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270 Engine in a CCKW or DUKW
Many people have asked about other engines or upgrades for more power or speed
for a CCKW or DUKW. There are a few choices and options that the owner has if
they wish to pursue one of these venues.
Many people have asked about other engines or upgrades for more power or speed for a CCKW or DUKW. There are a few choices and options that the owner has if they wish to pursue one of these venues.
The original 270 series engine was first produced in a smaller displacement in 1939. The original engines series introduced over a couple years were 228 cid, 248 cid and 256 cid. The 270 engine was first produced at the request of the government. They liked the earlier versions of the deuces (ACKWX-353) but wanted more power for the later, large production orders. In 19 days, GMC modified the 256 engine and created the 270. This was not all that easy. They had to modify the water jackets to allow for the larger stroke along with creating different connecting rods. The original 270 engine had all of 91.5 horsepower and it was governed at 2750 RPMs. Always remember that all WW2 engines, GMC’s included are long stroke engines that do not like or tolerate high RPMs for long. The 302 increased the bore over the 270 and changed the engine piston wise quite dramatically. They went from domes pistons in WW2 to flat pistons in the later engines. This also necessitated a different head design. Always remember too that the pedals are attached to the bell housing, so if you do not utilize the Chevy or GMC bell housing from the WW2 era, you have some serious issues to resolve
Later 1950’s 270 engines had up to 145 horsepower and were governed at up to 3250 RPMs. Using one of these engines, along with a 58% increase in raw horsepower, yields an 18% potential increase in speed which translates to 54 miles per hour versus the original 45 miles per hour. These engines have advantages and disadvantages. They are not too hard to come by being smaller in displacement and used by pickup trucks. They have the disadvantages of usually only having a smaller clutch with the stock flywheel. An advantage is that the flywheel will fit into the CCKW/DUKW bell housing. The real disadvantage is that the later 50’s engines are missing the foot under the front pulley where the front motor mount is. This foot is actually part of a plate behind the timing chain cover. It is almost the first part installed on the block! To install a plate with the correct foot, assuming you have an engine to donate one requires that you remove the following parts: Damper pulley, timing chain cover, oil pan, valve cover, lifter galley cover, rocker arms, pushrods, distributor, oil pump, fuel pump, lifters, and cam shaft. Quite a project! Buy an engine rebuild gasket set.
The clutch being smaller is not an issue I believe unless you are really using the truck hard. If the smaller clutch was good enough for a pickup truck, it would just wear out a little faster with the CCKW/DUKW. You still have the same horsepower regardless of clutch size so clutch slippage would not be an issue.
There are a number of 302 GMC engines that you can install in the CCKW or DUKW.
The military 302 used in the M211, M135 and (2 used in the) M59, are of a different block design than the civilian 302 or any 270 engine. The military 302 has a different water pump, damper pulley, carburetor, generator mounting and block. The block incorporates mounting for an air compressor and a U-bastard raised boss on the side of the block back by the starter. When used in the military trucks, the bell housing and starter, the starter is mounted lower on the bell housing. This boss is just above the starter in the stock version. When you install a CCKW or DUKW bell housing onto a military 302, you cannot install the starter unless you grind off this boss. The generator is mounted on the other side of the engine and the water pump may not be compatible with a CCKW’s short engine compartment. In a DUKW, you have more latitude. I have also heard that rebuilding kits for the special Holley carburetor are not available. Because this engine had an automatic transmission, there is no flywheel for mounting a clutch at all. You will need to find a civilian 270 or 302 one
Early Civilian 302:
Early Civilian 302:
The first series of civilian of 302’s are the easiest to use. This is mainly because they have the mounting foot in the front of the engine under the timing chain cover and utilize a Holley 2 barrel carburetor. The Holley carburetor is a very nice one that runs quite well under all conditions. This motor just drops in with the installation of a flywheel that will fit into the CCKW bell housing.
Later Civilian 302:
Later Civilian 302:
The later civilian 302 has 160
horsepower which is a 75% increase in power along with a higher
Large Truck 302
Large Truck 302:
There is another civilian engine and that version has a different block incorporating an air compressor mount cast into the side of the block. This casting also includes the oil drain for the compressor. This engine also has the disadvantage of not having the pivot for the gas pedal bell crank on the side of the engine behind the manifold. If you have a donor engine, you can install the gas pedal bellcrank in the side of the block. Be careful as this is the only bolt on a GMC engine that goes into the water jacket. This engine utilizes the Stromberg carburetor and does not have the front mounting foot.
There are two different 302
manifolds, the military and late civilian 4 carburetor bolt manifold which has
round runners and the early 302 civilian intake manifold (Holley carburetor)
with the more flattened out runners. The newer ones with the mostly round
runners have a tendency to develop cracks and actually loose chunks of casting
in the box below the carburetor that allows exhaust gases to heat the
carburetor. It is very common and I have seen/have many manifolds that have been
repaired by welding the area shut to prevent the exhaust from passing up into
that boxed area and leaking. If you have this problem, take it to a professional
welder for repair!
All these large truck 302 engines have another common disadvantage/challenge. They all used a 13” Brown and Lipe clutch. This very large clutch had a larger flywheel. This larger flywheel will not fit inside a CCKW bell housing. You need to replace the flywheel with a 50’s Chevrolet flywheel. All the 302’s have a 6 bolt crankshaft. The WW2 CCKW engine has a 4 bolt crank. You cannot safely re-drill the CCKW flywheel as there would be very little virgin metal left in the bolt circle area.
Also remember that there is a 6 volt ring gear and a 12 volt ring gear on all these GM engines. The ring gears are replaceable using a good automotive machine shop. You MUST have the correct ring gear to mesh with the starter (6 or 12 volt) that you are using. There are also 12 volt lever action starters like the original CCKW design.
Any other engine, later model GM 6 cylinder or V8’s present instant bell housing problems or in the case of the V8, additionally a potential steering column interference. Whereas the pedals are attached to the bell housing, this starts getting into extensive changes. The later model 270 or 302 civilian engines are really the way to go for more power and/or speed.
Always remember that going faster or with a more powerful engine requires stopping occasionally. Make sure your brakes are up to it and also remember about the tandems in the back being in 8WD all the time. I now cruise at 45 with an occasional spurt to 50 without having to worry about the engine being at the governed red-line all the time.
© 2005 Stephen Keith
© 2005 Stephen Keith
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Site opened 1/7/2005 Last modified: 03/12/2013 17:52