Some other Axle Tech Tips
Advance Design (1947.2 – 1955.1) 1-Ton Truck
Overview. In a nutshell, this is one of the easier "major" upgrades you can do to your 3/4 or 1-ton Advance Design truck to achieve more comfortable modern speeds. It's easily done in your home garage with a basic tool set. You don't even have to jack the truck up!
In fact, this upgrade works for any truck with an Eaton HO52 or HO72 axle (3/4 or 1-ton trucks, 1947 -1972!)
All we're doing here is disconnecting the driveshaft from the rear axle, undoing the eight axle flange bolts and sliding the axles out five inches or so, removing the rear cover from the axle (10 bolts) and then removing the 10 bolts that hold the center carrier section in place. Then, lift out the center section, replace it with the one you sourced from the donor truck, put everything back with two new gaskets and two new axle flange gaskets, and reattach the driveshaft. That's really all there is to it!
The hardest part about this task is finding the 1968-1972 donor center section...
SAFETY WARNING -- This upgrade provides a significant speed increase. PLEASE make sure your brakes and steering can handle that! Your truck was not actually designed for these speeds and a lot may have happened to the steering and brakes over the last 60 years. If you aren’t sure of these critical components, rebuild them before performing this upgrade.
News Flash! Did someone say brake upgrade?? Read on -- If you can upgrade the center carrier section, you can easily upgrade the brakes ... at the same time!! Read On!!
Sustained operation under load at high rpm’s is not recommended for non-full pressure oiled engines (i.e., the babbitted 216’s and pre-’54 235’s).
Although this tech Tip was written for the 1947 (Second Series) through 1955 (First Series) 3/4-ton (3600 series) trucks with the Eaton HO52* axle assembly (4.57:1 ratio) and 1-ton (3800 series) trucks with the Eaton HO72 rear axle (5.14:1 ratio), the swap works with *any* GM truck with the HO72 or HO52 axle (1947.2 through the end of the 1972 model year when the axle was discontinued). These two axles share the same center “Carrier” section which interchanges with the HO52 axles found installed in 1968-1972 3/4-ton trucks with automatic transmissions – the ones with the 4.10 ratio ring gears in the carrier section. The receiving truck keeps its complete axle housing and original axles -- just the center carrier section swaps out.
* NOTE: Spicer axle ring and pinion sets/center sections will NOT interchange with the Eaton axles -- make sure your ring and pinion set/center section is from an Eaton HO72 axle!
To start with, we had a mostly original and unrestored (but very good condition) 1949 1-ton pickup. It has a splasher (babitted) 235 engine that replaced the original 216 in the Spring of 1950 (engine is a 1949 manufacture, BTW). The tranny is the original SM420. The rear axle is the original HO72 Eaton axle with the 5.14 ratio. And the wheels are the 17-inch bias plys on 3-piece rims. The truck could cruise at 55 in stock form and sprint to 60. But it was at max RPM and screaming to do it. The easiest way to give this truck modern highway speeds, without altering the originality much, was to swap out the rear axle center section (“Third Member” or “Pumpkin”) for a later one with a better ratio for highway speeds.
Finding the correct Donor
The easiest truck to look for, with the identical HO52 Eaton axle containing the desired 4.10 ratio, is a 1968-1972 3/4-ton GM truck with the automatic transmission. The HO52 4.10 axle was offered with the new 350 engine that became available in 1968. But you have other options as well -- when going to the junkyard to look for '68 to '72 trucks,here are some additional pointers:
Part One -- Harvesting the donor third member
To remove the donor third member, the axle housing does not have to be removed from the donor vehicle (although it makes it easier!). Very helpful is if the junkyard guy flips the donor vehicle on its back for you. Makes the job very easy …
Step 1 – Securely block the front wheels of the donor truck.
Step 2 – If you can’t comfortably get to both sides of the rear axle (to disconnect the driveshaft and to remove the rear axle gear cover plate), then jack up the truck and securely support with blocks or jack stands (or both). Drain the oil from the axle and dispose of it in an environmentally friendly fashion.
Step 3 -- Remove the rear cover plate (10 bolts). Now is a good time to check the numbers on the ring gear to make sure this gear set has the desired ratio. (See the picture). If it has “10-41” stamped in it, then it is a 4.10:1 ratio.
Step 4 – With the truck securely in place on the jack stands, remove the rear wheel hub caps (if present) and remove the axle flange bolts (the eight bolts in the hub) on both sides. With the axles unbolted, simply pull the axles out 5-10 inches and let them rest there. There is nothing to unfasten at the ring and pinion gear. Note: When preparing the receiving vehicle, remove the axles completely as you will be replacing the axle gaskets.
Step 5 – Slide around to the front of the axle and disconnect the drive shaft from the pinion (input) shaft by removing the two trunnion bearing “U” bolts from the rear yoke. Let the driveshaft rest to the side and out of the way.
Step 6 – You will want to have help standing by for this step (and the next). Remove the 10 bolts that attach the center section to the axle housing. Be advised! Once those bolts are removed, nothing is holding the center section in place except the stickiness of the old gaskets, so be careful as you remove the last ones and support the center section in place so it doesn’t come free on its own and attempt to escape.
Step 7 -- Pull the center section free from the axle housing. It is very heavy so get some help for this step! Once it is free and sitting on the bench …. Congrats! You’ve successfully harvested the donor center section! If you are going to rebuild it, replace the pinion seal or anything like that, now is a good time to do it, or have it done professionally. At least check the lash and run out. Because of the ruggedness and durability of the Eaton HO52 and HO72 axles, it’s rare for them to need to be rebuilt or to need new bearings. More often than not, they can be used as is. Perhaps you may need to adjust the backlash (see next step).
Step 8 – To check the backlash, mount a dial indicator on the carrier and check the backlash between the ring gear and the pinion. It should be between .005” to .008”. If the backlash is more than .008”, loosen the right adjusting nut one notch and tighten left adjusting nut one notch. If less than .005”, loosen the left adjusting nut one notch and tighten the right one one notch.
Part Two -- Preparing the receiving vehicle
Step 9 – Prepare the receiving vehicle by repeating the first 7 steps. Except in Step 4, completely remove the axles as you will be replacing the axle gaskets.
Step 10 – Clean and prep the gasket faces / surfaces on the axles, hubs, both sides of the axle housing, the rear cover plate and the “new” center section.
Part Three -- Installing the harvested center section
Step 11 -- Before assembling the cover or inserting the pumpkin, run a file -- 'laid flat -- , over all the mating surfaces to remove any burrs or high spots from mishandling. Laid flat, a file won't take anything but the high spots off (if there are any) and will true up a machined surface.
Step 12 – Use two of the center section bolts to locate the gasket on the center section, and, with help, carefully install the center section in the axle housing. The shop manual says to “install lockwashers and [bolts], and tighten securely.”
Step 13 -- While you’re there, you may as well re-attach the driveshaft. Now might be a good time to install a new u-joint.
Step 14 -- Using new gaskets, re-install the axles. Remember, they are side specific (you probably noticed that when you removed them). Carefully reinsert them into the axle housing and push them in until the splines enter the differential side gears. You should be able to relocate their original positions, but as long as the holes line up, it shouldn’t matter. Torque to 85-95 ft-lbs.
Step 15 -- Replace the rear cover plate, using the thick gasket (RDS5088).
Step 16 -- Refill the axle with 6 pints of SAE 90 “multipurpose” gear oil and lubricate the rear U-joint.
Step 17 -- You are done at this point. Lower the truck off the jack stands and move the truck out of the garage. Go about 50 feet to check that everything is secure, then go for a test drive. Be conservative and don’t go all out on the first drive -- let’s make sure everything is secure first!
SAFETY WARNING -- As stated earlier, this upgrade should allow a significant speed increase. PLEASE make sure your brakes and steering can handle that! Your truck was not actually designed for these speeds and a lot may have happened to the steering and brakes over the last 60 years. So be careful and expect things to be a little wonky the first time you exceed the “AD Barrier” (60 MPH).
Step 18 – If everything checks out (no heat in the driveline components or leaks, everything looks and sounds good), then celebrate! You’ve done it! You can now sell the trailer.
Step 19 (Bonus Round) -- Sell your old center section to a rock crawler or dirt track racer and make all your money back.
See you out on the highway with a 65 mph smile!
BIG THANKS to Sweet, Denny Graham, Amtrakjoe, John Lucas, Barry Charon and the rest of the crew at Homecoming for help with this tech tip!
Money can't buy experience -- just parts and labor. Doug "ad hawk" Evans