Cylinder Head Castings

Numerous cylinder heads are available for the AMC six.  All utilize the same bolt pattern from 1965 to present Jeep 4.0L heads.  Heads from 1964 to 1981 and all 4.0L heads use a 1/2" diameter head bolt, while heads from 1982 to 1988 for the 258 use 7/16" diameter head bolts.  Heads manufactured during a given model year were used on all engines of that model year; there is no difference therefore between 1975 232 and 258 heads, for example.  Valve head size and stem diameter are the same for all heads except the 4.0L which uses a larger valve head and smaller stem diameter.  Rocker arm configurations vary from year to year, but the majority of heads use a non-adjustable bridged pivot design with stamped steel 1.6:1 rocker arms.  Some older heads utilize a shaft mounted rocker arm system with ductile iron 1.5:1 rocker arms.  Port sizes vary; older heads are typically considered “large port” heads, while newer heads are considered “small port”.  The 4.0L head utilizes a revised port design of larger intake port cross section than the small port heads but comparable exhaust port cross section.  The 4.0L head also utilizes raised intake and exhaust port locations for increased flow.

Large port heads were definitely used from 1965 to 1970, although there are rumors that they may have been used up to 1974.  This is very unlikely due to the fact that AMC spent thousands on flow development of the newer small port design introduced in 1971.  All large port heads I have found utilize shaft mounted rocker arms with an oil passage in the head that transfers oil from the camshaft bearing oil supply to the rocker shaft.  Severe scoring and wear between the shaft and rockers is a common occurrence with this arrangement, and although there are multiple proposed solutions to the problem, none will result in the reliability and longevity required for serious performance applications, except a complete replacement of the system with a custom roller rocker arm arrangement.  Large port heads feature semi-closed or fully open chambers, depending on the model year.

Small port heads were used from 1971 until the end of AMC production.  The small port heads utilize very small exhaust ports with a substantially higher floor than the large port heads.  Small port heads use either a small, open combustion chamber design or a larger, semi-closed chamber depending on the model year.

  MODEL YEAR   ENGINE     CASTING   ROCKERS       PORTS      CHAMBER      CC’S 
    1964-67    199/232    317071     SHAFT        LARGE    SEMI-CLOSED     56
    1968-70    199/232    318988     SHAFT        LARGE        OPEN        62
     1971      232/258    3211040    ?????        SMALL        OPEN        62
     1972      232/258    3213809    ?????        SMALL        OPEN        62
     1973      232/258    3213809    ?????        SMALL        OPEN        62
     1974      232/258    3213809    ?????        SMALL        OPEN        62
                          3222652    ?????        SMALL        OPEN        62
    1975-76    232/258    3224490    STUD         SMALL        OPEN        62
                          3224491    STUD         SMALL        OPEN        62
     1977      232/258    3217259    STUD         SMALL        ????        ??
     1978      232/258    ???????    STUD         SMALL        ????        ??
     1979      232/258    3227258    STUD         SMALL    SEMI-CLOSED     74
     1980      232/258    ???????    STUD         SMALL    SEMI-CLOSED     74
     1981      232/258    ???????    STUD         SMALL    SEMI-CLOSED     74

Note:  Casting numbers are based on the NHRA list of approved castings, and thus correspond to model year.  It is possible for a particular casting to have been installed on a model year vehicle up to one year previous or one year after those listed in the table, depending on the actual date of the casting and AMC’s remaining stock of previous years heads during the assembly of the vehicle.  Combustion chamber cc's for 317071, 318988, and 3227258 were measured from heads in my posession.  All other volumes are from outside sources.
 

 

Here's some shots of casting 317071 (1966-67 199/232).  The chamber is very tight, and even the area near the spark plug does not come out to the same radius as the cylinder bore.  The ports are massive in the as-cast configuration.

 

This is casting 318988 (1968-70 199/232) with some decent home porting.  The chambers are fully open, and the ports are again massive.  This is the head I am running right now.  Note how close the edge of the valve head is to the edge of the chamber - this valve shrouding is only increased by larger valves, making them a dubious performance modification.  I'm running stock valves in this head.
 
 
This is the chamber of head 3227258 (1978 258).  The only port pictures I have are severely ported and don't give you an idea of how small these ports are.  They are smaller than the 4.0L's, which are shown in the next picture.
 
 
Here's the ports of a 1998 4.0L.  Check out the miniature exhaust port - not exactly drag racers paradise in the stock configuration.  Compare this to the ports in my 1968 head and you'll see why I haven't switched to the 4.0L head.  The potential to outflow the old, large-port heads definately exists, but it typically takes a flow bench to determine how to achieve it.
 
 
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