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Old 11-25-2022, 06:56 AM
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What years did GMC install Pontiac engines in their truck lines?

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Old 11-25-2022, 07:27 AM
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1955 through 1959 in pickups. They also used Oldsmobile engines in larger trucks during that period. https://en.wikipedia.org/wiki/GMC_V8_engine#Pontiac

Pontiac
Prior to developing its own engines, smaller GMC's used the Pontiac V8 engine. They used the Pontiac 287-cubic-inch (4.7 L) engine for 1955 and 316-cubic-inch (5.2 L) engine in 1956, but advertised the engines as the "GMC 288" and "GMC 316". They used Pontiac's 347-cubic-inch (5.7 L) in 1957. For 1958, GMC reduced the bore of Pontiac's 370-cubic-inch (6.1 L) to 3.875 in (98.4 mm), resulting in a displacement of 336-cubic-inch (5.5 L). In Canada. For 1959, as the Pontiac engine's stroke was lengthened to 3.75 in (95.3 mm), a further bore reduction to 3.78 in (96.0 mm) was done to keep the displacement at approx. 336 cubic inches (5.5 L) (336.7).

Oldsmobile
Starting in 1955, for the larger trucks, the Oldsmobile Rocket V8 was available. In 1955 and 1956, it was 324 cubic inches (5.3 L). Power listed for 1956 was 210 HP @ 4200 rpm; torque was 305 lbs.ft. @ 2400 rpm. For the 1957 through 1959 model years, it was upped in bore and stroke and called the 370. Power listed as 232 HP @ 4200 rpm and torque as 355 lbs.ft. @ 2600 rpm.

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Old 11-25-2022, 01:15 PM
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fascinating stuff...

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Old 11-25-2022, 08:13 PM
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The Pontiac versions of the GMC engines had lower compression (dished pistons) and Bendix-Stromberg 2 barrel carbs instead of Rochesters. They were painted red. 3 speed transmissions were a mix of Muncie SM-318&319 and Borg Warner T85 and T89. A Muncie SM-420 four speed was also offered. The automatic tranny option was a Dual-Range Hydra-Matic 4 speed that was almost the same as Caddy, Olds and Pontiac GM car options from 1952-56.

Other than pistons, 95%+ of engine parts interchange.

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Old 11-25-2022, 09:26 PM
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Sorry, I shouldn't have said SM-319 which is an OD 3 speed. It was NOT offered in the GMCs from 55-59.

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Old 11-26-2022, 01:44 AM
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Also in the 1950's GMC medium sized trucks use a Buick V8. Many years ago I worked on a fire truck that was Buick powered. Back then we were checking emissions on everything. If you had a hydrocarbon reading around 150ppm it was considered very good. This truck had a reading of 86ppm.

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Old 11-26-2022, 01:47 AM
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Quote:
Originally Posted by Stuart View Post
... 1955 through 1959 in pickups...
Not just pickups- all 100 series up through 500 series trucks.

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Old 11-26-2022, 11:45 AM
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Quote:
Originally Posted by Goatracer1 View Post
Also in the 1950's GMC medium sized trucks use a Buick V8.
I'm pretty sure that the Buick 332 V8 engines were used in mid-sized Chevy trucks, not GMC trucks, in USA manufactured trucks. My GMC Master Parts Book that covers USA made 2nd series '55 through '60 all size trucks from 100 series (1/2 ton) through 970 series (the biggest GMC made) has no references to a Buick-type engine.

I have a 1955-57 Canadian Chevrolet/GMC truck parts catalog that shows the 19xx model (2.5 ton) Chevrolet and 99xx model GMC trucks used 332 cubic inch V8 engines in '56 and '57. The drawings in the catalog look like nail head Buick engines to me. Maybe your fire truck was born north of the border?

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Old 11-27-2022, 08:56 PM
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Quote:
Originally Posted by Bill Hanlon View Post
The Pontiac versions of the GMC engines had lower compression (dished pistons) and Bendix-Stromberg 2 barrel carbs instead of Rochesters. They were painted red. 3 speed transmissions were a mix of Muncie SM-318&319 and Borg Warner T85 and T89. A Muncie SM-420 four speed was also offered. The automatic tranny option was a Dual-Range Hydra-Matic 4 speed that was almost the same as Caddy, Olds and Pontiac GM car options from 1952-56.

Other than pistons, 95%+ of engine parts interchange.
Bill, are there any differences or issues installing the later(75 455) engines in the 55 GMC half ton trucks? I'm concerned about engine and motor mounts location. And were the GMC trucks 12 volts at that time?

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Old 11-27-2022, 09:54 PM
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I stand corrected.

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Old 11-27-2022, 11:46 PM
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The Pontiac V8 engine equipped GMC trucks were only from second series 1955 through 1959. These engines mounted with a single mount in the front that connected the truck's frame to the bottom of the timing cover and two side mounts that connected the frame to the bellhousing just like the 55-59 Pontiac cars. Pontiac changed to two side motor mounts and a single mount at the rear of the transmission in 1960.

Second series 1955 GMC trucks were 12v negative ground except for those equipped with six cylinder engines and stick shift transmissions which were 6v positive ground.

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Old 11-28-2022, 12:20 AM
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Swapping a later Pontiac V8 and later transmission into '55-59 GMC trucks only requires front engine mounting points added to the frame, and fabrication of a crossmember for rear of transmission mounting. Exhaust clearance to the steering box may require fudging the front of the engine slightly off the truck's centerline.

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Old 11-29-2022, 02:07 PM
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Quote:
Originally Posted by Bill Hanlon View Post
Pontiac changed to two side motor mounts and a single mount at the rear of the transmission in 1960.
Pontiac changed to the two side motor mounts and rear transmission mount in 1959. The 1959 and 1960 engines still had the motor mount tab on the front of engine timing cover and had the same rear engine block interface as earlier V8s. So a 1959-60 engine can be easily bolted in to earlier Pontiac models (I've done both). I assume it would be the same for GMC engines.

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Old 11-29-2022, 03:48 PM
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1961 had the mount tab on the engine timing cover too, but the cover was aluminum so it may not be up to the task like the earlier ones.

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Old 11-29-2022, 07:59 PM
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Quote:
Originally Posted by Safari Larry View Post
Pontiac changed to the two side motor mounts and rear transmission mount in 1959. The 1959 and 1960 engines still had the motor mount tab on the front of engine timing cover and had the same rear engine block interface as earlier V8s. So a 1959-60 engine can be easily bolted in to earlier Pontiac models (I've done both). I assume it would be the same for GMC engines.
Larry is correct about the '59 Pontiac engine using two engine mounts and one at the rear of the transmission like the '60 and newer engines. My mistake was in looking at page 56 of the 1960 Pontiac MPC which has a picture titled "1960 Engine Mounting Parts". Because it didn't mention 1959 models I assumed (there's that word) the '59 was the same as earlier 1 forward and two bellhousing mounts. Further reading in the MPC shows '59 uses the same mounts as '60. My bad.

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Old 11-29-2022, 08:51 PM
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I put a 1959 engine in a 1955 Pontiac. I used the cast iron timing chain cover/water pump from the 1955 engine and the coolant went thru the front of the heads. Reverse coolant. Used the 1955 trans that was built by HyderMotive trans. Car ran good.

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Old 11-29-2022, 08:52 PM
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Quote:
Originally Posted by Stuart View Post
1961 had the mount tab on the engine timing cover too, but the cover was aluminum so it may not be up to the task like the earlier ones.
The interface to the rear of engine also changed in 1961 (from 1955-60) so you can't bolt it up to earlier bell housings to get the 2 rear mounts on the bell housing.

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