52in to 56in spring swap

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BigSexy78

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I've got a 1979 GMC k15 and want longer rear springs for better ride and more load capacity, would really like 64s but 56s seem much easier, is it true that you can buy a lift shackle and just swap the side it's on to swap in the 56s? I'd like to go with the ord 2.5in shackle flip if possible
 

theblindchicken

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If I remember correctly, that's all you need to do. Swap them side to side so they're angled back for the 56's. Angled forward for the 52's.

I can't remember if it's the same idea with the stock shackle hangers or if they're physically moved back on the frame.
 

HotRodPC

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Go with the 56's. It's easy to do. The frame is already punched for the holes. Drill out the rivets from your rear hangers. Use grade 8 bolts and locktite red and use the holes that are 4 inches behind the current holes and done. I'm doing the same to my K10. I'm using C6P C20 rear 56in springs with 2in lift blocks and installed a 14b Full Float 3/4 ton axle. The front is staying 10 bolt but getting 8 lug 3/4 ton brakes, backing plates and 8 lug hubs, under 4 inch lift springs. In short, I'm converting a K10 to a K20 with just rear spring swap, rear axle swap, and 6 to 8 lug conversion on the front axle.
 

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Go with the 56's. It's easy to do. The frame is already punched for the holes. Drill out the rivets from your rear hangers. Use grade 8 bolts and locktite red and use the holes that are 4 inches behind the current holes and done. I'm doing the same to my K10. I'm using C6P C20 rear 56in springs with 2in lift blocks and installed a 14b Full Float 3/4 ton axle. The front is staying 10 bolt but getting 8 lug 3/4 ton brakes, backing plates and 8 lug hubs, under 4 inch lift springs. In short, I'm converting a K10 to a K20 with just rear spring swap, rear axle swap, and 6 to 8 lug conversion on the front axle.
That basically what I'm wanting to do with my k15 but with trying to uprgade the brakes a to 1 ton specs to set it up best for pulling, since one day I'd like to use it to pull about a 25ft gooseneck around with hay and farm equipment
 

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To get to 1 ton spec you'd need to upgrade to hyrdo boost.
 

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Okay, thanks wasn't sure about that part, from what I've read hydroboost is a pretty straight forward swap?
 

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Okay, thanks wasn't sure about that part, from what I've read hydroboost is a pretty straight forward swap?
brake pedal, brake booster, and master cylinder will need to be changed. not sure if the firewall bolt pattern and pass through is the same though. I'm working on a hydroboost brake system now. there are recent threads where I've linked to a hydro boost swap, although I don't have any experience with the swap as mine came with hydro boost.
 

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Jrgunn5150

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brake pedal, brake booster, and master cylinder will need to be changed. not sure if the firewall bolt pattern and pass through is the same though. I'm working on a hydroboost brake system now. there are recent threads where I've linked to a hydro boost swap, although I don't have any experience with the swap as mine came with hydro boost.

This is correct, I've even done it with Astro van parts.
 

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That basically what I'm wanting to do with my k15 but with trying to uprgade the brakes a to 1 ton specs to set it up best for pulling, since one day I'd like to use it to pull about a 25ft gooseneck around with hay and farm equipment

Are you going to swap the rear brakes to the 3.5" wide shoes/drums a 1 ton has or leave the standard 3" that 3/4 ton 14 bolts come with? I've done two 1/2 to 3/4 swaps on square bodies. The last was on my 79 K15. I can say, now that I have a K30, the brake set up is heavier than the 3/4 tons I was used to.

Since I wasn't going to do any major wheeling with my K15, I installed a 14 bolt SF rear. I used a master cylinder for a 79 3/4 ton (Napa NMC M2085 - 1979 GMC K20 w/“JB7” 13” rear brakes). It was a simple bolt on to my power brake booster. The only thing I had to due was reverse the brakes line locations on the master cylinder as they were reversed from the 1/2 ton MC.

After the swap, under panic stops, the rear brakes locked pretty easy. This could be due to the fact I had a short bed. I was going to install and adjustable proportioning valve for the rear brakes, but I sold the truck before I could.
 

BigSexy78

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Go with the 56's. It's easy to do. The frame is already punched for the holes. Drill out the rivets from your rear hangers. Use grade 8 bolts and locktite red and use the holes that are 4 inches behind the current holes and done. I'm doing the same to my K10. I'm using C6P C20 rear 56in springs with 2in lift blocks and installed a 14b Full Float 3/4 ton axle. The front is staying 10 bolt but getting 8 lug 3/4 ton brakes, backing plates and 8 lug hubs, under 4 inch lift springs. In short, I'm converting a K10 to a K20 with just rear spring swap, rear axle swap, and 6 to 8 lug conversion on the front axle.
I was looking at my frame and didn't see the holes to slide back the hanger? I'll try to get pics next time but didn't see anything
 

77 K20

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I've been researching this lately as I want to remove my 3" blocks from the back. And since I'm re-doing my suspension again I want to make sure I'm happy with it.

I found this website:
https://www.generalspringkc.com/Leaf_Springs_Chevrolet_GMC_Truck_2500_1973_1987_s/1829.htm

I was just going to do a shackle flip and be done with it- but I have 52" springs. Now I'm trying to figure out if it would be worth it for the 56" springs.

As per the website above I have 7 leaf 52" springs. 2,775 lb spring capacity.

I'm looking at the 9 leaf 56" springs with a 2,600 lb spring capacity.

In theory they should ride better and have more flex- but will they? A 40 year old spring gets soft- so are my old springs better to keep?

The new springs should provide some lift I'm thinking. Maybe 1-2" I'm guessing. Is that correct to assume?

With the shackle flip I've read the same as what the blind chicken said. Angle the hangers forward for 52", back for 56".

HOWEVER- I've seen different things online for what length shackle to buy. I wanted ORD's "normal" length ones that are 4.5". Some say that those are too short for the 56" springs and they won't be at the right angle. They then say you need the 6" shackle- but ONLY on a heavy vehicle. On a light vehicle the shackle can flip the wrong way.

Anybody have any input to shackle length? EDIT: I did find for a fact that ORD themselves do not recommend the 4" shackle flip with the 6" super shackles.

OP: didn't mean to hijack your thread. I just figured it was quite similar to your questions and maybe it would be good for you too...
 
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HotRodPC

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If we can get him in here, @crazy4offroad probably has more knowledge on the shackle flips and options available.
 

theblindchicken

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I've always heard that the best shackle angles tend to be right around 22-25° from vertical towards the rear.
 

HotRodPC

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I've always heard that the best shackle angles tend to be right around 22-25° from vertical towards the rear.
That sounds about right if you think about how the springs operate and importance of the shackle pivot. I've never heard anything like that, but it seems to reason.
 

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