OE-type application.
The other snag in a potential block swap is the missing holes for the roller tappets' stamped-steel retaining spider in the lifter valley floor. Most A-blocks don't have enough structure above the cam tunnel to drill and tap, but the 318 (1985-1991) and 360 (1989-1992) engines come with these three threaded holes. (See the camshaft applications chart for the blocks cast with these features.) Fortunately, all of the Magnum cams that we have seen (including P5249549, MP's R/T replacement Magnum truck motor grind) still have oiling grooves in their No. 2 and No. 4 cam journals that will direct lube to non-Magnum heads.
At this point in the block conversion, both Magnum and non-Magnum cylinder heads can be made to work. Using Magnum heads calls for Magnum rockers and pushrods. Non-Magnum heads need non-Magnum rocker gear and special-length pushrods. -AK
Chrysler Small-Block: OE Roller Tappet Camshaft Usage
Years Engine (1) Version
1985-89 5.2L/318 Passenger car (2)
1988-1991 5.2L/318 Dodge truck (3)
1989-1992 5.9L/360 Dodge truck
1992-99 5.2L/318 Magnum Dodge truck/Jeep
1989-1999 5.9L/360 Magnum Dodge truck/Jeep
Note 1: For use with roller camshafts, these engine blocks were cast with taller tappet bosses and three fasteners in the lifter valley for the tappet-yoke retainer. All '88-and-earlier 5.2L and 5.9L V-8 A-engines were carbureted.
Note 2: Chrysler built its final rear-wheel-drive passcar models (Dodge Diplomat, Plymouth Gran Fury, and Chrysler Fifth Avenue) for the 1989 model year.
Note 3: The '89-'90 Dodge Shelby Dakota (N-body) truck came with a 5.2L/318 roller cam non-Magnum V-8.
Mopar Performance Magnum V-8 Crate Motors
Feature 360/380 360/300
Assembly part number P5249499 P5249498
Compression ratio 9.5:1 (advertised), 8.7:1 (JTE nominal)
Cylinder heads Cast-iron, 18-degree valve angle high-swirl intake ports, combustion chambers
Intake valves 1.925-inch diameter, 5/16-inch stems
Exhaust valves 1.625-inch diameter, 5/16-inch stems
Rocker arms Ball-stud pivot, 1.6:1
Camshaft Hydraulic roller tappet
part number P4876348, without lifters P5249663, with lifters
application MP replacement '89 OE 360
duration 288/292 degrees 274/264 degrees
lift 0.501/0.513 inch 0.385/0.401 inch
Valvesprings High-performance, 1.42-inch diameter
Valve seals Banded Viton rubber, static
Retainers Chrome-moly steel, 7.5-degree plunge angle
Oil pan Center sump, 5-quart capacity (with filter)
Intake manifold Single plane Dual plane
M1 aluminum, square-bore carb pattern with provisions for
multiport injection
Ignition system Mopar Performance electronic kit vacuum-advance distributor
Valve covers Cast-aluminum, black-wrinkle
Fuel Premium only Best available grade
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Hrdp 9809 10 O+magnum And Non Magnum Parts Exchange+pushrods
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Hrdp 9809 10 O+magnum And Non Magnum Parts Exchange+pushrods
Hrdp 9809 11 O+magnum And Non Magnum Parts Exchange+pushrods
Hrdp 9809 12 O+magnum And Non Magnum Parts Exchange+mopar 59L V8 Engine
Hrdp 9809 10 O+magnum And Non Magnum Parts Exchange+pushrods
Hrdp 9809 11 O+magnum And Non Magnum Parts Exchange+pushrods
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Author: Al Kirschenbaum
Contributors: Al Kirschenbaum
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