avatar Peter Chronis

The GMC V6 was not a special order option on 1960-69 GMC pickup trucks from 1/2 to 1 ton models, it was a standard engine in those trucks from 1960-63 in the 305 cubic inch displacement. My father had a 62 1/2 ton K series with the 305 V6, in 68 he replaced it with a new K series 3/4 ton with the 305 V6 which I have today now powered by the 478 V6. In 1964 the Chevrolet 230 inline six was offered as a step down option and in 1966 the larger 351 was offered in all pickups as an option. In 1967 the whole line of Chevrolet sixes and V8’s was added on to GMC’s line up of engines along with both V6’s, for 1970 the V6 was discontinued but then you could special order one and the dealer would do the installation, this lasted up to 1972.

avatar Bill

Being that Rocinante is a 3/4 ton, it would have probably come equipped with GM’s massive HO52 axle, which would have meant either 4.57 or 5.13 gears. This truck should also have the same transmission (SM420 which also came in dump trucks btw) as your ’66. This truck should have had some serious pulling power, but also a top speed of about 55. It would have made a scenic drive across the US for sure.

avatar Tim Frazier

Bill, you know your stuff! I’m fairly certain my Dad changed out the rear differential to a Spicer 44 in our ’66. She still had tremendous pulling power, and the original granny gear standard transmission, but she would also top out around 80 mph.

avatar Greybeard

I don’t know how familiar you are with the line of old GM V8’s Tim, but looking once again at the photo of the engine the valve covers remind me a lot of the 348 and 409 engines of the late 50’s and early 60’s. Wonder if this engine was the 409 with two cylinders removed?
(409/8 = 51.????. 409-102= 307.)

avatar Tim Frazier

From Wikipedia, the free encyclopedia:
GMC Truck produced a unique 60° V6 engine from 1960 through 1978. The engine was available in 305, 351, 401 and 478-cubic-inch (5.0, 5.8, 6.6, and 7.8 L) versions. In late production there was also a 432-cubic-inch (7.1 L) version with enlarged crankshaft journals. GMC also made a 637-cubic-inch (10.4 L) 60° V8 with twin balance shafts using the same general layout as the 305. Finally, there was a 702-cubic-inch (11.5 L) “Twin Six” V12, which was basically two 351s placed end-to-end with a common block and crank. A Diesel version was called the ToroFlow.

avatar Greybeard

60 degrees between banks?
Then it wasn’t a modified V8, ’cause most of them are 90 degree between.
Amazing stuff. I wonder how many of ’em are out there if you wanted one?

avatar Greybeard

A friend had a GMC truck with that V6 in it. The exhaust note WAS unique.
I never gave it much attention ’cause I was too wrapped up in V8’s. What was the displacement?
In the early 70’s I rode in a Buick that had a V6 in it, again not paying it much mind. I now wonder if that was an early version of the 3.8 – 4.3 liter in so many GM products now, or was it an oddball with the engine you refer to?
But we know this, don’t we.
The torgue and horsepower on that engine would be equal at 2,525 rpm! ;>)

avatar Tim Frazier

305 cubic inches.

The story Dad told me was that it was originally designed to be a diesel, and the GM engineers determined that with a few minor changes they could make it into an incredible gasoline workhorse for the retail market as well.

I don’t know if that was a story the salesman he ordered it from made up or if it’s the truth, but I do know it was exceptionally heavy-duty for a gasoline engine; there was plenty of room for future boring and sleeving if one could only find pistons large enough.

Combined with a granny-gear transmission and a Spicer 44 differential, we used it like a tractor at times. We’d run a log chain over a wheel rim to the base of a railroad tie set five feet into the ground as a fence post and yank that puppy out like Hercules pulling a weed.

Your email address will not be published. Required fields are marked *