Frame Differences

[ssapach]

I did a little searching for other threads on the topic, but I wasn't finding the answers to the question I have.

I have access to a 1981 GMC 3/4 ton 4x4 that is in relatively good shape for it's age and location. The rear end is a 14-bolt SF axle but the truck has 8-lug hubs all around.

Would this be a plain old 3/4 ton frame or is there a "light duty" 3/4 ton and a "heavy duty" 3/4 ton truck of this style?

My understanding is that there are differences in the material thicknesses between 1/2, 3/4 or 1 ton frames. I could also do a bit of measuring on it to find out for sure, but I don't actually know the differences between the frames.

[frog210]

if i remeber correctly from a previous post yes
i have a heavy duty half ton
so there probably is a heavy and light duty too...

[Roanoker494]

I have always been under the impression that a light duty 3/4 ton came with a "standard" type rear end and the heavy duty trucks had the full float rear. I have also been lead to believe that all GMC trucks came with the heavy duty package.

[hatzie]

Look at this thread in the FAQ.
http://67-72chevytrucks.com/vboard/s...d.php?t=382332

[andrewmp6]

From what i read the 1/2 and 3/4 is the same but the 3/4 has more crossmember tie ins.The 1 ton frame is thicker then the rest.As far as i remember light duty and heavy duty was more the springs nothing about the frame.

[ssapach]

Thanks for the info everyone!

[Desert1957]

Originally Posted by andrewmp6

From what i read the 1/2 and 3/4 is the same but the 3/4 has more crossmember tie ins.The 1 ton frame is thicker then the rest.As far as i remember light duty and heavy duty was more the springs nothing about the frame.

This is true EXCEPT for the 3/4 ton "Camper Special" ask me how I know?

I purchased my 83 camper special 3/4 ton with the idea of upgrading my 1987 1/2 ton. Well to my surprise the 3/4 to frame was quite a bit thicker everywhere I measured, long story short I wound up rebuilding the 3/4 ton an having 3 pickup trucks. It is a very touchy subject at my house currently.

Desert

[Your41Plague12]

The 1/2 and 3/4 ton frames have the same outside dimensions, the only thing that differs is the thickness of the metal. The 1 ton frames are 2 inches taller. The light and heavy duty 3/4 tons have the same frames, the difference is in the springs. All 80's 3/4 tons should have had a SF 14 bolt from the factory. What you have is factory, from what it sounds like. They quit using the FF 14 bolts in 3/4 tons in the late 70's some time, IIRC.

[Desert1957]

Originally Posted by Your41Plague12

The 1/2 and 3/4 ton frames have the same outside dimensions, the only thing that differs is the thickness of the metal. The 1 ton frames are 2 inches taller. The light and heavy duty 3/4 tons have the same frames, the difference is in the springs. All 80's 3/4 tons should have had a SF 14 bolt from the factory. What you have is factory, from what it sounds like. They quit using the FF 14 bolts in 3/4 tons in the late 70's some time, IIRC.

I have worked on two other 3/4 ton trucks that had frame thickness like my 1987 1/2 ton. They were not "Camper Special" optioned trucks, I wonder why the differences? One of the trucks is the original owner, so I know it hasn't be modified.

Desert

[ssapach]

Originally Posted by Your41Plague12

They quit using the FF 14 bolts in 3/4 tons in the late 70's some time, IIRC.

This I did not know.

I kind of figured they were similar to the GMT400 trucks, where you could have a 14-bolt SF or FF depending on the GVWR of the vehicle.

Again, thanks for the info!

[hatzie]

Originally Posted by ssapach

This I did not know.

I kind of figured they were similar to the GMT400 trucks, where you could have a 14-bolt SF or FF depending on the GVWR of the vehicle.

Again, thanks for the info!

You were right. 14 bolt FF vs SF is GVWR dependent just like the GMT400. GM modified the axle design in 1986 to make the drums removable without pulling the hubs... it's still a 14bolt FF axle. HD 3/4 ton trucks like the camper specials could have FF axles. LD 1tons could have SF axles.