Hudson Big Six Engine Tech
Guys I thought I would start a Hudson Big Six engine tech thread due to questions I have been receiving from quite a number of people. I'll start with the basics of the history and go into where to get the engine parts then to some of the best modifications to make these old monsters scream.
History:
NARROW BLOCK
Hudson introduced the Big six engine line in 1948 as a 262 cubic inch only. Bore is 3.5625" and stroke is 4.375". This is the Narrow Block engine and was produced as 262 cubic inch in 1948, 1949 and 1950. In 1950 a 232 cubic inch version was introduced for the Pacemaker line . The 232 version has the same bore as the 262 but used a 3.875" stroke crankshaft. Narrow block engines have different cylinder head water passages and the lower oil pan rail is straight from the front engine plate to the rear engine plate.
Engines made in 1948 and some 1949 used aluminum 2bbl intake manifolds.
All 232 versions used a 1bbl intake manifold. Cylinder heads were either aluminum or cast iron and were unmarked in 1948 and 1949. In 1950 the cylinder heads were marked next to the thermostat housing location "500" for the 232 version and "501" for the 262 version.
Valve sizes are 1.8125" for the intake and 1.5625" for the exhaust. These sizes remained as the standard valve size through the entire production run from 1948 until 1956.
All engines received hand fitted pistons and bearings during production. The information for the pistons was hand stamped into the block behind the lifter and valve covers on the right hand side of the block. All of these engines were assembled with the utmost care at the time of manufacturing.
Crankshaft flywheel flange uses 7/16" bolts pressed into the crank with nuts and a special lock plate. These have been an issue since day 1 of this engine. The flywheels for both stick shifts and hydramatics must be sealed to the crankshaft to prevent leaks with good high quality Permatex #1 sealer.
These engines are suitable for stock and mild performance rebuilds but not recommended for high performance usage. Cylinder heads and oil pans do not interchange with the 1951 and later big six engines.

WIDE BLOCK
1951 saw the release of the Wide Block Hudson big six. With this came the release of the 308 cubic inch Hornet engine. Cylinder head water passages were improved and the block has a kick out for the longer stroke crankshafts of the 308 cubic inch version.
Bore of the 308 is 3.8125" and the stroke is 4.500". The 232 and the 262 retained the same bore and stroke as previous years. The 308 version also has stiffening ribs added to the lower area of the block about 2" above the oil pan rail. Minor refinement was incorporated each year until 1955. Valve relieve area was improved each year to increase air flow over the cylinder.
All Hudson big six engines used 7/16" head bolts except for the 7X racing engine built by the Hudson Race Shop. These 7X engines used 1/2" head bolts for added head gasket retention. Details on the 7X engine will be covered down later in the information.
Twin-H was made available in the 1952 Model year. It consisted of a special cast iron intake, twin Carter 1bbl carburetors, two air cleaners, special linkage and an exhaust manifold with two choke stove pipes in the center two exhaust ports. Twin-H added 10 horsepower to the standard 262 and 308 engines. It was not available for the 232 version. The Twin-H was an assembly line option as well as being available as dealer add on item.
Aluminum cylinder heads were optional on all three engine displacements during this time. Usually the aluminum head bumped the compression by .5 and gave a slight increase in horsepower. Head gasket retention with the aluminum cylinder head has always been an issue so beware if buying a used aluminum cylinder head.
Camshaft selection changed several times over these years. Two of the performance camshafts Hudson used in most engines were the "742" and the "040". Both of these camshafts started out as 7X racing camshafts and are suitable for a daily driven Hudson. The "742" camshaft had the ramp opening and closing rates altered in 1955-56 for use with hydraulic lifters and work very well with mechanical lifter for street performance.
The actual specifications for these two camshafts have never been accurately listed except for the lift. The "742" has .354" valve lift and the "040" has .390" valve lift. Both lift specs are at Zero lash. New reground camshafts are still available that offer more performance than these two so keep that in mind.

7X
The 7X engine was released sometime during the 1952-53 for racing and severe usage. These were not assembly line installed but available for installation at the dealerships or as a crate engine. These engines were hand built and modified by the Hudson Race Shop on a very limited basis.
It was only available as a 308 cubic inch version.
These 7X engines started out as hand selected block cores for modifications. Each block was selected for the least core shift and the thickest cylinder walls for durability.
Each engine received larger 2.00" intake and 1.6875" exhaust valves. The valve ports and bowls were hand ported and opened to match the larger valve sizes. The relief area was hand machined to remove any flow restrictions over this area of the block into the cylinder.
The head bolt threads were enlarged to 1/2" diameter for better head gasket retention. All 7X engines used a 232 cylinder head for higher compression, 8.7 to 1 with the cast iron head and 9.2 to 1 for the aluminum version.
Camshafts started out as the "742" camshaft and then progressed to the "040" camshaft sometime during the fall of 1953.
Each 7X engine received a Twin-H intake, carburetors and a special exhaust manifold with a bolt on twin outlet elbow in place of the standard single outlet elbow. The exhaust manifold itself is different internally in the intake manifold heat chamber. The 7X version is totally sealed off from the intake so no direct exhaust gas flows into the intake. This offers a improvement in flow for the center two exhaust ports.
Pistons, bearings and crankshafts were hand selected and fitted to these 7X engines for the best clearances and lowest amount of rotational drag.
Horsepower ratings were never given as exact numbers due to the hand fitting and modifications but were listed as 210-220hp.
AMC years 1955 & 1956

Major modifications were added in 1955. Hudson had already prepared to make these changes for better durability. The only big still in production was the 308. Both the 232 and 262 were dropped for the 1955 year.
Cylinder head bolt holes were enlarged to 1/2 for better gasket retention. Cylinder heads were either the 308 cast iron or aluminum for 1955 and 308 cast iron but used the 262 cast iron in place of the 308 aluminum for higher compression in 1956.
The valve relieve area of the block was enlarged again for better air flow. These blocks have the best out of the factory unmodified air flow of all Hudson big six engines short of the 7X engine.
As stated above the lifters were changed at the end of the 1955 year to hydraulic along with the re-profiled "742" camshaft due to problems with adjusting the valves in the Nash chassis. A special slip-on fuel pump eccentric was added to the arm that is 1/2 moon shaped to reduce camshaft fuel pump wear.
Twin-H was still available for these two years. The intake changed to incorporate the balance chamber so it was possible to adjust the mechanical lifters in the Nash chassis. These intakes do not flow as well as the 1954 and earlier versions. The exhaust manifold flange on the intake was also changed so earlier intakes will not fit the 1955 & 1956 exhaust manifold.
The exhaust manifold takes a different elbow flange so the twin outlet elbow will not fit these manifolds.
The crankshaft flywheel flange is 1/2" shorter than the 1954 and earlier engines and had 1/2" threaded holes for bolts now. This crankshaft is a much stronger crankshaft and flywheel retention is no longer a problem as with the earlier style. A modern neoprene seal is used at the rear main location in place of the older rope seals.
Oil pans were changed to a center sump to clear the Nash chassis.
Thermostats housings take the modern style thermostats now as the bypass is blocked off with the mounting flange.

The next installment will cover modification to wake up these engines for performance and street use.
Part -2
I did get some today of a mild 308 Hornet rebuild we are doing right now along with some of the flywheels & clutches. I will be getting more for modifications when we start the next 308 or 262 but it may be a few weeks. Sorry about the delay on these.
first two pictures show the left and right side of the 308 block. the rib located 2" above the oil pan rail is only used on 308 blocks. the 232 & 262 do not used this. if your Hudson engine has this reinforcement rib on both sides its a 308 block.
the third pictures shows the crankshaft bolts used in all 1948 through 1954 Hudson big six engines. if you are building a performance engine using one of these early crankshafts remove the bolts, have a machine shop drill and tap these six holes to the next size bigger and use ARP flexplate bolts here instead of the stock bolts. Hudson's with this bolt and nut arrangement on the crankshaft have the habit of snapping the bolts & throwing the flywheel off and it takes most of the rear of the engine & trans with it in big sharp pieces.
the forth picture shows a 1954 308 block with a mild polish on the relief area. usually you would place a old head gasket on top of the block, scribe the outline for the valve chamber and relief area in the block and grind the entire area out at an angled depth of 3/16" at the cylinder then tapering up to .090" at the deck area behind the valves near the port side of the block. I will get good pictures of the 328 relief area in my engine once I get it apart and detail it out better. This mild polishing does help out but its no where as good as a full relief grind.
the fifth pictures shows a 9" flywheel with the drive lugs made in it on the left and a 10" flywheel on the right.
the sixth picture shows a 9" pressure plate with the three empty slots for the drive lugs in the flywheel to fit.
the seventh picture shows a 10" pressure plate with the drive lugs incorporated into the outer cover fitting into the slots in the clutch pressure plate.
the eight pictures shows 9" clutches with the heavy duty without springs on the left and the standard duty with springs on the right.
the ninth picture shows 10" clutches with the heavy duty without springs on the left and the standard duty with springs on the right.
the tenth picture is the chamber side of a 308 cast iron head showing the valve pocket, transfer slot and cylinder wedge. the 308 head offers the least flow resistance on a 308 engine but a lower compression ratio than using a 262 or 232 cylinder head. the major difference between all of them is the depth of the transfer slot (area above the relief in the block) and how far the cylinder wedge penetrates towards the center of the cylinder.
Compression makes power on a flathead just like on a OHV engine BUT there is a major trade off in loss of air flow to support high RPM. In a Hudson flathead its kind of a mute point because the 308 is all done by 4800 RPM due to the 4.5" stroke. My point is that a 308 head works the best for air flow and does not impeed airflow at its max RPM of 4800 but you can make a little more power with a deeper relief cut in the block and a 262 cylinder head. using a 232 head on a 308 creates a detonation machine, not worth the compression gain and loss of airflow.
More will be coming soon with information on oil system modifications first.
PaceRacer50

the left and right side of the 308 block. the rib located 2" above the oil pan rail is only used on 308 blocks. the 232 & 262 do not used this.

Hudson engine info pictures

more pictures for tech