Posted By: Grigg 14 bolt axle swap info - Fri Jan 05 2018 05:00 PM
Work in progress, what questions and ideas do you have, what is missing?

Purpose of this thread is to explain how to perform a 14 bolt rear axle swap, not skipping over the tricky parts, and including part numbers wherever possible.
Specifically a 14 bolt rear axle for use in 48-54 GM 3/4 and 1 tons. Also good for other years, but only discussing the AD trucks here and now.
There are already various scattered and watered down details and info in this forum and others about the 14 bolt swap.

Some other threads and resources:
14 bolt Wikipedia
Service manual:
Gear swap/setup instructions:

New parts sources:

The 14 bolt was introduced about 1973 and used through about 2000 in countless 3/4 and 1 tron GM products. It replaced the HO72 Eaton axles in these applications, and has some similarities. The AAM axles that followed the 14 bolt are also similar.
Available gear ratios are 3.21, 3.42, 3.73, 4.10, 4.56, 4.88, 5.13, and 5.38
14 bolts in trucks up through about 1987 used 9/16" wheel studs and thereafter used metric, vans switched to metric in 1996. The earlier axles are probably more desirable for our purposes. To identify the 1989 and newer axle housing look for lots of cast in fins on top, the older ones lack fins.

The Eaton axle used in 3/4 and 1 ton GM trucks 48-54 measures 62.625" from Wheel Mounting Surface to Wheel Mounting Surface. Same for single and dual wheel trucks.
The 3/4 and 1 ton AD truck rear springs are 41.5" center to center.

The 14 bolt comes in a few different housing widths, hub types, and brake sizes and type.
Primary concern for this swap is the 14 bolt axle installed in "Cab & Chassis" trucks, which could be a pickup cab with a service body, an RV, or a step van (and other similar applications). This axle would have come with "dual wheels" and large drum brakes.
The WMS to WMS measurement is 62.75", It's basically a very narrow dual wheel axle, narrower than any other version of 14 bolt therefore a good fit for our old narrow trucks. Remember AD 1 tons even with dual wheels use same axle width as the single wheel trucks. The C&C 14 bolt axle can be run with single wheels or dual wheels.
In other words the 14 bolt C&C axle is perfect for any AD 3/4 or 1 ton.
Spring centers are 40" + or - 1/8" on 14 bolt C&C axles I've measured.

Tangent discussion the 14 bolt axle under a single wheel pickup has same housing width and axle shaft lengths as the C&C axle, but has different hubs making for a wider and unsuitable WMS width. The "dual wheel type hubs" can be swapped to the single wheel pickup axle housing, making it again ~63" WMS to WMS measurement, but brake backing plates don't bolt on in the right place... The spring center measurements on the single wheel pickup axles are 40.5" for the 1 ton, and 42.5" for the 3/4 ton. Further tangent a dual wheel axle from under a pickup bed is just way too wide. Van axles are also different... or kind of the same... hard to find reliable info.

--The C&C axle as it comes needs spring pads moved outward a little to match spring center measurement on your old truck.
--Clearance between springs and the brakes leaves little room for u-bolts, and nearly no room for bleeding brakes.
Following post will address these issues and solutions

Tangent/caution Many places (catering to the 4x4 truck crowd) sell "disc brake brackets" to convert a 14 bolt to disc brakes, either bolt or weld on, and if using the right (expensive) caliper you can even have parking brake too. The components used are usually from a 1/2 ton or 3/4 ton truck, they're not very robust in comparison to the later model 3/4 and 1 ton disc brakes. I have not used this type of disc conversion for the rear of a 14 bolt, but it is my opinion that it's a step backwards in safety and braking ability in comparison to the 13x3.5" drum brakes. I will not be covering this possible brake option but will cover a different disc brake swap.
Posted By: Grigg Re: 14 bolt axle swap info - Sun Jan 07 2018 05:14 PM
The C&C axle as it comes needs spring pads moved outward a little to match spring center measurement on your old truck.
1948-54 3/4 and 1 ton spring centers measure 41.5" (1.5 and 2 ton with 2.5" springs measure 42")
14 bolt Cab & Chassis spring centers are nominally 40"

The spring pads/perches on the 14 bolt are also for wider springs than the 2" springs the AD 3/4 and 1 ton trucks use.
So may as well remove and replace the spring perches or pads on the 14 bolt axle. Except they're kind of not available for 2" springs and to fit 14 bolt axle tubes.

One option is to harvest the pads off the old Eaton axle, but the 14 bolt has a 3-3/8" axle tube, a little larger than the Eaton, the pads would still need a little grinding/sanding to fit the tube diameter.

I chose to buy new aftermarket fabricated steel pads for the 2" wide springs and still modify the radius to fit the larger 14 bolt axle tube diameter, I used part number 2-Perch from Ruffstuff
Picture attached of what needs machining or grinding to fit.
These spring perches also have alternate holes in them for the spring center bolt, so you can move the whole axle a little back to center tires in the fenders, and or to make the new axle work with the original length driveshaft.

New U-bolts should always be used, they're usually made to match your original (or new length) while you wait at a truck and trailer parts place and or suspension shops, ask around.

You can reuse the original U-bolt saddle under the axle, like the perches it'll need the radius adjusted a little to fit new larger axle tubes.
Picture album of that process, not difficult.

Alternately I've found that the u-bolt saddles from an early 60's Ford 1 ton (probably 3/4 ton too) is an even better match and takes little if any work to fit the axle tube. These are cast with Ford part number C1TT5796Z
See attached photo of each with dimensions.

Before welding your spring perches to the axle now is the time to carefully consider and set or correct driveline angles, specifically pinion angle.
First read the manuals in this Driveline Set Up tech Tip Remember driveline angles should be checked with the vehicle at it's normal weight.
For reference (check yours) I measure the original spring perches on my 51 GMC 1 ton are level when the pinion is tipped up 4.2° from level. This may or may not be what yours was or needs to be, please READ the Tech Tip!

Attached picture IMG_0749.JPG
Attached picture IMG_0679.JPG
Posted By: Grigg Re: 14 bolt axle swap info - Sun Jan 07 2018 05:15 PM
Having now moved the spring perches to fit the wider spring spacing on an AD truck...
The clearance between springs and the brakes leaves very little room for u-bolts, and approximately zero room for bleeding brakes.

The original brakes to the 14 bolt C&C axle are 13x3.5" (few exceptions may exist) and have a deep offset drum, remember these axles came with dual wheels on them.

Bendix # PDR0619, GM # 18006042, NAPA # 440-1651, is a drum with same 13x3.5" brake size, bolt pattern, yet with 1.25" less offset. Photo attached of the two side by side. This drum comes on many 1 ton pickups (both 2wd and 4wd), vans, and step vans from 1974-1999.
The shallower drum works by spacing out or relocating the backing plate flange on the axle tube 1.25" closer to the wheel so the original backing plate and all brake hardware fits in the shallower drum. Now plenty of room between backing plate and springs for bleeding brakes.

This drum fits great in single wheels and also fits real nice in the original 18" dual wheels (photo attached), and also plenty of room for 19.5" dual wheels. Probably won't fit 16" dual wheels.

Couple ways to move the backing plate:
--Machine 1.25" thick spacers with both pilots and counterbores, see drawing attached. (thumbnail is real faint)
--Reuse, purchase, or make new square flanges to weld onto the axle tubes but 1.25" closer to the wheels.
--DO NOT use washers or pieces of pipe as spacers, not safe and strong. (see thumbnail below)

The new backing plate flange should be 4-11/16" from back side of hub where the drum mounts. Photo attached of original location for the deep drums, at 5-15/16".

Attached picture 14 bolt 13x3.5 drums.JPG
Attached picture 14 bolt C&C backing plate 5.9375.JPG
Attached picture backing plate spacer, not a good idea.png
Attached picture 14 bolt backing plate spacer.jpg
Attached picture shallow 13x3.5 drum, 18 wheel.JPG
Posted By: Grigg Re: 14 bolt axle swap info - Sun Jan 07 2018 05:15 PM
The 14 bolt (and original Eaton) uses a 2"-16 thread spindle nut.
like the old Eaton axle they usually have a flat washer and two nuts with a tab locking washer in between. This works OK, but can be aggravating to adjust and lock and unlock the tabs, the washer is also not forever reusable, the tabs will eventually break.

A very nice upgrade as comes on some later model 14 bolts and or AAM axles is a single nut with a key and spring clip to retain it.

I found the OE parts very reasonably priced from
AAM part numbers:
Nut 341509
Key 40082234
Clip 341511

Alternate Dorman part numbers, available at many auto parts stores (possibly more expensive than the OE parts)
Nut 615-132
Key 615-140
Clip 615-141

Just for info:
Dorman part numbers for the old fashion washer, nut, lock tab, and nut solution.
Flat washer 618-048
Spindle nut 615-130 (need two per side) also known as Dana # 33732
Spindle lock washer 618-049

Other axles that use same spindle thread and may benefit, Our original Eaton HO52 and HO72, many Dana 70 rear axles, and Dana 50 and 60 front axles.

Attached picture IMG_0710.JPG
Attached picture IMG_0699.JPG
Posted By: Grigg Re: 14 bolt axle swap info - Sun Jan 07 2018 05:16 PM
NOT NECESSARY if drum brakes are used as above, but another option and upgrade is adapting modern disc brakes (with parking inside) to the 14 bolt axle.
See this Previous thread working up to the following:

13" rear disc brakes with internal drum parking brakes from a ~2007 GM single wheel AAM 11.5" axle, (3/4 ton pickup). These brake parts are also used on many years and models of GM 3/4 and 1 ton trucks, vans, and SUV's roughly from 1999-2017.
I've checked fit of these brakes adapted to a 14 bolt hub/axle in 18" dual wheels and 19.5" dual and single wheels, no issues. These brakes on a 14 bolt should fit easily in the wheels you have on your AD 3/4 or 1 ton, but it'd be smart to test first. (Dual wheels however will not fit the modern AAM axle originally fit with these brakes, explanation below)

Caliper brackets, same left and right side, GM part number 15949892 or 88965717 come complete with dust shields, studs, and parking brake lever boot.

You'll need a flange to weld on the axle tube with the appropriate bolt pattern for the new caliper bracket.
It's an odd mounting pattern, drawing attached, center bore needs to be (likely) larger to suit your axle tube. I had flanges waterjet cut to these dimensions and will bore the last little bit to suit, and turn the pilot.

The flanges are welded to the axle tipped at seemingly odd angles, the following is how they were on my donor axle. This assumes, but have not confirmed, that the spring pads on the donor were close to level with the truck in normal operation (which is the case for an AD 1 ton). The idea/result is caliper bleeding screws are on top and also provides convenient routing for parking brake cables. It's not real critical and should be close enough to duplicate these angles on your new axle.
With spring pads level and while viewing the caliper bracket mounting flange on axle:
--View from driver side the drivers side flange has the widely spaced holes up, and tipped clockwise, 22.0° from horizontal.
--View from passengers side the passengers side flange has widely spaced holes on bottom, and tipped clockwise, 8.0° from horizontal.
****(revision 5-30-22) The above as original for the new model truck has the clippers forward of the axle with parking brake cables pointing back then U-turn to go forward. I now believe it'd be best to put them on opposite so the parking brake cables go forward being simpler and better looking like the old truck brake cables were (other model trucks have calipers on the rear of the rear axle, so this is not odd or problematic). To do so set the pinion angle as needed and with the truck level orient the calipers so they're straight up and down, so the bleeder port is at the top. Not the bleeder screw, but the passage in the caliper where it intersects the piston bore.

GM # 15712803, Bendix # PRT5260, ACDelco # 177-861, or NAPA # 86828CR is the new 13" rotor with ~4.836" bore.
It does not fit the pilot on back side of 14 bolt hub but there is room to install a bushing on the hub to correct the fit and properly center the rotor.
After extensive searching no other possible rotors are a satisfactorily snug fit on the old 14 bolt hub, some have less slop, but are still not accurately piloted.

An adapter bushing is the proper way to go, duplicating the hub to rotor fit GM designed for this modern brake setup.
The bushing would be a shrink fit on the hub with a nominally 4.593" inside diameter.
Outside diameter ~4.830" aiming for 0.004-0.005" clearance in the rotor.
Approx 3/8" thick, or same as pilot projection on back of hub.

Finally the donor rotor originally fit over the AAM 11.5 hub just under the wheel, and we're going to install it behind the hub, held on by pressed in studs same as the old 14 bolt drum was fitted to the hub. This is why dual wheels don't fit over these brakes on the newer AAM axle but will fit over them on a 14 bolt hub/axle; the rotor and caliper are moved further from the wheel.
To make it work out right the backing plate mounting flange gets welded onto the axle tube 4.3375" from the inside rotor/drum mounting surface of the hub (not the wheel mounting surface, and not necessarily that precise, but if you're following the math that's the answer). See angles above for orientation of backing plate. For reference or curiosity, the donor axle with these brakes had the caliper bracket flange set in 3-15/16" from inside mounting surface of the rotor (outside of the hub mounting flange). To that dimension I've added AAM rotor flange thickness of 0.400" to arrive at 4.3375" in our case with rotor mounted inside of the hub, not slid over the hub.

Parts list:
Caliper bracket flange, have made, drawing attached
Centering bushing for rotor on hub, have made, dimension considerations above.
Rotor GM # 15712803, Bendix # PRT5260, ACDelco # 177-861, or NAPA # 86828CR
Caliper bracket with dust shield, studs, parking brake boot GM # 15949892 or 88965717 (difference in ABS sensor mounting or not, we don't care, buy the available or cheap ones)
Caliper with carrier and guide pins _______
Pads ______
Brake hoses ______
Brake hose clips ______
Brake hose bracket left ______
Brake hose bracket right ______
Bolts, brake hose brackets ______
Nuts, for caliper bracket studs ______
Parking brake shoes ______
Parking brake boot alone GM # 12473096
Parking brake lever GM # 88965732
Parking brake adjusters and hardware ______
Parking brake cables ______

Further details to come as I figure them out

Attached picture AAM 11.5 Brake flange pic.JPG
Attached picture AAM disc brake mounting flanges.JPG
Posted By: Grigg Re: 14 bolt axle swap info - Fri Jan 12 2018 09:38 PM
Info on connecting driveshaft.

**Don't forget to check and set pinion angle before welding on spring perches, Please use these instructions and manuals
That done you can now accurately make or reuse driveshaft.

The original AD 3/4 and 1 ton U-joint is a Spicer 1350 series. (1350 also used on 1.5 and 2 ton)
The 14 bolt axle always comes with 1350 series yoke (exceptions would be very few if any).

The 14 bolt yoke doesn't use U-bolts like the original Eaton yoke does.
Instead it uses straps and bolts. It's said that the aftermarket parts aren't quite right and you're better off getting the genuine GM parts, from GM, and they're not expensive.
Straps, GM # 3920486
Bolt, GM # 458300 (short) or 14018700 (long)

Or there are aftermarket U-bolt type yokes to fit the 14 bolt, either 1350 as original, or 1410 for a size larger Try this link

The 14 bolt I believe is a tad longer from yoke to center of axle, so driveshaft may need to be shortened.
Or, if you used aftermarket spring perches with multiple spring center bolt holes you can move the axle back some and driveshaft may fit just fine.
Moving axle back also makes wheels look better centered in the fenders on a pickup, unexpected bonus to picking the easy solution.

In any case confirm that the slip joint won't bottom out at any point during full rear suspension travel, and is not extended to far either.

I haven't gotten this far to iron out the details, but will update when I can.
Posted By: Grigg Re: 14 bolt axle swap info - Fri Jan 12 2018 09:39 PM
Still to come..
Some thoughts on re-gearing a 14 bolt, ideas and parts resources.
Service manual
Tech tip on gear swap/setup
Solid spacer, to replace crush sleeve (if you choose)
Pinion yoke holder tool, to remove or torque pinion nut
Reusable cover gasket
Posted By: Grigg Re: 14 bolt axle swap info - Fri Jan 12 2018 09:45 PM
More info for later,
What's missing already?
Posted By: 95xlc2 Re: 14 bolt axle swap info - Sat Apr 27 2019 06:17 PM
I am doing a 14 bolt CnC conversion on my 55 GMC 250 8 dually. I have been doing research on remote brake and release bearing bleeders.

I first came upon this Jaguar remote bleeder set up

I checked the 80s wheel cyl bleeder specs and they are m8 1.25. That led me to this banjo fitting,

The bleeder surrounding mounting surface is flat and wide enough to accept this fitting but it might not be square to the bleed hole or machined to a quality to make a good seal with the copper gasket for the banjo fitting. The Jag fitting uses a banjo bolt with the same shape as the bleeder.

Hydraulic fittings are not my specialty. So I would need the banjo fitting, the proper unknown brake line, the proper unknown bleeder receiver fitting, and a bleeder or speed bleeder.

Any suggestions?
Posted By: Grigg Re: 14 bolt axle swap info - Sun Apr 28 2019 10:56 AM
Before going to all that trouble see if you can possibly bleed the brakes with the normal bleeder. It’s tight but probably not impossible.

Or move the backing plate flanges and use the drums with less offset as explained above. This also looks better because the drum is not so far outside the wheel.
Posted By: SS396CV Re: 14 bolt axle swap info - Tue Jan 07 2020 05:55 PM
I am doing the 14 bolt conversion on my 1951 GMC 253-22, I acquired a 1976 C&C axle for $100. I am going to utilize the drum brake conversion to single wheel 13x3.5, I am in the process of having two spacers made for the backing plate. I found, through Summit, Currie spring pads, 2 inch width, 3 inch tube size, also having them machined to 3.375. The hardest part so far has been the removal of the old spring pads. The rear came with 3.73 gears but unfortunately water got inside and ruined the gears, so I am converting to 4.10. Also the spider gear cross snapped an end off and took out the carrier, so all new parts inside.

I will provide an update once I have the spacers, to see how everything is going to line up. The biggest problem I have with the HO72, are the brakes, the real reason for the 14 bolt swap, the rear is in really good shape otherwise.

My only concern about the 4.10 gears will be the loss of power from 5.13 gears but will like freeway speeds better. The current engine is a 248 bored .060 over.
Posted By: Grigg Re: 14 bolt axle swap info - Thu Jan 09 2020 03:47 PM
Sounds like a reasonable plan.
I doubt you feel a loss of "power" with the faster 4.10 gears. Actually I expect you will appreciate the improvement in how it drives on the highway so much you then wish you had the 3.73 gears you wanted to start with.

When you get a chance please start a new thread so we can follow along with your progress.
If you learn things that need incorporating in this thread then we'll add them here later.
I've noticed technical "how to" threads become less and less useful the longer they get.
Posted By: SS396CV Re: 14 bolt axle swap info - Thu Jan 09 2020 11:07 PM
Thanks Grigg, I will start a new thread. documenting with pictures. I received the spacers today and they fit perfectly, so well done on the drawing.
Posted By: 53 green 1- ton Re: 14 bolt axle swap info - Fri Jan 10 2020 01:50 AM
Wow Grigg!
Amazing research and engineering on your part! I have a headache just reading through it! The time and dedication you put into this well appreciated!
You never cease to amaze!
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