Rear end question

[Stepper86]

Looking for some advice

I was offered a 12 bolt rear end with 3.42 gear from an 1988 GMC 1500 today, the price is not bad and the original 10-bolt in my 86 stepside is a bit noisy, so I think I will have to do something about it soon.
Hopefully someone can help me with a feve questions.
How wide is the 88 axle compared to my 86, and is there anything else except from possibly the brackets that I would need to modify?

I all so have a lead on a 94 Camaro rearend complete with diskbrakes and diff, but from what I can understand that axel is too wide for the truck and would need to be shortend, and then there is the different bolt pattern.
So to me it sems like a whole lot of fabrication to get it under the truck.

What do you guys think would be my best option?

[Rich84]

1988's didn't come with 12 bolts..

[Stepper86]

So 1988 is 10-bolt? I just went with what the seller told me, I have not seen it yet, Im supposed to go and have a look at it tomorrow.

[Ski-me]

Originally Posted by Stepper86

So 1988 is 10-bolt? I just went with what the seller told me, I have not seen it yet, Im supposed to go and have a look at it tomorrow.

Yea, most that era are 10 Bolts. This would be a new body style axle so the spring perches probably won't match up to your older rig either. You would need to remove the perches and shock tabs and install new ones.

(Going from my 89' K5 4x4 blazer research on 14 SF's.)

Wheel mounting surface to Wheel mounting surface (WMS) is about 64" on my K5 (standard 10 Bolt).

Many that I found were 67" WMS to WMS so they would be wider....Just depends on what you want.

I don't know if there are different widths in the 10 bolt but at least this will give you a starting point if you go and look at it.

Also, check the bolt pattern. Aren't 2wd trucks 5 lug vs. 4wd trucks 6 lug??

[Stepper86]

I had a look at it this morning, 5 lug pattern , no diff 3.42 gears 67"
Looks okey but with no diff and slightly wider I might as well just get new bearings and restore the original axle.
Thanks for the help guys.

[Rich84]

Get a 30 spline posi carrier and and thirty spline axles..That makes a good strong 10 bolt.

[BigBlocksRule]

The only one worth swapping in, IMHO, is one of the 14-bolt, 6-lug semi-floaters from the '88 & up heavy half Z-71's. Huge honkin' axles, 9.5" ring gear and a factory gov-lock that actually works. You'll have to have spring and shock mounts moved, but it'll be bullet proof. I paid $40 apiece to have the old lug holes welded up and new ones drilled for the 5-lug pattern.
Otherwise, rebuild your 10-bolt.

[Ski-me]

Originally Posted by BigBlocksRule

The only one worth swapping in, IMHO, is one of the 14-bolt, 6-lug semi-floaters from the '88 & up heavy half Z-71's. Huge honkin' axles, 9.5" ring gear and a factory gov-lock that actually works. You'll have to have spring and shock mounts moved, but it'll be bullet proof. I paid $40 apiece to have the old lug holes welded up and new ones drilled for the 5-lug pattern.
Otherwise, rebuild your 10-bolt.

I'm doing this route as well. I got the 14 SF from the junkyard for $100 and am cleaning it up and then welding on new perches. A good way to go if you can. I also am using the more narrow, and harder to find, 64" WMS width.

Now, if I could only learn to weld.....

If you go the 10 bolt route, I wouldn't expect to pay anything over $100 for it. There are a lot out there. I think the spline count mentioned above can be found on the 89-91 square body trucks.

[Firebirdjones]

Originally Posted by Rich84

Get a 30 spline posi carrier and and thirty spline axles..That makes a good strong 10 bolt.

Yep, 30 spline became factory in the 8.5" 10 bolts around the 88 model year.

Once this is done, the 8.5" 10 bolt becomes a better piece than the factory truck 12 bolts.

The truck 12 bolts have a very small 1.438" pinion diameter that isn't all that difficult to snap.

The 8.5" 10 bolts have the more common 1.625" pinion that you see in alot of the other heavy duty rears like the passenger car 12 bolts, dana 60's, ford 8.8's.

Why GM chose the smaller pinion for the truck 12 bolts is strange in my opinion.

[Firebirdjones]

Originally Posted by BigBlocksRule

The only one worth swapping in, IMHO, is one of the 14-bolt, 6-lug semi-floaters from the '88 & up heavy half Z-71's. Huge honkin' axles, 9.5" ring gear and a factory gov-lock that actually works. You'll have to have spring and shock mounts moved, but it'll be bullet proof. I paid $40 apiece to have the old lug holes welded up and new ones drilled for the 5-lug pattern.
Otherwise, rebuild your 10-bolt.

I like the 14 bolts (I have the full floater in my 79 1 ton) they are actually a 10.5" ring gear and nearly bullet proof.

But they pose some issues that I didn't want to deal with when I swapped the 12 bolt in my 72 blazer for yet another 12 bolt.

I am somewhat of a weight savings nut (racing backround ) and I didn't want that big honker under my daily driver robbing HP.

I also didn't want those massive brake drums in there. I know on the 14 bolt in my 79 it won't fit a small wheel (anything less than 16" won't fit) so that also made it a "no go" for my blazer as the 15's have to stay. I also wasn't interested in having another vehicle that takes $130 brake shoes and a specialty shop to turn the drums
I also didn't want to deal with 8 lug axles on the blazer.

Otherwise I like the FF 14 bolts. Maybe these draw backs aren't apparent in the semi float 14 bolts with 6 lug axles? Smaller brakes that aren't an issue with 15" wheels? I wouldn't know as I never see the semi floaters around here.

[Firebirdjones]

Ya know what we really need is a good "sticky" that lists all the truck rears with their width, dimensions, and adaptability issues between the years.

I've been told there are some 14 bolts out there that are the proper width for things like blazers but applications and years vary.

[BigBlocksRule]

The semi-floater is a lot lighter than a full floater. I have a couple of the narrow ones for 454SS clones and finally found one of the wider ones off of a 4-wheel drive Z71. GM used a narrow 10-bolt in Tahoes/Blazers that makes the rearend 3" narrower than the front. The 4WD 14-bolt SF is the same width as the front end of a Tahoe and will make it track straight. It's a true bolt-in, 6-lug strong as hell swap. You can run a disc setup on the narrow ones and have the axles shortened and re-cut for C-clips and cut a smooth 3 1/2" off of the rearend width. Almost 2" of extra wheel/tire on each side make it a worthwhile swap, IMHO.

[Firebirdjones]

Originally Posted by BigBlocksRule

The semi-floater is a lot lighter than a full floater. I have a couple of the narrow ones for 454SS clones and finally found one of the wider ones off of a 4-wheel drive Z71. GM used a narrow 10-bolt in Tahoes/Blazers that makes the rearend 3" narrower than the front. The 4WD 14-bolt SF is the same width as the front end of a Tahoe and will make it track straight. It's a true bolt-in, 6-lug strong as hell swap. You can run a disc setup on the narrow ones and have the axles shortened and re-cut for C-clips and cut a smooth 3 1/2" off of the rearend width. Almost 2" of extra wheel/tire on each side make it a worthwhile swap, IMHO.

Semi float applications would help,,,,where to find and which ones are narrow and wide? How wide exactly are the narrow versions? I'd like to compare that to my 72 blazer 12 bolt.

I'd prefer to keep the drum rear, but I'm uncertain of drum sizes on the semi float applications. May not accept a 15" wheel?

[Stepper86]

Wow I never expected all this usefull info when I started this thread, the collected knowlege on this forum is outstanding.
Now Im tempted to go and pick up a 1989 4x4 Z71 stepside that has been advertised localy for a while and pick it for parts. Only problem is, knowing my self I will probably want to keep it once I bring it home

I secund the idea of a sticky with rear end specs, it would be a very usefull source to have at hand.

[Firebirdjones]

Sorry for blowing your thread into a large rearend discussion, you just really got me thinking about it. I hope you don't mind.

I actually started my own thread about which rear fits what over in the 69-72 blazer section because we had discussed it in detail a few months back so I'm hoping those guys will pitch in, and maybe get this sorted out and make a list for a sticky.