• 2015

Rebuilding Magnum Engines

LarryShepardThe V8 Magnum engines were built from 1992 through 2003 and came in two sizes — the 5.2L V8 and the 5.9L V8. The Magnum family of V8s is the third generation of Chrysler/Mopar® small blocks and replaced the earlier “LA” engines. While the 318 LA engine is the same size as the 5.2L Magnum engine and the 360 is the same size as the 5.9L, there are some unique aspects to the Magnum V8s.

All of the Mopar small blocks are very durable and high mileage is common. Beyond high mileage, there are many reasons for rebuilding an engine — breakage, burning oil or smoking, poor performance or bad noises may warrant  a rebuild. Compression tests and leak tests can give you an idea of where or what the problem is.

Once you decide to rebuild the engine, you have more decisions to make: do you pull the engine or rebuild it in the vehicle? Do you want to change something in the engine? Do you want more performance? And then there’s the important question of the cost of repair. Of course, this part of the balance sheet may not be known until you take it apart.


The Magnum V8s are a very high-tech family of engines. They are all MPI — multipoint injection — which means they all have ECMs (engine control modules). The ECM controls both the fuel injection system and the ignition system. There are all kinds of sensors (about 6 or 7 main ones). The sensors can be damaged or broken, but typically the ECM doesn’t need to be replaced at an engine rebuild. Sometimes replacing the sensors is required.


At an engine rebuild on a high-mileage engine, you can assume that the air cleaner element should be changed. Stock replacement or upgrade to a low restriction unit is recommended. Also, the oil filter should be changed, but don’t throw the old one away just yet. If you plan on painting the engine, you can install the old oil filter to cover the opening during painting and then throw it away (drain oil before using).

An engine rebuild will require a complete engine gasket set and new bearings – both rod bearings and main bearings. While some bearings may look like new, it is not worth the gamble to re-use.


Beyond replacing the spark plugs at an engine rebuild, the distributor cap and rotor (P4876255AB) should also replaced. Magnum parts are unique from the LA engine parts. The plug wires are a grey area — they could be fine, but it’s wise to replace them if the engine has over 150,000 miles on it.


When taking an engine apart, one of the key issues is the condition of the short block. You will need the help of your engine shop at this point to determine if the block is damaged or if it can be rebuilt. The Magnum blocks are thin-wall, cast blocks so you should try to limit the overbore to .020″ or .030″ overbore. If one cylinder is damaged (scratched), then it may be sleeved. If the block is broken and can’t be repaired, it will need to be replaced.

With any overbore, always check the availability of pistons and rings in the desired configuration before any machining is started. If you plan to overbore a block more than the .020″/.030″ then you should sonic test the block before any machining is done!

Generally the crank can be re-used as is or by grinding the journals .010″ or .020″ undersized and using matching bearing sizes. Since the engine is apart and if more performance was desired during your “decisions,” then replacing the crank with a longer stroke crank could be considered.

There are many options like the small main 3.58″ stroke crank P5007257 for the 5.2L engines (344-inches at stock 3.91″ bore) or the large main 4.00″ stroke crank P5007258 for the 5.9L engines (402-inches at stock 4.00″ bore). There is also a 4.00″ crank for the 5.2L engines. The rods can be re-used but consider replacing the rod bolts. The 5.2L engine uses a flat top piston while the 5.9L engine uses a dished piston. Almost all rebuilds will require the pistons and rings to be replaced. The pistons are selected based on the final bore size, crank selection and rings. The 5.9L Magnum pistons are much lighter than similar LA engine pistons. Magnum engines use 1.5mm rings. The other aspect of the piston is its compression ratio – stock is about 9-to-1. You don’t want to increase this number if you plan on using pump gas.


Generally, you can’t tell much about the camshaft until you take the engine apart. All Magnum engines use the hydraulic roller cam design. If you find a broken tappet at disassembly, you can replace it – P4876054 individual tappet.

While you want to keep the tappets with the cam if the cam lobes are OK, then the tappets can probably be re-used. If the lobes are damaged, the cam and tappets should be replaced.

The cam can also offer more performance. As an upgrade, consider hydraulic roller cam P5155561 for the 5.2L engines, which has 258/264 degrees duration and .480″ lift, and P5155562 for the 5.9L engines, which has 264/270 degrees duration and .480″ lift. Some Magnum engines come with a silent chain cam drive. Consider upgrading to a double-roller chain drive like P5249267 (with 3 matched keyways). As a general rule on high-mileage engines, plan on replacing the cam drive set (the chain and two sprockets). Unless there has been an oiling failure, you can generally re-use the pushrods and rocker arms.


All Magnum engines are fuel injected. There are eight injectors — one per cylinder. They should be cleaned but can be re-used. The intake manifold is made of aluminum and can be used as is. The production intake is somewhat round and is nicknamed “the beer barrel.”


If there was an engine failure, then the cylinder head (one or both) is most likely damaged. But, if it can be repaired your engine shop can help you with that analysis.

So, let’s assume that the heads can be rebuilt. The key issues, after the basic casting, are the valve guides and the valve seats. Typically, high-mileage heads have worn guides and they must be replaced. Likewise, the valve seats can be worn and will require rework.

One solution is to install large (oversized) valves like 1.97intake P5249877 and/or 1.625″ exhaust P5249876. Both are stainless steel valves. The large head diameter allows the engine shop to grind the seat on a larger diameter. This doesn’t always work.

Another option is new seat inserts which can be installed by your engine shop. The Magnum uses 8mm valve stems and your gasket set should include new valve stem seals – be sure to use them! If the engine was severely overheated, then the valve springs should be replaced.

A higher lift cam can also require new springs. A set of HP springs P5249464 with retainers P4452032 could be the solution. Some Magnum engines use beehive or conical springs, which require the spring and retainer to be looked at as a set.

Many Magnum engines were installed in trucks and it was popular to install a supercharger on these engines. If this modification is planned, be sure to drop the compression ratio from 9 (stock) to about 8.

If the engine is already built, or the pistons are already purchased, then the solution is the Cometic extra-thick head gasket (about .090″ vs. about .040″ for the stock gasket). They also have extra-thick intake gaskets to go with it.

So, what if the head is cracked or broken and must be replaced? This can be tricky because new cast iron heads are somewhat in demand. No worries, Mopar® offers cast iron Magnum heads (P5155469) and aluminum Magnum heads (P5153847).


According to the Engine Rebuilders Council, having a damaged engine rebuilt costs an average of 87% less than the cost of a new car. Rebuilding is good business for your shop — and reduces expenses for cost-conscious customers.

When rebuilding a Magnum engine, remember that the A-engine water pump runs in the reverse direction from the Magnum pumps so they can’t be swapped. The Magnum intake manifolds have a smaller thermostat housing/gasket than most A-engines. There are examples going both ways so check it out early.

There are also some unique one-piece damper and front pulley systems on the newer Magnums that can cause problems with the fan belt drive system (upgrades mainly). There are also valve cover (10-bolt) issues usually solved by re-using your original set.

These are just some of the more popular options that you may encounter in a normal rebuild. For more information and added details, please contact the Mopar Performance Tech Line – 888-528-HEMI (4364) or go to the website – www.mopar.com.



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