DIY: 3/4 ton rear springs on 1/2 ton

[MalibuSSwagon]

I seen plenty of threads regarding Heavy Half/Big 10 rear springs, I figure I'd share my conversion. I found one of my leafs was broken at the eyelets, so decided I'd do this conversion since I tow and haul heavy items often with my 86 shortbed. Yes, some guys say they don't like the stiff ride, I personally prefer a vehicle that doesn't feel like it's tipping over on every corner.

What you need:

Big 10 / C20 rear leaf springs. Get the 2600lb rated 8/1 springs.
U-bolts. Your spring shop should be able to spec what you need and bend them on site.
Eye bolts, you may have to cut a couple of the old ones.
quantity 10; 3/8"x1" Grade 8 bolts, lock washers, and nuts. If youre near a TSC, you can get them for 3.50.



My bill from the spring shop, that spring part number is for an off the shelf Dayton springs. The u-bolt spec is on there as well, altho you could get away with a 1" shorter height.



Some tips to make this easier:

Remove the bed! Otherwise you'll be working on your back and contorting to get the grinder in awkward places.

Soak everything in PB blaster once a day a week in advance. It'll help to loosen things up.

So set the truck up on jack stands. I put two high lift stands under the receiver hitch and two short ones under the axle.

Remove the u-bolts. Best method I found was to slice the nut with the 4.5" grinder, which will relieve the tension on the bolt. Then you can cut the u-bolt in half with out it snapping at you. Obviously, make sure it's supported by jack stands before removing all the u-bolts.

At this point ,the axle should be loose and the u-bolts and spring plates removed. Now, if you are lucky, the eye bolts should come right out with an impact. I got 2 right out. Otherwise, its sawzall and grinder time! Get those bolts removed and the old leafs out!

Now the tricky part. The old 1/2 ton leafs are 52" long, the new 3/4 ton leafs are 56". Well, GM discontinued the Big 10 in 1979, but they didn't stop punching the holes for the hangers in the frame. You should have 5 holes in the frame exactly 4" rearward of the hangers current location.

Removing the rivets is semi-easy. I chose the air hammer method. You could also grind off the heads if you don't have air.

First, slice the heads, cross is best.



Next, take an air hammer with a sharp chisel and go at them. then use a point bit to knock out the rivet heads.



Once you got the bracket off the frame, bolt it back on in the rear ward holes using the 3/8 bolts. There are only 5, the 6th hole would interfere with the bumper bracket.



Ok now you are ready for the new springs. This step is easier with some help, the springs fit pretty tight into the hangers. Also, if you want to lower the truck, now is the time. You will gain 1.5 to 2" of height with these springs.

Lower the rear end. Slide the eyelet into the front hanger. I also found out, the center pin is not in the middle of the spring, make sure it lines up with the spring pad on the axle before going further. Once you get a bolt thru the front eyelet, line up with the rear shackle. Unless you loosen the shackle bolts, it's going to need some persuasion from a BFH to line up with the eyelet. This is where the helper comes in. Once both springs are lined up, jack up the rear axle so it's resting against the bottom of the springs, then slide it so it snaps into the center pin. Install u-bolts and springs plates, torque u-bolts to factory spec, put wheels on and you are done!



Keep in mind this is only 1/4 of a Big 10 conversion, they also had HD coils up front, 1.25 in thick rotors, and wider drums out back. I would also recommend LT load range C tires. I personally am running an All Terrain 235/75/15 on 15x8" Rally wheels.

[MalibuSSwagon]

Originally Posted by Keith Seymore

From another website:

"You are correct about the Big 10’s being the same as Heavy Halfs. In fact, the Heavy Half was the GMC equivalent to the Chevrolet Big 10. Both were identified by the option code “F44” and contained equipment allowing higher GVW’s than those of the regular Chevy C10/GMC C15.

The 1979 model year F44 package included a choice of 1,625 or 1,700 lb front springs (1,475 and 1,550 lb front springs were commonly used on standard C10/C15’s), 2,000 lb capacity rear springs (1,550 lb rear springs were standard on regular C10/C15’s), heavy duty power brakes (manual and light duty power brakes with smaller rear drums were offered only on standard C10/C15’s), larger tires in a choice of 1,790 and 1,905 lb capacities (several smaller capacities were also listed for regular C10/C15’s) and the 305 V8 (the 250 six was standard on regular C10/C15’s). With the exception of the 1,700 lb front springs, all of the F44 components shown above could be ordered separately on the regular C10/C15. Depending upon the equipment selected, the regular C10/C15 offered GVWR’s of 4,900, 5,300, 5,400, and 5,600 lbs. However, by selecting the F44 package, the buyer received all of the above equipment, plus the bodyside decal designating the C10/C15 as a Big 10/Heavy Half, offering GVWR’s of either 6,050 or 6,200 lbs (depending upon the tire and front spring capacities selected). (You can determine your Big 10’s rating by checking the label affixed to the driver’s door frame.)

A few other notables should be mentioned. For 1979, the U.S. EPA established fuel economy standards for trucks up to 6,000 lb GVW. To meet these standards, GM equipped the standard C10/C15 with a special hood seal and underbody air dam, and also offered a selection of rear axle ratios biased in favor of economy. The Big 10/Heavy Half GVWR’s exceeded the EPA’s 6,000 lb threshold, and accordingly, those models were not equipped with the hood seal and air dam. Also, the F44 package allowed the purchase of lower axle ratios, and the 454 V8 engine option. In contrast, the 350 was the largest engine available on the regular C10/C15.

Prior to 1979, the EPA considered all trucks under 6,000 lbs GVW to be light duty emission class vehicles. With few exceptions, these vehicles required undesirable catalytic converters and the use of unleaded gasoline. The F44 option was introduced to allow buyers the possibility of buying a heavier duty ½ ton truck that could use cheaper leaded gasoline. However, for the 1979 model year, the EPA expanded the light duty classification to include all trucks up to 8,500 lbs GVW. Thus, the Big 10/Heavy Half (and all ¾ ton Chevy C20/GMC C25 trucks, as well) lost this particular advantage. Only 1 ton C30/C35 series remained exempt from catalytic converters and unleaded gas.

Excepting the differences discussed above, the Big 10/Heavy Half was identical to the standard C10/C15. A 3.14 section modulus frame, 3,750 lb rear axle, and 15 inch wheels were common to both. These components were much more robust in the ¾ (20 series) and 1 ton (30 series) models."

I didn't verify for accuracy before posting but can follow up if you have any specific questions.

Here's an earlier thread with some discussion: http://67-72chevytrucks.com/vboard/s...d.php?t=369184

K

I was hoping to gather together the scattered knowledge on this subject, answer peoples question on this conversion. I will also post some more DIYs in the future on other mods I have done to my truck; electric fans, bucket seats, TPI; if there is interest.

[Willowrun]

Nice job on the write up and pics. Looking forward to seeing your other upgrades. I'm sure I would have taken the "easy" route and not removed the bed if doing the same swap as you only to find myself crouched under the bed and fumbling with everything ultimately taking longer than just removing the bed in the first place. Interested in seeing what kind of rake your truck has once the bed is back on.
Posted via Mobile Device

[Jonboy]

My '74 SWB had the HD suspension, so it came from the factory with these springs. When I converted my 3/4 ton to 1/2 ton, I went ahead and used them as well. Not sure about how much it might have raised, as a flip kit was used on both trucks.

[KQQL IT]

My 73 has gone from short then long then back to short springs again.
Posted via Mobile Device

[SCOTI]

Originally Posted by Jonboy

My '74 SWB had the HD suspension, so it came from the factory with these springs. When I converted my 3/4 ton to 1/2 ton, I went ahead and used them as well. Not sure about how much it might have raised, as a flip kit was used on both trucks.

The difference is typically 1" when comparing the Lt duty 52"/HD 56" spring set-ups. The flip kits w/lt-duty 5-leaf springs yield about 5" drop & the HD 8-leaf pack was 6".

[MalibuSSwagon]

Originally Posted by SCOTI

The difference is typically 1" when comparing the Lt duty 52"/HD 56" spring set-ups. The flip kits w/lt-duty 5-leaf springs yield about 5" drop & the HD 8-leaf pack was 6".

The stock 3/1 spring pack is 1.5" tall at the axle pad, the new 8/1 pack is 3 1/16". So you will gain at least the difference when the springs settle, if not more.

Also, I had the bed off to repair the cab corners and paint, when I noticed the left spring was cracked at both eyelets.

[izzy_Britton]

Originally Posted by MalibuSSwagon

You should have 5 holes in the frame exactly 4" rearward of the hangers current location.

forgive my ignorance... you said the 3/4 ton springs are longer? and if i have read that correctly you said to move the shring hanger rearward by 4 inches? but in the picture you show the spring hanger bolted to the frame more forward of the original spot? but it does not appear that the hanger is mounted a full 4 inches fore or aft... or is this an optical illusion?



TIA for the explanation of what i am obviously missing.

-Izzy

[8lugnutz]

Looks like the first couple of pictures are of the left side, the last picture is the right side. If you look at the end of the frame, and you can tell it's left/right. If you are using the cross brace as a reference point, then yes, it looks like it.

[izzy_Britton]

Originally Posted by 8lugnutz

Looks like the first couple of pictures are of the left side, the last picture is the right side. If you look at the end of the frame, and you can tell it's left/right. If you are using the cross brace as a reference point, then yes, it looks like it.

AHHH! exactly right.

it would also appear, only from the last photo, that you could move the spring hangers back one more notch, maybe for an extra long spring? of coarse without crawling under my own rig i cannot tell from the photos if there is another further back mount point. just curious about that. I am planning to throw some 3/4 burb springs under my truck this weekend if time permits. now i have the info needed when i would likely face a head scratching problem.

-Izzy

[Jonboy]

I don't think there is that long of a spring, save for a custom one. Those other holes are for the bumper brackets.

[izzy_Britton]

yeah i don't need a longer spring, just making an observation.

-Izzy
Posted via Mobile Device

[MalibuSSwagon]

Izzy,

In the picture you quoted, you can see the mark on the frame where the rear of the spring hanger originally resided, under the hanger. The holes you are thinking are the hanger holes, are for mounting the bumper.

[Axle]

If you want heavy duty springs without wanting to move your spring hanger, spring# 22-485 6+1 leafs, 2775 pounds, 52" long spring.

[BONUSCAB]

I see so many threads with people trying to make their half tons and turn them into super-duper double extra heavy duper-duty tow rigs. And I just don't get it. Small brakes. Short wheelbase. Screams unsafe. just sayin...

[MalibuSSwagon]

Originally Posted by BONUSCAB

I see so many threads with people trying to make their half tons and turn them into super-duper double extra heavy duper-duty tow rigs. And I just don't get it. Small brakes. Short wheelbase. Screams unsafe. just sayin...

Well, the hauling capability of my 1/2 ton was pretty bad in stock form, 3/1 leaf springs meant it sagged with a couple of 2x4's in the back haha. Obviously those leafs were not up to the task since the main leaf was cracked at both ends, with only 95k on the odo.

Don't get me wrong, I don't plan on moving mountains with this truck, just household chores and towing cars. Should be more than up to the task.

Also, I can't see how the brakes would be an issue.

1986 C10 HD rotors vs 1986 C20 rotors


Less than an inch larger diameter and same thickness.

Rear brakes are also fairly comparable.

1986 C10 HD drums vs 1986 C20 11" drums


Really not a big gap. No I have not yet upgraded the brakes, but it's coming.

Also IIRC the 1/2 ton and 3/4 ton frames are the same thickness, according to Keith Seymore. I could wrong.

[izzy_Britton]

Originally Posted by BONUSCAB

I see so many threads with people trying to make their half tons and turn them into super-duper double extra heavy duper-duty tow rigs. And I just don't get it. Small brakes. Short wheelbase. Screams unsafe. just sayin...

my Truck is an 86 with the 1.25 (1.29) inch thick rotors and the heavy duty half ton shoes in the rear. this was not an upgrade done by me but as far as i can tell it was done either at the factory or at the stealership.

the thing that i dont think you are seeing is that myself as well as MalibuSSwagon (as he has mentioned) are not out to move mountains. but rather a stiffer suspension on our trucks is more desirable to us.

I prefer a little stiffer suspension front and rear, and i like the handling of these trucks with the heavy sway bars installed. my truck is also a 3+1 leaf pack and sags with a fairly light load in the back. I was planning on simply adding an extra leaf or two to my existing pack but this seems much more like a better option.

my MAIN reason for wanting the heaver spring pack in the back is because i do tow a small pop-up camp trailer. it doesnt weigh much but in stock form my suspension complained alot with it hooked up. so the decision and the benefits are two fold, better ride and slightly better towing capacity.

-Izzy