Editing GMC straight-6 engine

{{Infobox automobile engine
| name = GMC straight-6 engine
| image = 
| image_size = 
| alt = 
| caption = 
| manufacturer = [[General Motors]]
| designer =
| aka =
| production = 1939–1962
| configuration = [[straight-6]]
| displacement = {{convert|228-503|CID|L|1|abbr=on}}
| bore =
| stroke =
| block = cast iron
| head = cast iron
| valvetrain = [[Overhead valve engine|OHV]], 2 valves per cylinder
| timing = 
| compression = 
| idle = 
| redline = 
| operating principle = 
| supercharger = 
| turbocharger = 
| turboboostpressure = 
| fuelsystem = [[Carburetor]]
| management = 
| fueltype = [[Gasoline]]
| oilsystem = 
| coolingsystem = [[Water-cooled]]
| power = 
| specpower = 
| torque = 
| length = 
| width = 
| height = 
| diameter = 
| weight = 
| emissions level = 
| emissions control = 
| predecessor =[[Buick Straight-6 engine]]<br>[[Pontiac straight-6 engine#Flathead Six|223 cu in Pontiac straight-6]]<br>[[Oldsmobile straight-6 engine#230|230 cu in Oldsmobile straight-6]]
| successor = [[Oldsmobile V8 engine#324|324 cu in Oldsmobile V8]]<br>[[GMC V6 engine]]<br>[[Chevrolet Turbo-Thrift engine]]
}}

The '''GMC straight-6 engine''' was a series of [[gasoline]]-powered [[straight-six engine]]s introduced in the 1939 [[Model year#Automobiles|model year]] by the [[GMC (automobile)|GMC Trucks]] division of [[General Motors]]. Prior to the introduction of this new engine design GMC trucks had been powered by straight-six engines designed by the [[Buick]], [[Pontiac (automobile)|Pontiac]] and [[Oldsmobile]] divisions of GM.

The new engine family featured a [[Overhead valve engine|valve-in-head]] design, pioneered by Buick and also used by the [[Chevrolet]] division's contemporary [[Chevrolet Stovebolt engine|"Stovebolt Six" engine]]. Many [[engine displacement|displacements]] were produced using three [[Engine block|block]] sizes: "Group 1" (small), "Group 2" (mid-size) and "Group 3" (large).<ref name="meyer"/>{{rp|15}}

The straight-6 engine was replaced by the [[GMC V6 engine]] in 1960, remaining in use only in certain light-duty models of the P-series [[Multi-stop truck|step van]] until 1962.<ref name="meyer"/>{{rp|pages=27–28}}

==Group 1==
The Group 1 engines were the smallest in displacement and outer dimensions, and differed most significantly from the larger Group 2 and Group 3 engines by having only four main bearings (whereas the Group 2 and 3 engines had seven)<ref name="meyer"/>{{rp|15}} and a different [[firing order]] (1-5-3-6-2-4, whereas the others are 1-4-2-6-3-5).<ref name="motor"/>

===228===
GMC replaced the [[Pontiac straight-6 engine#223 Pontiac/GMC|Pontiac 223]] with their own {{Convert|228|CID|L|1|adj=on}} '''228''' in 1939.  This OHV ([[overhead valve]]) engine was produced through 1953. With a cylinder bore of {{Convert|3.5625|in}}, this is the smallest low-deck engine. All four low-deck engines have a stroke of {{Convert|3.8125|in}} and used 7 inch long connecting rods.

===236===
The {{Convert|236|CID|L|1|adj=on}} '''236''' was introduced in 1941 and only in limited production through [[World War II]]. This is a low-deck engine, with {{Convert|3+13/16|in}} stroke and {{Convert|3+5/8|in}} bore. Production ended in 1946.<ref name="motor"/>

===248===
The {{Convert|248|CID|L|1|adj=on}} '''248''' was released in 1939 alongside the 228. The 248 was similar to the [[#236|236]]. The bore was {{Convert|3.71875|in}}. Stroke is 3.8125. Power in 1955 listed as 100 HP @ 3100 rpm; torque 202&nbsp;lb ft @ 1000 rpm. The 248 was discontinued in 1955.

===256===
The {{Convert|256|CID|L|1|adj=on}} '''256''' was similar to the 236 and 248. It was also an OHV/pushrod engine, and was built for just two years, 1940 and 1941. This is the largest low deck engine. Bore was 3.78125 and stroke was 3.8125. Power listed as 91 Net HP @ 3000 rpm; torque 201 Net lb ft @ 1000 rpm.

===270===
The last GMC-only straight six was the {{Convert|270|CID|L|1|adj=on}} '''270'''.  It was produced from 1941 through 1963, and was an OHV/pushrod engine. This is a raised-deck engine. The bore was 3.78125" (same as 256) and stroke was {{convert|4|in|1}}. Power listed in 1963 as 133 HP @ 3600 rpm; torque 244&nbsp;lb ft @ 1300 rpm.

===302===
The {{convert|301.6|CID|L|1|adj=on}} GMC inline six was produced from 1952 to 1960, when it was replaced by the V6. It has a [[Stroke ratio#Square engine|square bore/stroke ratio]] of {{convert|4x4|in|1}}. This is the largest raised-deck engine. It was originally designed for the GMC military M135 and M211. It was used in military 2{{frac|1|2}}-ton trucks with the [[Hydramatic]] transmission; however, the engine was a sealed engine for snorkel/submersion use, had an electric fuel pump, and other features such as a deep sump oil pan. From 1952 to 1959, GMC manufactured the civilian 302 engine, which was not sealed, had a mechanical fuel pump, and used a standard oil pan. Power listed in 1959-160 HP @ 3600 rpm; torque 268&nbsp;lb ft @ 1600 rpm. This engine was popular with hotrod enthusiasts because it delivered tremendous power for an inline six engine at the time (although now obsolete), is built with a heavy cast block, and can take quite a bit of abuse.

==Group 2==
===278===
The {{Convert|278|CID|L|1|adj=on}} '''278''' was released in 1939 alongside the 308, sharing the same {{Convert|4+1/2|in}} stroke but with a {{Convert|3+5/8|in}} bore. It produced a maximum 110&nbsp;hp @ 3000 rpm and 230&nbsp;lb•ft @ 1000 rpm. The 278 was discontinued in 1946.<ref name="motor"/>

===308===
The {{Convert|308|CID|L|1|adj=on}} '''308''' was released in 1939 alongside the 278, sharing the same {{Convert|4+1/2|in}} stroke but with a {{Convert|3+13/16|in}} bore. It produced a maximum 122&nbsp;hp @ 3200 rpm and 241&nbsp;lb•ft @ 1000 rpm.<ref name="motor"/> The 308 was discontinued in 1949 and replaced by the 318.

===318===
The {{Convert|318|CID|L|1|adj=on}} '''318''' was released in 1950, replacing the 308. It shared the same {{Convert|4+1/2|in}} stroke as its predecessors but with a slightly larger {{Convert|3+7/8|in}} bore. It produced a maximum 136&nbsp;hp @ 3400 rpm and 245&nbsp;lb•ft @ 1400 rpm.<ref name="motor"/> The 308 was discontinued in 1954 and replaced by the [[Oldsmobile V8 engine#324|324 cubic inch Oldsmobile V8]].<ref name="meyer"/>{{rp|24}}

===360===
The {{Convert|361|CID|L|1|adj=on}} '''360''' was released in 1950, replacing the physically larger 361 "Group 3" engine. The 360 and 361 shared the exact same {{Convert|4+1/8|in}} bore and {{Convert|4+1/2|in}} stroke, but the 360 used the mid-size "Group 2" block. It produced a maximum 127&nbsp;hp @ 3000 rpm and 289&nbsp;lb•ft @ 1300 rpm.<ref name="motor"/> The 360 was discontinued in 1955 and replaced by the [[Oldsmobile V8 engine#324|324 cubic inch Oldsmobile V8]].<ref name="meyer"/>{{rp|24}}

==Group 3==
===361===
The {{Convert|361|CID|L|1|adj=on}} '''361''' was released in 1939 alongside the 426 and 451. The 361 was discontinued in 1949 and replaced by the 360 "Group 2" engine. The 360 and 361 shared the exact same {{Convert|4+1/8|in}} bore and {{Convert|4+1/2|in}} stroke, but the 361 used the larger "Group 3" block. It produced a maximum 136&nbsp;hp @ 3000 rpm and 273&nbsp;lb•ft @ 1000 rpm.

===426===
The {{convert|425.6|CID|L|1}} with a bore and stroke of {{convert|4.25x5|in}} GMC inline six appeared in 1940s 4x4 Cab Over Engine (COE) trucks made in [[Pontiac, Michigan|Pontiac, MI]]. Power ratings for 1955 were 190 HP @ 3200 rpm; torque 350&nbsp;lb ft @ 1000 rpm. It also appeared in large GMC trucks in the 1950s ending in 1955.

===451===
The {{Convert|451|CID|L|1|adj=on}} '''451''' was released in 1939 alongside the 361 and 426. It was the largest in the GMC straight-6 line until the 477 debuted late in the 1940 model year. It shares the same {{Convert|5|in}} stroke as the 426 and 477, with a {{Convert|4+3/8|in}} bore. It produced a maximum 146&nbsp;hp @ 2400 rpm and 350&nbsp;lb•ft @ 1000 rpm.<ref name="motor"/> It was replaced by the 477 and ceased production at the beginning of 1942.

===477===
The {{Convert|477|CID|L|1|adj=on}} '''477''' was released late in the 1940 model year, replacing the 451. It was the largest in the GMC straight-6 line until it was replaced by the 503 in 1950. It shares the same {{Convert|5|in}} stroke as the 426 and 451, with a {{Convert|4+1/2|in}} bore. It produced a maximum 154&nbsp;hp @ 2600 rpm and 385&nbsp;lb•ft @ 1000 rpm.<ref name="motor"/>

===503===
The {{convert|502.7|CID|L|1|adj=on}}; {{convert|4+9/16|x|5+1/8|in|1}} GMC inline six was more numerous than the 426 inline six, starting in 1950 and ending with the 1959 model year. In the 1957-1959 model years this engine was listed as 225 HP@3200 rpm and 436&nbsp;lb ft torque @ 1200 rpm. The GMC 630, 660, 720, and 750 Series of the 1950s offered the 503.

==See also==
* [[List of GM engines]]
* [[GMC V6 engine]]
* [[GMC V8 engine]]
* [[General Motors Atlas engine#LL8 (Vortec 4200)]]
* [[Duramax I6 engine]]

==References==
{{reflist|refs=

<ref name="meyer">{{cite web |format=pdf |title=The First Century of GMC Truck History |last=Meyer |first=Donald E. |url=https://www.gmheritagecenter.com/docs/gm-heritage-archive/historical-brochures/GMC/100_YR_GMC_HISTORY_MAR09.pdf |publisher=General Motors Heritage Center}}</ref>
<ref name="motor">{{cite book |title=Motor's Truck & Tractor Manual: 5th Edition |editor-last=Blanchard |editor-first=Harold F. |date=1953 |publisher=The Hearst Corporation |location=New York City |pages=557–559}}</ref>
}}

{{DEFAULTSORT:GMC straight-6 engine}}
[[Category:GMC engines|Straight-6]]
[[Category:Straight-six engines]]