The truck is an '85 GMC C2500 2WD longbox with the semi-floating rear axle (I think it's a Dana 70, but I am not sure). I was reading that the 1-ton SRW trucks came with a full floating axle. I am considering upgrading to the full floater. I have had problems with my existing axle (lubrication troubles, etc.) but it works fine now. I was wondering if anyone has done this swap. Info, pics, other modifications neccesary, what all is involved?
Also, about how much do you think the total cost of doing this would be (considering that I would do all of the labor)? Are the full floating axles hard to find?

Any help would be appreciated.

I've been thinking about this one for a while; I can't tell you cost, but can help with some info.

The swap out should be easy, springs should line up, brake line should match (auto parts can help you there), park brake cable should fit (if dissconnected at the Y) and brake proportioning should be the same (if not a simple switch to a 1 ton prop valve will take care of it). The only deal would be a shorter driveshaft with the correct u-joint attached; may be a dirrect swap with a 1 ton also.

To me, I think I would spend the extra $$ for a newer style diff with the disc brakes IF the leaf springs would drop in the same spot and a proper brake prop. valve is available.
Just my $.02

The only deal would be a shorter driveshaft with the correct u-joint attached; may be a dirrect swap with a 1 ton also.


So the driveshaft is shorter on the full float axle?

Also, I have 3:43 rear gears at the moment. Can I get a full floating 1-ton axle with these same gears?

As far as disc brakes go, I would like to have them, but I think the swap would get a little too complex and expensive. I am trying to keep this simple and relatively cheap.

The driveshaft will probably (not 100% on this) be shorter because the rear diff is phyically larger than the other; the snout should be bigger and longer, u-joint should be larger also.

As far as 3:42's, I think you will only find 3:73 as the highest gear, with 4:10's being the most common gear with that rearend; may be lucky and find 4:56's for some real towing.

The semi-floater is a 9.5" considered to be of GM manufacture. Do you have an 8 lug semi-floater as some 2500 trucks were actually 6 lug. Full floaters will be 8 lug for sure.

I have the 8-lug semi-floater. I never heard of a 2500 with a six-lug axle, but I'll take your word for it.

The driveshaft will probably (not 100% on this) be shorter because the rear diff is phyically larger than the other; the snout should be bigger and longer, u-joint should be larger also.


I guess that I would then have to get the driveshaft assembly out of a 1-ton and bolt it in. I am guessing it is probably a bolt-in swap.

As far as gearing goes, I would want the 3:73 if that is the highest you can get. I want to keep some fuel economy!

I might have missed it? But what axle are you looking to install? A 14 bolt would be the best choice!! I swapped one into an 81 12 ton! now it's eight lug! it was cake everything was bolt in! save for the drive line modification that I'm not sure you would require! Scott

I have the 8-lug semi-floater. I never heard of a 2500 with a six-lug axle, but I'll take your word for it.



I guess that I would then have to get the driveshaft assembly out of a 1-ton and bolt it in. I am guessing it is probably a bolt-in swap.

As far as gearing goes, I would want the 3:73 if that is the highest you can get. I want to keep some fuel economy!

Starting in 88, you could get a light duty 3/4 ton with the 6 lug wheel (7200GVW) which was basically a glorified half-ton :rolleyes:

At any rate, you shouldn't have to get a different driveshaft, provided the length is correct. There are many, many different combination ujoints out there to make this swap happen.

If you want to go with disc brakes in the rear, you might consider looking for a P-30 rear axle..some of them had rear disc from the factory. :)

It is a easy swap that can be done in a few hours. You would want to swap in a 14 bolt FF. These are cheap and parts are cheap as well. You may or may not have to shorten your rear driveline. In my Jimmy I have swapped out my 10 bolt for a 14 bolt ff and swapped the 14 bolt ff for a DRW Dana 70. The dana 70 is a little stronger then the 14 bolt. http://www.off-road.com/toyota/tech/dana70discbrakes/

If I was to do it again and had a corp 14 I would of used that instead. The locker for the 14 bolt is allot cheaper then the 70 locker.

You can do a disk brake swap on the 14bolt or the 70 pretty cheap as well

So if I was to look at a 14 bolt full floater, I would just measure the distance from the axle centerline to the u-joint and that would tell if I would need to upgrade my driveshaft, right?

I guess I'll look for a 14 bolt axle next time I'm at the junkyard, and get some measurements.

How do I identify a 14 bolt axle? I know it's a full floater, and it has 14 bolts retaining the rear cover.

One other thing: Can I keep my existing (stock) rims with the full floater?

The 10.5 14 b came in many vehicles and may not just bolt up to your truck. Look for one that was in a suburban of the same era 73-91 and it will bolt in no problem. Get one from a 3500 truck same era and the spring pad dimension is different. The 3500 used different spring hangers which were closer to the frame rails than your set up. Unless you lift it the drive shaft will be to long no matter where the axle comes from.
As far as a 70 being stronger, I don't see how, they both have pluses over each other.

So the suburban had a different axle than the truck even though both axles were heavy duty full floaters?

I had already written this post earlier and it erased it. second time around.

as far as I know, stock rims will work on the full floater. I was also told suburban axles are a differnt width. most 3/4 surburbans came with gm 14bolt semi. a few does have a 14bolt ff. never seen one but they are around. I know for fact van axles are wider.

I want to do the same to my K20 pickup. a 1 ton single wheel gm14 ff will not work without modification. the spring perch and shock brackets will have to be cut off and properly welded to match a 3/4 ton. the shocks are on the inside of the frame for a 1 ton, outside for a 3/4. also, the brakes are differnt. the 1 ton 14 bolt ff using a larger brake setup then the 3/4, somthing might have to be converted to work.

also, alot of 1 ton 14 ff came with lockers, and there is a carrer break in the design. so if you choose to go with that, find the one that your 3.42 gears will fit in. they are actually different cases so not all gears will fit. the carrer break is 4.56 and down, and 4.56 and up. so if you find a ff with 4.56 and up, take caution.(you can still keep your 3.42 gearing if you want, some companies do have gears of that ratio for the 14 ff)

your best bet is to find a 3/4 ton pickup 14bolt ff axle. they were made from 1978-1996 I think(ones from 78-88 would be best). they will fit right under, no cutting and weilding. also brake size will be the same as the semi floater I beleive, so no mods done there. I found a 1 ton single wheel ff and I gave up on that, im trying to find a 3/4 ton ff instead. less work since I would also have to swap gears and put a posi unit in. although gm 14bolt ff are supposed to be a dime a dozen, I have a hard time in this area finding.(if anyone has a source for good condition GM 14 bolt ff axles..let me know please)

to tell what axles really are GM 14 bolt ff, you need to find one that the cover matchs, as well with the axle shafts protruding from the rims.
this link has a lot of good information I reccomend reading.
http://www.pirate4x4.com/tech/billavista/14b_bible/index.html

You can use a 14bff out of a 3500 73-91 old style you just need to use the spring hangers off the frame as well. They come off easy with a grinder and a punch. Bolt the 3500 hangers on your 2500 or even 1500 frame and bolt the axle right up no welding no nothing. You will still need a driveshaft mod though.

When I swapped a 14 bolt FF into my 1/2 ton K5 Jimmy. I did not have to grind and reweld the spring perch's. I have seen 3 of these done and none of them have the perchs moves. Although if you are swapping in a dodge or DRW axle the spring perch's will need to be grinded of and rewelded on. The dodge it going to be out 3/4" not a big deal. The DRW 70 will be way out though

Never heard of taking the spring hangers off the frame for a axle swap. You are still using your springs and not longer ones so you will not have to relocate the hangers.

If you do have the grind and reweld the prechs on you can always set the pinion angle to where you want it before you weld it.

Okay, Theres a lot of conflicting info here so lets straighten this out. If
you get a 14 bolt full floater from a 3/4 ton pickup, or a suburban. The spring perches will match right up. If you get it from a one ton, you gotta move the perches or get the hangers. Depending on the year of the 14 bolt full floater, The yoke may be different. But 90% of the ones i have are the same size joint, full float or semi float. The drive shaft legnth is the same from full float to semi float. I have measured it, and a 14 bolt has the same shout legnth as a ten bolt, beleive it or not. Brake lines will fit fine, as will e brake cables. Some 3/4 ton 2wd full floaters came with 3.42 gears, but the majority came with 3.73 or 4.10s (depending on auto or stick). Hope this helps.

The 3/4 ton 10.5" 14 bolts had small drum brakes. You can actually fit 15" wheels over them. You can't go wrong with this axle!

Thanks for the replies, everyone.

I have only one question. Driveshafts: do I need a shorter one or not? Different people have said different things. I would like to know before I go looking at the various available axles.

As far as spring perches go, I would like to keep this a bolt-in job, so I'll be looking for an axle with the same dimensions as my current axle. I am sure I can find one around here somewhere. Every one of my nearby scrapyards has a huge pile of full float axles. Gotta be one in there somewhere!

Yep 14 bolts on the cover! and bolts holding the axle in! just don't get the dually version here is a decent review! http://www.pirate4x4.com/tech/billavista/14b_bible/ Scott

As mentioned any 3/4 (pickup) or 73-91 (suburban) 10.5 14 bolt full-floater will bolt right in to your 3/4 ton and any 1/2 ton for that matter. All 3/4 ton pickups and burbans had this rear from 73-80/81. After that the small blocks got the 9.5 semi-floater and only the big blocks got the full-floater. The 1 ton single rear wheel version will not bolt right in because the spring pads are placed in closer to the differential on the tubes. Boyce military equipment will build you a 14 bolt f.f. with new everything ready to bolt in, with any gear and locker combination you could want. Don't know the website off top of my head but it's there somewhere.

I keep hearing surburban is differnt width. we need measurements

http://www.boyceequipment.com/axles.html

burb's p/u & blazers all are the same width, EXCEPT 1 tons. The easiest way to measure is from the brake backing plate to the spring perch. You will find that the 1 ton single & maybe the duelly have the perches closer together than 1/2 tons. I've just gone thur this on my project. It's an 85 ex military with 25,000 mi on it I picked it up without the drive train. I have a completely rebuilt 82 6.2 red block in it with a sm 465 4speed with a 2 speed aux (25% over) when I put the rear axle 3/4 full floater in I found at that time the differance in spacing! I found that that the spring hangers are differant, after much thought I decided that cutting the perches off and rewelding the would be better in my case as the frame looks like new. One thing that hasn't been mentioned is the price of new brake drums for the full floater:eek: WOW!! i went and switch over to rear disc's I think it's pretty neat, if any one wants to see it I'd be happy to post some pic's as soon as one of my kids show me how:o: !

High price for rear full floating axle brake drums? I looked it up here and for most model of drums, its a few dollars cheeper the the semi version.

Are the UV joints the same for the semi and full floating GM 14bolt axles?

Does the full floater take different lug nuts then the semi?

YES some of those old drums are very pricy. Well I guess it all depends on what you can afford, and I can't afford a new truck thats why I drive an 84 with a 6.2 with 300,000 or so miles on it.the axle in my project truck had 13" drums & shoes that where about 2.75" wide and very heavy. I bet I saved 100 lb with the brake swap

U-joints are the same- 1350 series. Unless it's out of a motorhome, usually 1410 series.

Disc brakes work better, but good luck setting up a parking break if you need one. Sure, Eldorado calipers work, for a little while.

3/4 and 1 ton drums are the same diameter, but the 1 ton has 3/4" (maybe more?) wider drums per side.

You can change the lugs to any style you want, lugs are about 2 bucks each. They will PROBABLY be the same, but if you're shopping used it's usually luck of the draw.

A 14 bolt goes for about 50 bucks. A set of u-bolts goes for that. A new u-joint and new straps should be less than that. After you replace the lube, you could be in it for less than 200 bucks plus about 2 or 3 hours labour. You'r talking about purchasing an axle that's going to be at least 14 years old, or 17 if you get it from a pick up. I'd rebuild it no matter what. I'd look at a complete rebuild, but that depends on how serious you are about having it last forever, or just rebuild stuff as it wears.

I am still wondering about the driveshaft. There seems to be no definitive answer, so I guess I'll have to go measure two different axles.

You'r talking about purchasing an axle that's going to be at least 14 years old, or 17 if you get it from a pick up. I'd rebuild it no matter what. I'd look at a complete rebuild, but that depends on how serious you are about having it last forever, or just rebuild stuff as it wears.

I am not planning to rebuild the axle. I could, but if I was going to rebuild anything on this truck, the axle wouldn't be my first choice. Rebuilding the axle (even the parts alone) would cost more than I paid for the truck.

i went and switch over to rear disc's I think it's pretty neat, if any one wants to see it I'd be happy to post some pic's as soon as one of my kids show me how

I'd like to see that. What do you do for a parking brake? How about a proportioning valve? Did you change master cylinders? Also what did you get the discs off of?

the pinion on a 14 bolt full floater sticks out exactly 1 inch more than a semi floater 14. You MAY or MAY NOT have to shorten the driveshaft, it seems to be a luck of the draw thing.

I have had many sets of disc braked 14 bolts. You need the rotors off of a 3/4 10 bolt or Dana 44. Also, you'll need calipers; You can use either eldorado calipers with a parking brake, which never seems to work for too long, or you can use the same calipers off of the 10 bolt/44. You'll need to get a backing plate to mount the calipers, (there's a bunch of outfits out there, just search on google) or, if you're good with a torch you can make the backing plates off of a 10 bolt work.

In my experience, I have not had to change the parking prop valve or MS, but if you DID have to, search for a firebird/camaro with 4 wheel discs, should be a direct swap (booster+master cylinder and prop valve)

http://www.oliverdiesel.com/tech/1tonbrakes.htm

seems to be good info on a guy modifying his 3/4 full floating axle to use 1 ton brakes.

only thing that gets me is he said it was a 3/4 ff with 11 x 2.5" brakes. I dont think thats right.

I have done a lot of these swaps. 1) the 14 bolt semi floater is about 1 inch longer at the pinion than the full floater, so your driveshaft will work, but it will be extended a little. 2) The brakes are interchangeable and were available in 3 sizes. The early 3/4 tons (73-75) used a 11x2.5(about) the later 3/4 tons used a 13x2.5 and the one tons used a 13x3.5. I currently use the 13x3.5 one ton brakes on my suburban.
The one tons do have different spacing on the spring perches for clearance to run dual wheels. A one ton axle will bolt up to 70 to 72 Blazers and 1/2 ton pickups and suburbans. Not sure about 3/4 tons of that era. The 3/4 ton version is interchangeable for all years between 73 and 87 with a few small changes. 2wd axles may have different angles on spring pads and may need to be shimmed to get propper pinion angle. 2wd also have different shock mounts that hang down below the axle tube, where 4wd shock mounts are flush with the bottom side of the tube. ;)

2wd axles may have different angles on spring pads and may need to be shimmed to get propper pinion angle.

My truck is 2wd. Would I still have to shim the new axle (assuming that it came out of a 2wd)? I would think that the 4wd axle would be the one that would need to be shimmed.

Yes, my point is to check your pinion angle of your current axle and then make sure the new one is the same. In my neck of the woods, 2wds are rare.

You'll need a 14FF from another 2500 to do the (bolt-in) swap. The 3500 ones have different spring pad widths. The ujoint is bigger, there are conversion joints available. Not sure on length of driveshaft. Otherwise the brakes will all connect just like normal, and your wheels will fit. The 14FF has the acorn shaped diff cover with 14 bolts yes, and the hubs are sticking out.