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Magnum Head LA Block Hybrid

There seems to be more interest in this conversion with recent articles published in High Performance Mopar and Car Craft. What follows is a collection of my experiences in making this conversion and a collection of information and advice from various sources including Moparts members.

Converting a flat tappet LA motor to Magnum heads offers several advantages. The small fast-burn combustion chamber (60 cc) with large squish area allows increased compression on pump gas and reduces the need for excess timing lead (31 degrees works well). These heads also offer an excellent exhaust port and a low volume high swirl intake port (153 cc) that provides great low-speed throttle response and enough flow when mildly ported to support nearly 450 HP.

I recently completed a hybrid LA/Magnum 360 build up. In its current configuration, I used Weisco flat-top pistons set at zero deck with the valve notches increased slightly for additional volume. With combustion chambers polished (61 cc), the compression ratio works out to around 10.6:1. With a Comp Cams XE268, it runs fine on 92 octane fuel at sea level.

There are a few challenges to overcome when installing these heads on a flat tappet LA block. These challenges include:

Intake Manifold - The Magnum heads have a vertical mounting bolts. MP offers a single and a dual plane intake for this combination. LA-type intakes can be adapted by re-drilling the intake or re-drilling the heads. I used an RPM Airgap intake and chose the latter method because this intake didn't have enough material in the outboard holes to drill vertically.

Rocker Gear - Magnum heads utilize stud-type rocker gear that is oiled through the pushrod. MP offers special length pushrods for this conversion as well as Jeep/AMC style lifters. Many of the aftermarket Mopar lifters already have the oiling holes such as Comp Cams #822. You will notice these lifters are also recommended for AMC applications. The stamped rocker arms (1.6:1) are available through MP and are inexpensive. These are the same rocker arms as used on the Magnum crate motors. Aftermarket SBC roller rocker arms can also be used although I have no first-hand experience with them.

Valve Covers - Magnum heads use 10 bolts to secure the valve cover to the head. LA style valve cover bolt-holes will align properly, but the gasket sealing area is not an identical match, but would probably work.

Intake Ports - The intake ports are smaller than a typical 340/360 LA head but can be ported to match an LA intake.

Exhaust Ports - The exhaust ports and flanges will mate to LA style headers. However, they are not asymmetric like the LA ports: the Magnum ports don't include the "point" on the floor of the LA port. When mated to set of LA headers, this overlap provides some anti-reversionary effect.

Head Bolts - The Magnum heads use a unique set of bolts. I found after having my block decked that some of the head bolts would bottom slightly when the heads were installed without a head gasket. The holes were deep enough, they just needed to be tapped a few turns deeper.

Heat crossover - There is no exhaust heat crossover provision in these heads.

Here's a list of what you will need to complete the swap. I found the price through MP for rockers, pushrods, head bolts, and assembled heads to be surprisingly reasonable.

Magnum Head Assembly MP# P5007086 (two required)
Magnum rockers, stands, guide plate, and bolts: MP# P4876050 (eight required)
Special length Pushrods: MP# P5007477
Mopar/AMC lifters: Comp Cams #822 or MP4529220
Magnum head bolts: MP# P4876759
Magnum valve covers (desirable, maybe not required)
MP Magnum intake OR
Modified LA intake OR
Bushed and re-drilled heads (to adapt LA intake)
Spark plugs for Magnum heads - I used Autolite #3923
Standard LA gaskets
Throttle cable bracket from MP (only used with vertical drilled intakes)MP# P4876850
Throttle return spring bracket from MP (only used with vertical drilled intakes): MP# P4876313

Additional sources of data:
February 2002 Car Craft: Magnum head flow #'s and comparison to earlier heads.

October 2001 High Performance Mopar: Converting LA block to Magnum heads.

I had purchased a set of heads from a boneyard and decided to make this conversion.
I bought the appropriate retainer(P4452032) and went with the high lift spring (P4876062)
However, with the casting of my head was off such that the install height was only 1.60" instead of the recommended 1.64" height. This made spring seat pressure with the purchased spring being 149lbs.
Then I purchased and tried the weaker medium lift spring which was near the limit of my cam I was using. (0.520 Lift)
However at the 1.60"install height the seat pressure was still a whopping 139 lbs. Way too much for a hydraulic cam.
So in searching for another solution I wound up going with Crane as it appears to be the onlyother alternative.
Part numbers are:99840 for the spring and 99947 for the retainer. Using the MP locks as the 8mm valves are radial groove and not square groove.
This gives a install height of 1.58" which gives a seat pressure of 108 lbs. I did have to grind the guide some to get a proper clearance of seal/retainer, but wasn't expensive to get done. So, this is a solution for those that have poor castings that aren't to mopars supposed specifications.

Jim E


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