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Chrysler Small Block Cylinder Heads:
Casting Differences and Porting

by Paul M. Pitcher


This report will compare several modern small block Chrysler heads with those used on the earliest LA blocks and in 340 High Performance applications. Chamber configurations and volumes, valve sizes, and port volumes will be compared. Some aspects of porting the modern heads will also be covered. The sources for information contained in this report are my observations; Larry Shepard's book called "How to Hot Rod Small Block Mopar Engines", published by HPBooks (1989); "How to Build Dodge and Plymouth Performance", edited by Larry Schreib and published by SA Design (1991); and the "Mopar Engines" book, 8th edition, also written by Larry Shepard. The SA Design book highlights the work of Bob Mullen, one of the principal designers of the W2 cylinder head.

Rebuilding the Chrysler LA engine offers some interesting choices in modern cylinder heads and performance manifolds. Probably the biggest incentive to consider using modern heads is the presence of hardened exhaust valve seats, enabling the use of unleaded fuel without appreciable erosion of the exhaust valve seat. Induction hardening of exhaust valve seats was begun in the early 70s. In addition, the latest heads were designed to optimize combustion to enhance fuel economy. These so-called "swirl port" heads were introduced sometime during the 1985 model year, and were probably offered in versions for 318-2bbl, 318-4bbl, and 360 (truck) applications. Donor cars for these are becoming plentiful in the salvage yards, and with the information contained in this report, the reader can make some informed choices.

A few words on condition are warranted. Sometimes, heads found on junkyard cars are cracked. Cracks typically form in the area between the valve seats as a result of engine overheating. Therefore, fleet vehicles which have been abused in city driving (like taxis) may not make the best donor vehicles for cylinder heads. Often valves in used heads are reusable. Check the flatness of the end of valve stems as a crude indication of the amount of wear on the valve. Heavy carbon deposits in the exhaust ports are an indication that the engine was running rich, so the likelihood of burned valve seats due to an excessively lean air- fuel mixture might be lower. My observation is that considerable variation in chamber volume can be expected, especially in later castings, which tend to be of lower quality than earlier ones. Differences of one or even two cubic centimeters (ccs) between chamber volumes in the same head can be seen in later heads. Since it is recommended that chambers vary no more than 0.3 cc, it is advisable to check chamber volumes and consider equalizing them before planned machine work is started. Often it is a single chamber that is unequal to the rest. Another indication of casting quality is the surface texture in the chamber, which is much rougher in later castings. Finally, casting or machining defects are sometimes seen. If the holes for valve cover bolts are not jigged accurately, the drill can penetrate into the intake port, resulting in a potential vacuum leak that should be welded shut.

The Heads:

Production heads used on 1964-66 LA engines have closed chambers 57-65 cubic centimeters (cc) in volume. The pair I examined had chamber volumes averaging 64.5 cc. Exhaust ports average 60cc in volume and intake ports average 127cc in volume. The valves are 1.78 inches in diameter (intake) and 1.50 inches in diameter (exhaust).

For 1971 and 1972, 340 and 360 engines were equipped with 3418915 heads, so-called "J" heads, because of the cast-in J in three locations (backwards in one place) on each head. These were either equipped with 1.88 inch/1.60 inch valves, or 2.02 inch/1.60 inch valves, and have "open" chambers with volumes of 65-73 cc. Open chambers have a circular margin. The heads I examined had the larger valves and an average chamber volume of 71 cc. The ports are large, averaging 69 cc (exhaust) and 149 cc (intake).

Police sedans were equipped with a high performance 318 engine, which was equipped with 360 heads and a 4 barrel carburetor (Thermoquad through 1984, Quadrajet from 1985). The earlier ('80- '84) heads have a 4027596 casting number and a cast-in "360" on the top of an intake runner. It has an open combustion chamber with a volume of between 66 and 72.5 cc. The pair of heads I measured averaged 71 cc. The valves are 1.88 inch/1.60 inch and the ports are large, averaging 65cc on the exhaust side and 149cc on the intake side. Note how similar the volumes of these heads are compared to the performance heads of the early 1970s. Police engines are equipped with flat top pistons with no valve relief's. Calculated compression ratio is 8.4:1.

Beginning in 1985, police sedans were equipped with a different 360 head, 4323345, with larger pushrod holes, 11/16 inch in diameter, instead of 1/2 inch as found on all earlier heads. The larger pushrod holes are to accommodate hydraulic roller lifters, which were introduced in 1985. The chamber is open and its volume is slightly larger than the earlier 360 heads, 69 to 77 cc. The two heads I measured averaged 74 cc. Port sizes are very similar to earlier heads, and the larger pushrod hole does not narrow the intake port relative to the earlier heads. These heads also have 1.88 inch/1.60 inch valves. Because the piston pin height is 0.020 inches greater, the calculated compression ratio is similar to earlier engines, even though the chambers are larger. It has been suggested that the 345 heads are a swirl port design, but the port and chamber shape is indistinguishable from earlier heads.

It may be problematic to use the 345 head on certain early applications because of the large combustion chamber. In order to preserve the compression ratio, excessive amounts may need to be planed off the deck surface of the head or off the block deck. For any open chamber head, the chamber volume is reduced about 0.2 cc for each 0.001 inch planed off the deck surface of the head. If more than 0.010 inch is removed from the deck surface, the intake surface will need to be milled to allow the intake manifold to fit. Mill 0.0095 inch from the intake surface for each 0.010 inch milled off the deck surface of the head.

The 318-2bbl heads (4323302) used from 1985 on are a swirl port design with a closed (heart-shaped) combustion chamber design with a chamber volume of between 56 and 65 cc. The 4 heads I examined averaged 62cc in volume. The 302 head has 1.78 inch/1.50 inch valves and small ports averaging 54cc on the exhaust side and 118cc on the intake side. The intake ports have a more severe dogleg than earlier heads because the holes for the pushrods are larger - 11/16 inches. Cars equipped with the 302 head have a dished piston to keep the compression ratio from being too high. Some cars left the factory with nail head exhaust valves in 302 heads, others with semi-tulip exhaust valves, which add 0.6-0.75 cc to the chamber volume. There is an interesting excerpt in "Mopar Engines", page 72, describing how such a head was ported and made to flow as well or better than other small block cylinder heads. Apparently, this experimentation resulted in the master for today's Mopar Performance P4452758 cylinder head.

Table 1. Chrysler Small Block Cylinder Head Volumes (cc)

Casting Number Year Chamber   Exhaust
2465315 / 2658920 1965-1966   64.5 60 127
3418915 1971-1972   71 69 149
4027596 1980-1984   71 65 149
4323345 1985 -->   74 62 150
4323302 1985 -->   62 54 118

Porting Small Block Heads:

The impetus to consider modifications to the ports of modern small block heads comes from the need to match performance manifolds to them. As a footnote, it is interesting that production exhaust manifolds on 1985 and newer M bodies (Diplomat/Gran Fury) have openings that are similar in size to the 340 HP manifolds of '68- '70 or '71. Thus, when these or 340 HP exhaust manifolds are mated to the 4323302 heads, a huge "step" exists at the transition between the head and the exhaust manifold.

Other performance-enhancing modifications besides port-matching manifolds include 1) removing the "steps" at the transition between the valve seat and the combustion chamber and at the transition between the valve seat and bowl, 2) smoothing the transition between the exhaust bowl and runner, and 3) polishing the exhaust bowl and runner. Each of these modifications will be described. A cautionary note is in order: modifications to the port configuration will not necessarily result in performance gains. Unless the home-porter has access to a flow bench for evaluation of modifications, attempts to extensively modify the port configuration should be avoided. Instead, simple line-of-sight smoothing and removal of gross obstructions to flow should be the objective. Keep in mind the direction of gas flow and consider inertia of the flowing gas as cuts are planned. That said, for intake ports, bigger is generally better.

One other easily solved problem arises when mating modern heads to early applications. That is the air injection port present on the late heads. Simply tap this opening with a 1/4 inch-20 NC tap and thread in a socket-head set screw to plug it.


The equipment needed by the home-porter is readily available and not prohibitive in cost. A pneumatic die grinder is essential. The "mini" designs, such as Ingersoll-Rand 307A offer greater maneuverability and control. Die grinders consume a lot of air, and a higher capacity air compressor is desirable. A selection of carbide burrs is required. The most useful configurations are: 3/8 inch cylindrical, 3/8 inch round nose, taper nose, 3/8 inch ball, and 1/4 inch round nose. One bit of each type is sufficient to do several heads. Sandpaper "tootsie rolls" are also needed to polish the ports after cutting is completed. These are offered in taper and straight cylindrical shapes. About 15 each coarse and fine grade taper rolls and 5 of each grade straight rolls are needed for each pair of heads. A mandrel is needed to mount the rolls in the grinder. Good lighting is essential. A dust mask and goggles are recommended as safety equipment. The total cost of this equipment is between $100 and $200.

Porting Procedure:

Before investing many hours in grinding and polishing, take the heads to a machine shop for cleaning and magnafluxing to detect cracks. It makes no sense to invest a lot of time on a cracked head, unless you're just doing it for practice. If you end up with a cracked head, don't just scrap it, have it sliced up with a power hack saw through the ports so that you can observe the thickness of the walls in critical locations.

The first procedure for the uninitiated should be to remove casting flash on the outside of each head to be modified. This will provide the porter with familiarity with the equipment and its characteristics. Use the 3/8 inch cylindrical burr exclusively for this procedure, and save the wear on the other burrs. It will take between 1 and 2 hours to remove all the flash, including the flash adjacent to the pushrod holes, at the "window" between the rocker gear side and the block side of the head. There are 3 rules in using a die grinder: 1) use two hands on the grinder at all times, 2) keep it moving to avoid gouging the metal, and 3) take light cuts at high RPM for the smoothest surface texture and best control. Be aware that using the end of the cutter can result in the grinder spinning out of control and damaging a valve seat, for example.

Next, the manifolds to be used should be matched to the port openings of the head. First, clean up the openings on the manifolds, simply straightening the edges of the openings and removing minor casting flash. Then bolt up the manifolds to the heads. Two bolts in each manifold-head junction are sufficient. To port match the heads to the manifolds, you'll need a shop vacuum sweeper, duct tape, and a can of spray paint. The procedure described here is an easier substitute for traditional gasket matching. Tape off each valve seat, except the one for the port to be marked. Wrap duct tape around the nozzle of the vacuum sweeper as a sort of gasket so that the nozzle will seal well to to the valve seat of the port to be marked. Now, turn on the vacuum, place the nozzle in the valve seat, and when flow is established, spritz some spray paint in either the exhaust outlet, or the carb mounting flange. Just a 2 or 3 second burst is sufficient. Proceed to mark all 16 ports the same way. Let the paint set for a few minutes, then unbolt everything. You should see nice, clear impressions of the inside edge of each manifold opening around each port opening on the head. You should also see that the "roof" of each port will not need to be cut very much at all to match it to the manifold.

To port match the head, simply cut to this line of paint. Now, blend back into each port about an inch to create a smooth transition between the port opening and the port runner. Use the 3/8 inch round nose cutter for this job. Do not be concerned about the large amount of metal that needs to be removed from the lower edge of the exhaust ports. However, do not try to create a straight line from port opening to the valve seat. It is likely that the water jacket will be cut into if this is attempted. Chrysler small block heads do not flow exhaust gas particularly well, due to a retrograde flow along the floor of the exhaust ports. This "riptide" sets up turbulence in the port and impedes flow. To minimize this phenomenon, leave as much metal as possible on the floor of the exhaust ports, particularly about 1 inch in from the exhaust manifold mounting flange. This will create a sort of dam which will help prevent the retrograde flow. When the experts modify small block heads for maximum performance, some metal may even be added in this area by brazing.

There are several obstructions present in the ports which can be dealt with to varying degrees. On the intake side, there is a bump in the roof of each port near the opening. This is a cast-in boss for one of the valve cover bolts. It can be completely removed without problems unless the bolt holes are inaccurately drilled. If the holes are not centered over the web between adjacent intake ports, it is likely that the hole will be penetrated attempts are made to completely remove the boss. There is a "dog-leg" in the intake ports resulting from the pushrod hole. The wall of this obstruction is only about 1/8 inch thick on the '85 up heads, so it cannot be completely removed. However, some of it can be cut off to widen the port at this point. Do not be too aggressive, and eyeball the pushrod hole to get an idea of where you are. On earlier heads with the small pushrod hole, more of this obstruction can be safely ground off. For intake ports, the floor is critical to flow. Take care to clean casting defects from this area and the walls. Do not polish intake ports, the roughness helps to keep gasoline vapor in suspension by creating turbulence. On the exhaust side, there is a bump in the roof of each port. Do NOT try to remove this obstruction completely, or the water jacket will be penetrated. Similar to the intake dogleg, however, some metal can be removed from the highest point of this bump to make the exhaust port taller. Do not be too aggressive here either. Now that you've spent 8-10 hours on the grinder, you're developing some real skill in controlling it. You're ready to start cutting in the bowls themselves. This is the touchiest part of home- porting because of the supreme control and concentration needed to avoid gouging a valve seat and ruining the head. Make sure the burr is fully inserted PAST the valve seat before powering up the grinder. In other words, do not try to pass a spinning burr through the valve seat opening.

There are several areas that need cutting in the bowls and chamber. The heaviest cutting is to be done at the sides of the exhaust bowls, where they transition into the runners. First, feel the transition at the inside edges of the two exhaust bowls nearest the center of the head. If you were to cut the head in two, making two halves with two chambers on each half, the areas referred to here would be immediately adjacent to that cut. Compare the feel of these ideal profiles to the transition between bowl and runner in the rest of the the bowls. Feel the difference? The rest of the bowls have rather large bumps at this transition which may or may not be removed, using any burr with a round nose. My opinion is that these bumps represent obstructions to exhaust flow and should be removed, but this opinion is NOT based on flow bench results. Next, spend some time in the intake bowls with the 3/8 inch ball burr to remove edges where the valve guide was machined. Do NOT try to remove this obstruction, just round the corners off.

Probably the greatest single improvement to flow that can be made is to cut off the parting line that runs through each bowl. This parting line is parallel to the deck surface of the head, and is present at the transition between valve seat and bowl. There is a prominent corner where the outside edge of the exhaust valve seat transitions directly into the floor of the exhaust port. Cut this back so that the profile is a smooth curve. Not much metal is to be removed, but removal of this corner is one of the most important things you can do to improve the flow characteristics of small block Chrysler heads. Be VERY careful to not damage the valve seat during this procedure. The cylindrical burrs or the taper nose burr work best for this cut.

The final cut to be made is the most dangerous to the valve seats, and should be reserved until the porter's skills are maximized and NOT be tried at the end of a long day. A tiny amount of metal is removed in this cut, so fine control of the cutter is essential. The cut is made to remove the step that exists between the valve seat and the chamber itself. Some heads have very little metal to be removed in this area, other heads have a 0.040 inch to 0.050 inch step. The step is most detrimental to flow in the shrouded areas of the valve (302 heads). Use the taper or 1/4 inch round nose burr for this cut and do not try to remove the entire step with the cutter, come back with a sandpaper roll to take off the last few thousandths and finish the surface. During this cut, smooth the vertical parting line which exists at the front and back edge of each chamber. In high performance applications with radical cams, it may not be advantageous to remove this step from the intake side, because it restricts flow at low valve lift which is beneficial. If there is some restriction of flow at low valve lifts, intake charge velocity and inertial filling will be greater as the intake valve continues to lift, resulting in a greater intake charge overall. Since CFM is very small at low valve lifts, a change in flow at this point has a smaller potential effect on the total intake charge than a change in inertial filling at higher valve lifts.

OK, now the cutting has been completed, you're ready to switch over to sandpaper rolls to finish the surfaces. Start with the exhaust port runners to get a feel for how the grinder behaves. Use a coarse cylindrical roll to finish roof, floor, and sides, and a coarse taper roll to finish the corners of each exhaust port runner. You can do some slight smoothing of the intake port at the port opening, just to take off your mill marks from the previous cutting. Proceed to finishing the exhaust bowls, using taper rolls, or cylindrical rolls that have a round end as a result of use in the runners. Take off enough metal to completely remove the cast texture from the mold. Watch the valve seat as you polish to give the best avoidance of contact of the sandpaper rolls with the valve seats. After you've done everything with coarse paper, switch to fine and start over. Expect to spend almost as much time polishing as was spent cutting.

The last thing to be polished is the transition between the valve seat and the chamber. You won't be able to polish in the shrouded areas of the chamber, but try to take off mill marks from previous cutting in the rest of the chamber, and take off any sharp corners that may exist. The margin of each chamber can be rounded off slightly to prevent hot spots and preignition.

OK, now that you are finished porting, all that remains is to have the valve seats ground. The three-angle valve job is a standard in performance heads. You might need to come back with sandpaper rolls and remove any edges that arise from the seat grinding procedure, but for all intents and purposes, you're ready to hit the streets and kick some Brand X butt! The beauty of this procedure is that it is completely unapparent unless the engine is torn down. The Mopar Engines book says that the home porter can expect improvements from 5-10 HP, but that the potential exists to get 50 additional HP out of the 302 heads!

Paul's original document date unknown, (perhaps 2000). This document was originally linked to his Web site, which apparently went offline in early to mid 2003.

Be Careful! Danger! You risk severe injury or death doing mechanical repairs on your 1962 to 1965 Mopar. Don't take rash chances and don't shy away from careful use of jack stands, spring compressors, eye protection and other safety devices. Get a qualified professional technician to do the work if you are at all unclear about the repair procedures, or if you do not have adequate tools or safety equipment.

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Uploaded March 18, 2004

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