Differential swap out options
 

I currently have a 1970 GMC with leaf springs, 5 lug, disc brakes, non posi, 12 bolt differential with 3.08 gearing. I am interested in having a 3.73 gearing with posi/limited slip and keeping the 5 lug and disc brakes. I have looked into different options and seems like easiest would be to find a used differential that is already 3.73 and posi. Also, in the future I would like to convert to trailing arms. I have read that somewhere around 1970 the GMCs differential was a little different size than the Chevys?

My question is what would be a simple bolt in? Are there certain years that work better?

I found a 1998 10 bolt 3.73 posi differential someone is selling but I would have to buy 5 lug axles. Not sure though if it would fit.

Re: Differential swap out options
 

I have a 69 with leaf springs. Also interested in this answer.

Re: Differential swap out options
 

Mid year 1970 the axle length is longer than the early model. You can pull 5 lug axles from 71 and up and slide them right in to replace the 6 lugs. Trailing arm conversion is going to be a little more interesting to complete. If you want to change the diff, look for a 12 bolt in an early square and bolt it in. You do have to fix the shock mounts locations/possible park brake cable replacement. Axles can move between coils and leafs, but it is not just a bolt in. Mounting perches have to be done and adding or subtracting a panhard bar.

Best option to me is to re-gear your current axle. You have a lot of shops near you unlike me (I do it all myself). I'd pull the axle, clean it and drop it off. 3.73 from 3.07 will have the Gas Gauge doing the Electric Slide faster than it used to do it.

No matter which way you go now, the axle issue is going to come up twice to solve if you do a suspension change.

Smitty

Re: Differential swap out options
 

Thanks for the reply. I realize it will be work with relocating perches, shocks, possible drive shaft length, etc. You are probably right about just regearing. I have thought about regearing and posi myself - I have done everything else on my truck. Read a lot and watched a lot of videos on how to DIY and there would be a few areas of challenge for me as well as just screwing it up and now out a rearend. I think it would be funner to go with the posi/3.73 gearing but really not necessary and find I can easily cruise on the freeway going to Dino's Git Down going 75-85 mph keeping up with crazy fast AZ drivers! So I guess what I mean is I don't want gear changers remorse. Almost would like to have a 2nd differential so if I ever wanted to go back to the 3.08 it would just be my time.

I slid under the truck and got a rough measure of 59 inches from the inside of the mounting plates. Would an early square bolt right in if I kept leaf springs?

Re: Differential swap out options
 

If you have never been trained on how to rebuild a rear diff, I would not do it. Please do not watch videos on it, it just aint that easy. I have built a many 12 bolts and there are a lot of ways to mess it up. A ton of people do not know how to read gear tooth patterns and that is where you will mess up along with preload requirements on bearings. I have a whole drawer just for diff special tools. Crush sleeves aint nobodies friend either. I could go on and on about this subject...

I have a 74 squarebody leaf unit on my 68 stepper. Only change was the shock mounts. Everything else was the same. Have to think if the park brake cables matched (probably didn't). I can measure it tomorrow if you like, it will be a while, I got to go pick up O-G in the morning about 400 mile trip.

Talk to Tom Korinek in Mesa. He may have what you need on the ground already. He is on here (TKorinek) send him a message.

Smitty

Re: Differential swap out options
 

Thanks for talking me off the ledge. I get it. I'll probably just keep looking for another diff or save up to have mi e professionally done.

Re: Differential swap out options
 

No worries.

Smitty

Re: Differential swap out options
 

Yes, it would be a lot less painful to simply change the gears. In addition to making the gas gauge pointer drop quicker, 3.73 gears will make your tach pointer move 21% higher at any given road speed, and highway driving will not be fun, unless you have an OD trans.

Re: Differential swap out options
 

You should use a gear that provides the performance and comfort you desire. IME any gear change will not change milage. Have a 94, and 2000 as well as my current 72. Also had a past 72 my first vehicle. The 94 and 2000 currently driving vehicles average 13-14 MPG. I expect the same with the LSA swap in my 72. Both of my 72's averaged 10 MPG empty or loaded. The original longbed 3/4 ton had 4.10's and the current 1/2 ton shortbed had 3.73's when a driver. Did the gear change in my 94 a while back because of a failure, my first diff ever 3.42 up to a 3.73 and no milage change. Just better performance. Was not really difficult, but I attempt a lot of stuff some may say is difficult.

Re: Differential swap out options
 

This brings up some questions and back ground info on this truck.

As 1/2 ton 70 trucks were all six lug and drum brake be it either Chevy or GMC With no listings in parts catalogs for disk or 5 lug for 70 what's the scoop?

Historically Chevy came with the trailing arms and coil springs while GMC came with leaf springs but either could be special ordered the other way from what I understand.

If you were close enough I have a trailer made out of the back end of a 70 C10 Frame from a truck I rolled in 1989 that has all the coil spring pieces except the axle. I sold the axle after I totaled the truck and stuck the old axle from the guy I sold it to's truck under it. A very worn out stump pulling geared 66 axle. I've thought about putting the trailing arms under my 48 but the jury is still out on that.

Re: Differential swap out options
 

Montierth,
Hard to remember how to spell your user name...LOL
It sounds like you already have a lot of good going your way. You 70 has been converted to 5 lug and disc.
I advise folks to buy a 71 or 72 if they like this era of pickup, just because they already have front 5 lug disc. Conversion is not that difficult for many of use, but a hassle for folks less mechanically inclined, that just would like to enjoy one of these pickups without working on them. I've seen many of these that are real nice that still have the dreaded front drum brakes still on them....I have a 67 and 68. Both will be 5 lug disc when they are roadworthy again. I am using factory parts.

The gear ratio is an important thing. In the olden days you just ran what you had. The 3.07 and 3.73 are the most common gears in these pickups (I think, anyone?)
I read here, sometime back that the factory ratios available were 3.07, 3.42, and 3.73 (there were others, but I don't remember, these were numbers I was interested in. I recently purchased a 3.42 set. Is a great gear for overall driving performance or cruising. MPG's won't be the best, your driving desires can dictate the gear you want.

I understand your desire for rear coils. Both my 67 and 68 have rear coils. I've been doing a lot of work on a friends 71 C-10 LWB. It has rear leafs. I see no undesirable driving characteristics in it. It's a boatload of work to change to coils. Might be easier to buy a coil spring chassis and swap your stuff over. Your leaf spring rear endd connt be (easily) converted to coil spring use........

Re: Differential swap out options
 

Well then, GM must have made a huge mistake going to the 700R4 OD trans, or those crazy gear ratios like 2.41 and 2.56 thinking it would help them meet CAFE standards.

Re: Differential swap out options
 

I have a 1970 SWB trailing arm truck, and I have been interested in a similar question to OP's. My truck had the G80 option on the SPID, but that appears to be long gone, now. Its a one-wheel peeler.

The reason why I would be interested in swapping an entire axle is to kill two birds with one stone: getting posi while also going to five lug.

Are the 73 and up rear assemblies that much different from the 71-72? If I am reading correctly it sounds like it is just the axle perches that may be different?

Re: Differential swap out options
 

It was bad, but not as crazy as one thinks. The later the transmission, the lower the first gear.

I had a 2006 Corvette with 2.73:1 standard rear gears. It also had a 6-speed with double overdrive. The 6th gear had a 0.57 ratio, and the final drive ratio was 1.56.
It would pull a 5 Deg sloped hill without shifting down, in Drive, at 1400 rpm in 6th.
A light car with 400 hp helped. The dog engines of the 80s and 90s definitely struggled with the high ratios.

Re: Differential swap out options
 

sent you a Pm....

Re: Differential swap out options
 

tire diameter of probably 26" helps as well

Re: Differential swap out options
 

The posi in the 1998 unit is likely a Gov-Lock style of posi that has a centrifugal engagement mechanism that activates once there is a significant enough difference in RPM between the left and right axle shafts. It will then disengage at speeds over 20mph or so. I put one in my 1989 K10, and it worked well for me while doing some four-wheeling in Moab, UT a few weeks ago.

Re: Differential swap out options
 

Hey guys thanks for the input. I know I'm coming in late. We just moved and been busy fighting with the builder. I'm gonna crawl under the truck and measure the axle. I think I will try to find an axle with the specs I'm looking for so I can try out and swap back to old one if not happy?

Re: Differential swap out options
 

If you're worried about "gear changer remorse" as you accurately put it, I had the same concerns. I went from 3.08 open to 3.42 posi and love it. With a 700R4, it's fun off the line and local driving but still cruises nicely on the interstate. As a few have stated, 3.73 would likely be too steep for me.

I bought a spare 12-bolt coil spring housing to rebuild, but after figuring time and parts, I took it to a very reputable differential builder do it. It was the same price whether I drove the truck in to rebuild on a lift, or had them build the spare, so it was a no-brainer for me.

Re: Differential swap out options
 

3.73 x .70 OD is like running a TH350 with a 2.61 axle.
3.42 x .70 OD is like running a TH350 with a 2.39 axle.

Depending on engine powerband and tire height, the 3.42 may leave you wanting a lower gear, like the 3.73. However, I understand 3.73 was a common axle ratio for square body trucks with 700R4 transmissions. But those trucks were built with very mild cams and an emphasis on low-mid RPM torque.

Re: Differential swap out options
 

Personally I would have the current rear converted with new bearings and seals everywhere. A quality shop will also check the axles for straightness too.

Then you can enjoy the piece of mind that you good to go for the next 100k. Used axles are a mixed bag of good ones with unknown mileage and ones with bent axles or housings, toasted posi units, bad pinon bearings or leaks, missing gear teeth, and other hidden issues. Ugggg!

If you desire to go back to 3.07's it's much easier to find a used axle of that ratio. And the rebuilt one is worth premium pricing.

I agree with the no real change in mileage. I went from a 305 SBC with 3.07's to a 350 SBC with 3.73's both got 14 MPG on the highway.

The real world difference is the engine is turning more RPM so the truck becomes louder on the highway. Especially with a manual transmission. Do you do much highway driving?

Re: Differential swap out options
 

Hey Guys - Shout out to 53burb! He lived right aound the corner from me and just sold me a 1971 3.73 posi 12 bolt. I will just have to change the perches and will probably leave leafs for right now. This way I can try out the gearing without too much effort. If I like I will use then maybe rebuild/update it.

I will need to pull the axles and remove the drum brakes and add my disc brakes. It is already 5 lug so it'll be good

Re: Differential swap out options
 

Now need to try to find a 700R4. Or would a 2004R work? The only freeway driving I do is to go to Dinos Git Down. About an hour away.

Re: Differential swap out options
 

Good meeting you Steve!!

Re: Differential swap out options
 

2004r will not tolerate towing or heavy loads. They have a bad propensity to split the case when going down hills with a trailer or 1000lbs in the back.

So if you never hook a trailer or put the truck to work, a 2004r would work.