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Old 05-23-2020, 04:18 PM   #1
fishmunger
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Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

I got this 1972 K10, 350, auto, currently has original axles and 4.56 gears (open) front and rear. It's lifted 4" and has 35" tires on 15x10" 6 lug mags. Other than that, it's a pretty original truck, and I'd kinda like to keep it just the way it is. I like the vintage 6 lug mags in came with and want to keep them.

I'm fixing the truck up to be a daily driver/work truck. I'd be towing a 3500# boat around pretty regularly, and occasionally a 6000# boat. I'd also like it to be a fun off road trail truck, but not for crazy rock crawling. I 'plan' on keeping all 4 wheels on the ground and trying not to smash up the body.

I definitely want to have those detroit truetracs installed front and rear, and possibly change the 4.56s to 4.10s to get better mileage.

In my mind, the path of least resistance is to have both axles rebuilt at a shop when I have the truetracs and/or gears installed. I'd probably upgrade to aftermarket beefier 12 bolt axles at this time. This would also keep the truck original, which is maybe a good thing value-wise (and the fact that it's held up for almost 50 years this way)?

But it seems that the consensus on the internet is that putting money into a 12 bolt is stupid. So I've read a lot about swapping to a 14 bolt semi float or full float too. Sounds like the semi-float is the way to go if I want to keep my 6 lug wheels. If I went this route, I'd probably find a 14 bolt SF and then have the same shop rebuilt it and move the perches or whatever is necessary to make it fit.

I plan on leaving the front axle alone (other than rebuilding, gear swap, trutrac).

Concerns are:

Is the 14 bolt semi float 'that much stronger' than a rebuilt 12 bolt with upgraded axles? Is it necessary for my needs? Do 12 bolts break that easily?

Other than moving perches/shock mounts, am I going to run into other problems swapping the semi float in place of the 12 bolt? Since I don't plan on doing the work myself (can't weld), complications add $$. I worry about things like the E-brake, driveshafts, u-joints, etc adding up in a hurry.

Will my 15x10" wheels fit over the drums on a 14 bolt?

It sounds like the 14 bolt SF came in 2 widths? What would be ideal?

Alternatively, if I went with a FF axle, could I just run wheel adapters to fit my 6 lug rims and be on my way?

Appreciate all your opinions guys, thanks in advance!
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Old 05-23-2020, 04:55 PM   #2
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

Given your plans, I’d just stick with the 12 bolt. With a set of axles and gears it should be plenty strong as long you don’t drive like an idiot. I know my dad had a 71 K10 with 38’s and stock gears in the 90’s that never had a problem with the rear end. He mudded that truck from time to time with no issue. They are plenty strong for what they are. I believe the kiss principle applies here... keep it simple stupid! Both 14 bolts are stronger but I doubt you’ll need the extra strength vs. the extra pieces of the puzzle you’ll have to put together to make either rear work. If you were going to swap the front axle than I’d say 14 bolt ff/3/4 ton 44, ten bolt or even a 60. Given you don’t want to swap the front keep the 12 bolt.
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Old 05-23-2020, 09:21 PM   #3
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

id rebuild the 12 bolt, but I'd keep the 4.56s if you plan on towing.
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Old 05-25-2020, 10:31 PM   #4
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

Thanks for your guys advice. Nice to know that your old man didn't blow up his 12 bolt. If you get on pirate4x4 and read about 12 bolts, everyone makes it sound like as soon as you get 33" tires you're 12 bolt will grenade instantaneously.

b454rat, you think the lower gears are just going to make everything easier on the drivetrain?
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Old 05-25-2020, 11:11 PM   #5
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

I agree the 4.56 gears will definitely help you with towing. It helps take a lot of load off of the whole drive train by making the wheels easier to turn. A big part of making axles and drivetrains in general last is how they are driven. Spinning tires don’t break parts it’s when the spinning tires catch traction that problems result. If you use common sense when your driving and don’t let it wheel hop or catch hard traction in slick conditions you will last a long time. If you pin the throttle on ice and then hook it up on asphalt... well stuff is probably gonna grenade... like i said I wouldn’t hesitate to run a 12 bolt with what your trying to do. I’ve got one in my 3100 with 35’s and 3.07’s and it has been no issue at all.
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Old 05-26-2020, 12:22 AM   #6
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

For what you are doing a 12bolt should work. Definitely go for stronger axle shafts for it as that would be your weak link, even more so with a tru-trac.

I've busted the original 12b in my old '75 K5 just driving down the street around the corner from my house though. Nothing screwy other than it was like 4 degrees outside and everything was super cold heading out for a snow run. I never took it apart but by the size of the exit wound on the cover it spit a tooth off the pinon or a spider gear. I wheeled it hard for a few years (open, with 35's) and never had a problem before. I can only chaulk it up to 40 year old original parts that had probably been stressed to the limit and the extreme cold finally fatiuged something to the point of failure.
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Old 05-28-2020, 12:49 PM   #7
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

Thanks for weighing in guys. you've got me feeling better about it.

Any recommendations for a good brand to go with for the upgraded axle shafts?

I was looking at 12 bolts parts, I was thinking of perhaps also getting upgraded main caps and bolts. And i guess if i really wanted to go whole hog I could get one of those covers that has bolts that preload against the main caps. My research suggested most of the 12 bolts problems is that you get a bit of flex or movement on the caps and then the gears get a little out of wack and pow...?
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Old 05-28-2020, 05:23 PM   #8
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

I think the caps are designed for drag cars, where the instant shock of acceleration will move the cover, but just driving and towing heavy loads won't do much. Offroaddesign.com carries all axle stuff.
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Old 05-28-2020, 11:10 PM   #9
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

I towed a bunch of boats with my 72 k5. IIRC, it had a 12 bolt in it and other than shaking loose a ring gear bolt while I was camping out in the cuts, it towed just fine. I was also towing small 2K lb hot rod jet boats.

My '63 K15 had a Dana 44 rear axle (I think) which I swapped out to a Dana 60 - 8 lug rear with 4.10s. At the time I would tow a smallish 4,500 lb comp ski boat and it did fine but had better braking than the smaller axle.

The last incarnation involved installing a 14bff with factory discs, 4.11 and the Gov Lok. It has been in since 2017 or so and has done fine although I think that the Dana 60 might have had better braking. Maybe I just need hydro boost?

The boat that I pull now is around 5,500 lbs before the wife and kids throw all of the stuff in it (extra 1k lbs.)and no fuel. Usually have around 1,500 in the bed as well. Usual tow is 3-4 hrs through the valley and up into the foothills.

In my humble opinion, for braking alone, I wouldn't go back to a 12 bolt or a Dana 44 (I wouldn't even consider a 10b) but really think that the 14b with discs might have been a little over rated (for the brakes). Maybe drums would have been better? Maybe user error on my part in the set up. The 60 with the big drums was a real nice axle that started its life of abuse long before i got it and was abused as long as I had it. I tortured that thing with some of the loads it hauled. Never a peep from it and it always stopped real well.

Anyone know what size brakes are on the 12b vs the 14b SF?

The older I get, my tolerance for not having good binders while towing has waned a bit. Ive puckered plenty of bench seats into needing new upholstery .

Whatever youre gonna run, get the biggest brakes offered on them and keep them in tip top shape. Or get a real good set of rip stop seat covers and a kick arse upholsterer buddy! .
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Old 05-29-2020, 01:48 PM   #10
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

Quote:
if I went with a FF axle, could I just run wheel adapters to fit my 6 lug rims and be on my way?
Just convert everything to Full Float and 8 lug wheels and then you can run real E-range truck tires

I've had nothing but trouble with 15" wheels and radial LT tires on half tons. The old 700-15 bias ply did hold up pretty good.
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Old 05-29-2020, 10:12 PM   #11
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

I would do the 6 lug 14 bolt. And yes, it is that much stronger. The 12 bolt is rated 3300# GAWR and the 9.5" semi-float is rated at 5500#, equal to the Dana 60 in many ways and superior than 30 spline versions.

The 33 spline axles in the 9.5" sf are massive compared to the 12 bolt. I have run a semi-float 14 8 lugger in my Jimmy for the last 15 years. In the previous 15, I went through 4 or 5 12 bolts.

Before the truck was mine it was my dad's. And he hot rodded NOTHING. Drove it like the little old lady. And the 12 bolt broke. And so did the junkyard replacement installed circa 1981. Then when I started beating on it, it seemed to come apart all the time.

Mine has the Gov-Lok in the back still and it actually works OK for boat ramp type exercises that was the primary use for mine when it was a daily. But it's no fun for spinning tires in the dirt or whipping a donut in the snow. I recently bought a Yukon LSD that will replace it at some point. It's on the list...
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Old 06-02-2020, 09:29 PM   #12
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

The 14 bolt sf is the perfect rear for a half ton 4x4 and the drums are the same diameter as 12 bolt. I ran rallies on the one I had in my 72. The wheel bearings half nearly twice the surface area and 50% larger dia. The pinion stem on a 12 bolt truck is the same as the old 8.2 car 10 bolt 1.48 dia (weaker than 8.5 10 bolt 1.63) 14 bolt SF is 1.88 with massive bearings. They are 4 inches wider so the rear track will be the same as the front. They are cheap and plentiful, most are 4.10 or 3.73 geared many have Gov lock diffs which are much stronger than the infamous govbombs , later versions have better pinion seals. They have 1350 u joints but you will need to have your driveline shortened an inch. The breaks are similar and if you trade the wheel cylinders for the ones from your 12 bolt the original break lines will screw right in as well as the e break cables
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Old 06-02-2020, 10:07 PM   #13
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Re: Rebuild 1972 K10 12 bolt or swap in 14 bolt Semi Float or Full Float

Good mention about driveline length. On a lifted truck the stock driveshaft with a conversion shaft should work.

On both trucks I have done this on, we redrilled the perch to move the axle back 1" which makes the wheel much better centered on the truck. Another feather in the cap for the SF 14.
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