Learn Me: GM Truck Axles

So I need to put a new axle in a mid 90's G20 van. I will measure the spring perches tonight, but I think its somewhere in the 50" range center to center. What do I need to know before I go axle hunting at Pull A Part? Pretty much only requirements are disc brakes and bolts up to existing leafs. Should I be looking for something specific? Anyone have experience with a swap into a similar vintage GM truck? Any knowledge is good knowledge pls and thank you. 

Edit: I would consider ability to handle 400hp and burnouts in a 6000lb vehicle a secondary requirement.

Huh.  Disc brakes?  I thought some cars still had drum rear brakes in the 90's.  I figured most trucks did.

I think its only things like escalades and the 3/4 ton and bigger got discs at that point.

The GMT800s would have rear discs, 1999-06 2500-up pickups, 2000-06 Suburban 2500. Probably later ones as well. The 90's ones would be drum still.

Probably the only thing you'll bolt in is another gm van axle.  The good news is, you can chop perches off and weld new ones on if you are so inclined so a tape measure is your friend at the junkyard 

In reply to 1988RedT2 :

I apologize if that was a bit confusing. The van is original drum brakes. I am looking for an axle with discs because who likes dealing with drums...

In reply to 81cpcamaro :

Good to know, thank you sir

In reply to Patrick (Forum Supporter) :

Yeah that is definitely not the end of the world. I think I even have some new perches, I was just trying to avoid the guess work and headache involved with pinion angle and such. We will see what the junk yard gods have in store for me

81cpcamaro said:

The GMT800s would have rear discs, 1999-06 2500-up pickups, 2000-06 Suburban 2500. Probably later ones as well. The 90's ones would be drum still.

To clarify: by GMT-800 you mean the half ton variant right? Because my half ton Sierra has disc rears. I think after 06 they switched back to drums because they had problems balancing brake pressures but I could be wrong on that.

When did the GMT600 go to rear disks? That's probably the closest I would think.

young_boomer said:

81cpcamaro said:

The GMT800s would have rear discs, 1999-06 2500-up pickups, 2000-06 Suburban 2500. Probably later ones as well. The 90's ones would be drum still.

To clarify: by GMT-800 you mean the half ton variant right? Because my half ton Sierra has disc rears. I think after 06 they switched back to drums because they had problems balancing brake pressures but I could be wrong on that.

DEPENDS..... 2wd 1500 with certain cab styles went back to drum in 2004. All the 1500 99-03 had rear disc

How many lugs?  6 or 8?

Your van likely has either the 9.5" SF or 10.5" FF (semi-floater or full floater) version of the 14-bolt.  On incredibly rare occassions (mostly in the 80s) you could have a 12 bolt because it's a van.  Some of the vans got a lower GVW than a comparable 3/4 truck.

You need to get one of the same flavor unless you want to do a custom driveshaft or find a proper driveshaft for the application.  They all have different pinion depths.

Without googling I can't remember specifics, but I'm pretty sure the switch to rear discs came at about the same time as the switch to Savannah/Express.  IIRC, the new body came with a new frame and wheelbase so you might have trouble getting things to line up by just getting the later disc axle.  The good news is that with careful shopping you can get later discs from the same axle and swap them onto an older version.

Are you swapping because the old axle is toast, or because you're just looking for discs?  If just looking for discs, it might be wiser to convert your axle because of the differences early/late.

The important measurements:  wheel-to-wheel flange width, spring perch width, same center carrier (to avoid driveshaft length issues).  Don't fuss about the type of yoke that is on it.  You might buy an axle and realize that your old van used 1350 u-joints and the new ones are metric.  Just swap your old yoke on to the new axle.

One other thing to consider.  If you're going same-same, bolt it in and don't worry about it.  If you find one from (for instance) a 3/4 truck that fits, be prepared to get some wedge shims for the springs.  Some of the trucks use a different driveline angle than the van, so the perches are welded on at a different clock position to change the pinion angle.

This website has some good ID things if it's a 14-bolt.  All GM 14-bolt axles can be identified by the number of bolts on the cover.  It's 14.  When you get into 10-bolt and 12-bolt, things get a bit hazy.  95% of the time, a 10-bolt has 10 bolts on the cover and a 12-bolt has 12... but there ARE 12-bolts with 10 on the cover, and a rare few 10-bolts with 12 on the cover.  Most of those were 70s full size car (in the case of the former) and BOP (in the case of the latter).  The important distinction is that the axle's NAME comes from the number of RING GEAR BOLTS, not the cover bolts.  It just so happens that MOST of them follow the rule that cover bolts equals the ring gear bolts, but not always.

This website has info on IDing other axles.

.... and just for reference, I have a 10.5" behind a Duramax that puts 938 lb-ft and over 500 hp to the wheels and frequently tows 10k or more.  If you break it, you're a very special person.  

In reply to Curtis73 (Forum Supporter) :

Wow thank you, this is the thorough response I was looking for! I wish it was as simple as just wanting discs, but apparently the axles is toast. I really need to get under there and ID it and assess it before I go further though. Apparently the wheel bearings were replaced and within 1000 miles the wheel was wobbling like it was gonna fall off again. If it a 14 bolt full floater then it might be worth saving. As far as the driveshaft goes, I already need to get a custom one made since it now has a 6l80e that has a flange instead of a slip yoke. So I figured if I am getting a new driveshaft, then I can go ahead and throw whatever in the back. 

1988RedT2 said:

Huh.  Disc brakes?  I thought some cars still had drum rear brakes in the 90's.  I figured most trucks did.

Chevy mostly kept using drums on a lot of trucks, especially vans, because they are much lower maintenance, which the fleets like.

The went to disks on the pickups and that lasted about five-six years before they went back to drums.  What was interesting is they used non-servo brakes with the giant horseshoe monospring, like the fullsize FWD chassis used for years.

I have to say, I'm a bit flustered by th findings. It was  bit tough to count the top bolts because of the e brake cables, but I am pretty sure it is a 10 bolt. It also is 5 lug. shich is a bit tricky, because I doubt I am getting any newer disc axles that are 5 lug. 

Dunno what to do. 

Yup, looks like an 8.5" 10 bolt.  If the cover was almost perfectly round, that's what it is.

That is such a strange discovery since it is a 3/4.

The good news is that (since you mentioned your custom driveshaft anyway) the sky is the limit.  The 8.5" will handle a fair amount of power, or a fair amount of trailer, but not both at the same time.  Lots of guys put 500+ hp through them on sticky tires and they hold up, but the long, torquey dragging of a heavy trailer can't take tons of oomph at the same time.  They are also slightly limited in how much axle weight they can carry.  Adequate, but nothing to write home about.

There are plenty of truck 12-bolts out there with the 5-lug pattern, and they're cheap, but there is a reason.  The pinion shaft on the truck 12-bolt uses the same as the 10-bolt, so they're not quite as beefy as a car 12-bolt.  Still, it would solve the problem of the 8.5"... you would be able to give it plenty of twist, and plenty of trailer, and the bigger axle bearings would carry more weight.

Once you get to the 14-bolt, they are all 6 and 8 lug to the best of my knowledge.  The brakes and axle tubes are very different so I can't imagine any parts-bin swaps to get back to 5-bolt.

If you're willing to go 6 or 8 lug, start shopping.  If you find one of the 14bFF with the gov-lock differential, use the gov-lock's reputation as being terrible to negotiate the price down and then walk away the world's biggest winner.  Gov-locks got a bad name because folks would stuff  a truck 12-bolt under their 454-powered drag car, then wonder why they blew up when one slick got a little more traction than the other.  For street, towing, and truck stuff, a gov-lock is a really sweet piece.  I have pulled apart 14-bolt gov-locks with over 400k miles and the clutches look like new.  Gov locks (in case you didn't know) work like an open diff until one wheel spins X amount faster than the other, then a little counterweight and clockspring engages the clutches.  On 99% of the vehicles out there, that is a wonderful and reliable thing.  In a 750-hp with slicks, they try to launch (a time when it's open), they mash the pedal and one tire breaks traction and spins to the moon before it gets to lock.  Then it locks and big explosions happen.  Instant bad reputation for something that was just a poor choice for the application.

Alternate option:

8.5"s are pretty simple to do a disc brake swap.  Your bolt pattern is 5x5 which limits your options a bit, but all the HD B-bodies had rear discs after 93 or 94 with the correct pattern.

Think Roadmaster, Impala SS, Caprice, Custom Cruiser.  Scavenge brake parts.

Oh, when you get to proper proportioning, hit me up.  Swapping to rear discs without changing the brake proportion is a no-no.  Discs take way more pressure than drums to offer the same braking, so just swapping to discs alone will throw about 90% of your braking to the front when you actually want closer to 70%.  If your prop valve is one of the aluminum tetris-looking things under the master, it's pretty simple.  If not, we'll have to get creative.

Looks like a 1995 or older van there. Its a 2500 series upfitter model by the looks of it. These were basicly 1/2 tons with a small spring upgrade hence the 5 lug wheels. The "heavy' 3/4 got 6 or 8 lugs. You may be able to swap the front spindles and brake assemblies to match an upgraded  rear axle if you get a 6 lug. Check your ball joints for  interchange etc.

I agree the gov lock got a bad rap. Its meant for crawling out of a mud hole or climbing a snowy hill, not drag racing and drifting.

Curtis73 (Forum Supporter) said:

Alternate option:

8.5"s are pretty simple to do a disc brake swap.  Your bolt pattern is 5x5 which limits your options a bit, but all the HD B-bodies had rear discs after 93 or 94 with the correct pattern.

Think Roadmaster, Impala SS, Caprice, Custom Cruiser.  Scavenge brake parts.

Oh, when you get to proper proportioning, hit me up.  Swapping to rear discs without changing the brake proportion is a no-no.  Discs take way more pressure than drums to offer the same braking, so just swapping to discs alone will throw about 90% of your braking to the front when you actually want closer to 70%.  If your prop valve is one of the aluminum tetris-looking things under the master, it's pretty simple.  If not, we'll have to get creative.

I used a custom bracket, 93 LT1 camaro calipers and Imp SS rotors for the 12-bolt in my C10. That gives you an ebrake and off the shelf parts. 

http://www.67-72chevytrucks.com/vboard/showthread.php?t=382633

This bracket, 93-97 LT1 camaro calipers and Impala SS rotors. All off the shelf parts available from any parts store for future replacements. 

In reply to Curtis73 (Forum Supporter) :

Definitely tons of options considering it will mostly be custom anyway. Is the 'gov lock' the G80 locker? I was reading about those yesterday, they seem good for the application. 

I was considering just rebuilding the 10 bolt considering its probably strong enough, and leaving the bolt pattern would be easier, but the lack of locker is a bummer. It definitely seems like an easier solution to just get a junkyard axle that already houses a G80 and figure out the kinks from there. 

In reply to Gearheadotaku (Forum Supporter) :

That is super interesting. Sounds like the old version of the mid 2000's 1500hd vs 2500.

6 lug front swap was my next question. I am wondering what donor would house the best 6 lug swap. I also am wondering if its worth getting a whole spindle assembly, or trying to find a rotor/hub assembly that will work for the application. Like this: https://www.summitracing.com/parts/clp-cp4r-6?seid=srese1&gclid=Cj0KCQjwoaz3BRDnARIsAF1RfLe5OjTHmZoH2LlIx4i6OJgPLxO9hz3UqJuWeSGf7Zth1HpUD_DADEwaAiWrEALw_wcB

In reply to bobzilla :

I think that is a similar thing to some kits that have seen onine. I think summit has one for about $350 that includes calipers with built in e brake, rotors, and brackets. Definitely the easy button if I keep the 10 bolt. 

In reply to mgfoster :

I've done some research into putting a gov-bomb in my Sierra, if we can find a junkyard trucc with a G80 with the same ratio we can just wholesale swap the entire diff over with a little shimming.  Otherwise, I'm pretty sure we can just grab the locker if we don't find one with the same ratio.  They're a little expensive at pap but if we go to south we can probably trick them and tell them it's something cheaper since they don't know their ass from a hole in the ground.  We can do the Sierra at the same time since I've been planning it for a while.  They're in upwards 50% of trucks that I've seen there.  I think that's probably the best choice for this one unless you want to do it less sloppy...

What's the van ratio? I think mine is a GU6 (3.42) which is pretty common, it's in my Blazer and in my Sierra and it's the same axle in both except different widths and brake styles on both.

In reply to young_boomer :

Yeah that's not a bad idea. Shimming scares me, but if you know what you're doing even a little bit I'm sure we could figure it out. I'm still not convinced that just grabbing the whole axle wouldn't work better though. I measured last night, it's 50.25" leaf spring center to center. The axle in my dads truck (same as your sierra probably except it has a locker) is the same perch distance. Feels like getting the whole shabang is easier and knowing PAP pricing, probably not much more. But as I'm writing this I'm remembering the 5 lug dilemma... hmmm 

In reply to mgfoster :

So the 5-lug issue isn't a big deal. If the axle spline count is the same and the axle tube/pumpkin is the same, just reuse your old axles. Pop the cover, pull the c-clps, and away you go.

In reply to bobzilla :

Good idea. Then I could redrill rotors from the new axle, or find 5 lug rotors of the same dimensions.