1970s to 1980s Spring Differences?

[Kilian]

What are the major spring differences that would exist between a 1975 3/4 ton 4x4 frame and a similar 3/4 ton 4x4 frame from the mid 1980s? Shackle or mounting differences? Differences in spring lengths, etc.?

Parts catalog lists a ton of different springs through the model years...just wondering if someone has deciphered the changes and can give me the Cliff's notes version. Thanks.

 

[Bextreme04]

There are no shackle or mounting differences. They are all compatible with every year. The spring that came on the truck from the factory had to do with model and selected RPO’s, but they could all be interchanged. The biggest factory detail that will matter for spring selection if you are trying to replace factory springs is whether your rear spring mount is set up for 52” or 56” springs. If it is all the way at the rear of the frame it is in the holes for 56”. All frames came with the holes drilled for either spring length option.

 

[75gmck25]

It’s been my understanding that the 56” springs were used on 1 ton trucks, but maybe they also used them on the lower GVW trucks.

My ‘75 K25 is the highest 3/4 ton GVW (8400 lbs) in ‘75, with the camper special option and 7 leaf springs, and I have 52” springs.

The spring eye diameter is different on some springs as the weight rating changes, so it took me a while to figure out the right combination when I bought poly bushings. However, it’s easy to measure bushing diameter if you have any doubts.

Bruce

 

[bucket]

It’s been my understanding that the 56” springs were used on 1 ton trucks, but maybe they also used them on the lower GVW trucks.

My ‘75 K25 is the highest 3/4 ton GVW (8400 lbs) in ‘75, with the camper special option and 7 leaf springs, and I have 52” springs.

The spring eye diameter is different on some springs as the weight rating changes, so it took me a while to figure out the right combination when I bought poly bushings. However, it’s easy to measure bushing diameter if you have any doubts.

Bruce

Iirc, both my '86 and '90 3/4 ton 4x4 Suburbans came with 56" springs.

As far as 3/4 ton trucks in general, there were a lot of 2-wheelers with 56's. Even a bunch of C10's too.

 

[idahovette]

Got 56" springs from an 80 C10,so they are all over on different rigs.

 

[Kilian]

I checked the 75 K25 project frame that I have, and that has 52" springs, 7 leaves. The 87 V20 parts truck I have has 56 inch with 8 leaves (has the C6P option code). My running 83 C2500 has 56 inch springs, and the 85 K10 parts truck has 52 inch.

Are the 52 inch, 7 leaf springs similar in weight bearing to 56 inch, 8 leaf spring sets? I'd have to assume so based on the earlier reply of a 75 camper special having 52 inch/7 leaves.

 

[75gmck25]

GM changed the max GVW a little over the years because they were chasing emissions control requirements that were based on GVW. My ‘75 K25 is 8400 GVW, with 7 leaf springs, 14 bolt FF and 9.50 tires. It’s considered an HD emissions truck, so no catalytic converter, EGR, leaded gas, etc. I believe the only factory emissions devices it had were the PCV and the vapor canister for the gas tanks.

I’ve looked up replacement 52” 7 leaf springs for my ‘75 and they are rated for 2775 lbs per spring, but I don’t know if that is an exact match to the original springs.

When I looked up the springs for an ‘87 3/4 ton the highest rated replacements were 2600 lbs, but on a one ton you can go up to 10 leaf that are rated at 3955 lbs.

Bruce