10.5" Full Float Truetrac Differential Installation

[Schurkey]

1997 K2500 8-lug 10.5” Ring Gear Full-Float axle
Truetrac Differential Installation

Driveline Tragedy and Heartache, Ending in Success.​

Intended to install an Eaton Truetrac helical-gear style “posi” in the 8-lug '97 K2500 plow truck, 10.5” full-float rear axle last summer. Things got put off. Around November, I figured if the truck was going to plow snow, I'd better get off my ass and do this job.

It's now the end of March. I have said that it takes me a month to change valve-cover gaskets; I guess five months to do a differential is about what I should expect. A Stealership would probably get 5 hours labor to do this job under warranty.

Phase One: Remove driveshaft. I immediately break not one, but two driveshaft strap bolts. And I knew better—the first one took enormous effort even with a long-pattern combo wrench. I “thought” it loosened a little, so I go to the next one. That one merely twists off immediately. So I go back to the first one, and give it another lil' tweak, which proves it was not loosening, it was failing.

Photo 1, 2, 3. Broken bolts and tools

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The remaining two bolts came out pretty easy...AFTER I got Mr. Oxy-Acetylene Torch on the pinion yoke, and made it glow. Ran a 5/16-24 tap through those threads, which were really in pretty fair shape. Wasted heaps of time, effort, and enthusiasm trying to remove the broken bolt shanks. Used reverse-twist drill with a drill guide I made out of some brake tubing with the idea that the drill guide would help me drill through the center of the bolts instead of drilling crooked. Used a 1/2” air drill turning low RPM hoping the reverse drill bit would “catch” the broken bolt shanks and spin 'em out. No luck.

Marinaded the broken shanks with penetrating oil for about a month. No help. Used the torch with a big “rosebud” nozzle to make the iron yoke around the bolt shanks glow. No luck. Finally put a higher-speed air drill on that bit, and spun through the center of the bolts. More marinading, more torch. Two styles of “bolt removal” tools. Heck, I even used a small welding tip to blow the torch flame through the hollow center of the broken bolt shanks so that the broken shanks glowed. Nothing helped. My goal of unthreading the broken shanks evaporated. Eventually I just crammed a letter “I” drill bit (the tap drill size for 5/16-24) through the holes, afterwards I spun a starting tap until it came out the back side of the yoke. I have nothing to show for this effort except a small hill of drill swarf and metal-tapping chips. I hope that the tap cleared the broken bolt threads out of the existing threads in the yoke, but I have no proof that this happened. All I know is that the new bolts thread into the holes nicely, hold torque, and I'd have been time, money, effort, and enthusiasm ahead to have scrapped the yoke and bought a new one. I spent more money on electricity heating the damn garage than the new 1350 size yoke would have cost.

Photo 4. New Precision 492-10 bolt and strap kit on yoke

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This portion of the job is done. Next up...U-joint on the driveshaft.

 

[Schurkey]

Phase Two. Dealing with the driveshaft U-joint.

I'm not out of the woods yet. The front U-joint on the 180K-mile aluminum driveshaft feels nice 'n' smooth. The rear U-joint had just a hint of roughness. The rear U-joint usually wears-out before the front one(s). I guess they don't like the additional road-splash, dust, and cold-weather operation when the front U-joint is kinda shielded from debris, and warmed by engine/transmission and exhaust heat. The original U-joints are non-greasable, and the grease looks pretty petrified on the two caps I can easily remove.

Anyway, I discover that aluminum driveshafts require “special” U-joints that have “coated” caps to prevent corrosion between the steel cap and the aluminum of the driveshaft yoke. One Precision-brand 331C “Coated” 1350-size U-joint from O'Reillys comes home with me. The “Made in China” printed on the box is in letters so small I didn't notice them until I took a photo and blew it up. They're obviously ashamed of the Chinese origin.

Photos 5, 6. U-joint, U-joint box close-up.

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The original rear U-joint is a disaster to remove. One cap is so seized in the driveshaft yoke that the U-joint body pops right through the end of that cap.

Photo 7. U-joint with broken cap

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I did get the other cap to push out, using a big gear puller, a socket, and lots of harsh language. And a hand-held propane torch to warm and expand the aluminum yoke. I clean the corrosion out of the snap-ring grooves, I polish the inside of the yoke where the caps are held. Cleaned-up some damage I did when pulling the body of the U-joint. And discover that the grease zerk on the new joint has to face outward, 'cause it'll never clear the extra-bulky aluminum yoke of the driveshaft. I wanted a non-greasable U-joint, but couldn't find a coated, non-greasable 1350.

Photos 8, 9 (#10 in next post) Old, rough, U-joint bearing surface scored by bearing rollers, U-joint with no zerk clearance.

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(At the limit for photos. Continued next post.)

 

[Schurkey]

Photo 10. U-joint installed. Zerk under compression when driving forward.

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(Driveshaft installed after differential work, zerk clears iron differential yoke.)

Onward to the actual differential work.

 

[Schurkey]

Phase Three—finally dealing with the differential and gears.

I have the axle shafts pulled out of the differential, fresh Fel-Pro gaskets for the axle shafts at the hubs, (two of 55350) and rear differential cover (RDS 55063), a new GM differential cover, (22891940) 14 new "Grade 8" diff cover "flanged" bolts (10 of 3/8-16 x 3/4", 4 of 3/8-16 X 1.0") from Ace Hardware, a pair of new Timken “Set 76” differential bearings, the Truetrac, (915A545, made in Taiwan) 12 hardened 1/2" lock-washers, and four quarts of new GL5, 80W-90 non-synthetic axle lube.

Rusty? The COVER has rust holes in it. The cover bolt heads are so rusted and the threads so seized that I have to cram a 14mm 6-point impact socket onto some of them because a 9/16 6-point impact socket just spins.

Photos 11 and 12. Terrible-looking fluid, rust holes in cover.

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I discover that there's a Military service manual for CUCV vehicles that use the same 10.5” axle as what's under my truck (except they have the Gov-Lock differential.) The Military manual is easier to understand than the Genuine GM manual. Section 6-15 of TM 9-2320-289-34 (note that '34P is the Parts manual, which doesn't help us.)

tm9-2320-289-34 INTERMEDIATE DIRECT SUPPORT/GENERAL (7

Important Notes: The '97 GM Service Manual is WRONG:

1. When installing the ring gear onto the differential, they tell you to use left-hand-threaded dowel pins in two places, to assure the ring gear bolt holes align with the bolt holes in the differential. Ring gear bolts are NOT reverse- (left-hand-) thread, and
2. The screw-type adjusters for backlash and bearing preload are NOT captured or threaded-into the bearing caps. They're threaded into the axle assembly. That's why the bearing caps have provision for locking-tabs for the adjusters.​

I don't know if this is old information that's been carried-over after the axle assembly has been changed; or if this info has always been wrong. I have no reason to believe my axle assembly has been changed during the life of the vehicle...but I did buy it with ~175,000 miles on the thing. The axle U-bolts are rusty as hell, one so bad that it broke. IF (big IF) this axle is not original, it's been replaced a LONG time ago. Which seems unlikely. I think it's the original axle.

I am not dicking with the pinion gear. The pinion seal doesn't leak. I don't have to mess with pinion depth, pinion bearing pre-load, crush sleeves, or anything else having to do with the pinion. I'm going to transfer the ring gear from my open differential to the Truetrac. I do need to be concerned with pressing-on new bearings, getting differential bearing preload correct, and getting backlash correct. In a perfect world, the backlash with the new differential / used (original) gears would be exactly the same as before, so the established wear-pattern is not disturbed. This may not be possible because bearing preload and backlash are set using threaded adjusters, and they're locked in place using a retainer through a series of holes...and the “perfect” position could be in-between two holes. There are no differential shims on this 10.5” axle. BECAUSE I'm not dicking with the pinion, there's no need to buy a "differential installation kit"; which is why I bought two Set 76 bearings and avoided the pinion crush-sleeve, pinion bearings, pinion shims, etc. that come in the "kit".

First step is to loosen differential fill plug, remove diff cover, check for gear damage and metal shavings. Clean housing including the glued-in magnet. Measure existing ring 'n' pinion backlash in several places around the ring gear. Spec is .003--.012, .005--.008 preferred. Measure ring gear runout.

As it turns out, my backlash is about .014, and I can't do a measurement that I trust with regard to ring gear runout. I get .003-ish, but I don't believe that result.

Photos 13, 14, 15. Checking backlash.

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^^^ Looking "up" towards underside of bed.

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(At the limit for photos, continued below.)

 

[Schurkey]

Photo 16. Dirty/rusted cover gasket surface, filth in housing. This was cleaned before attaching the dial indicator, but the photo wouldn't fit in previous post.

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I covered the "guts" with aluminum foil, before scraping gaskets and running a tap though the cover bolt holes.

Photo 17. Getting ready to clean.

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Second step is to mark the adjusters, then play with finding “zero lash”, and returning to previous backlash measurement. Practice setting bearing preload.

This turns out to be easier than I expected. I loosened the bearing cap bolts about ½ turn each. The bearing adjusters turn pretty easily. I can slide the differential left and right with the adjusters without trouble. A tapered punch does a nice job of substituting for the official “special tool” bearing adjuster. I discover right away that my differential not only had excessive backlash, but it had no bearing preload at all. I play at setting the backlash and preload “to spec, by the book”. I have confidence I'm getting it right.

Third step is to mark the bearing caps, remove caps, remove differential.

Photo 18. "Empty", cleaned housing.

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Fourth step is to press bearings onto Truetrac differential. I got them started by tapping with a hammer. Pounded them most of the way on with a BFH and a piece of 2X4. Moved them the remaining few thousandths using the same hammer and a punch on the inner race. The Set 76 bearings fit perfectly.

Fifth step—Remove ring gear bolts, then allow the ring gear to drop off the old differential. I guess the ring gear sometimes requires being tapped off with a small hammer. I didn't need to. Don't let the ring gear teeth smack the concrete when it drops off the differential. I dropped it onto a couple pieces of wood. The old differential is held together by the ring-gear bolts, so I crammed a couple of hardware-store nuts and bolts through it to keep it together “for later”. Ran "bailing wire" through the old diff to hold the outer bearing races in place.

Photos 19 and 20. Old differental, ring gear removed.

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(At the limit for photos. Continued next posting.)

 

[Schurkey]

Install ring gear on Truetrac using original bolts and new lock-washers. Torque bolts to 120 ft/lbs. I set the differential assembly onto an old riding lawnmower tire, then held the differential in place using a pry-bar, while torquing the bolts.

Photo 21. Torquing ring gear onto Truetrac.

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Sixth step—Install differential into axle housing. It's heavy—be careful lifting it into place. Put caps on “just snug” so it won't fall on you as you make the adjustments. Adjust for zero lash, then loosen left adjuster approx. 2 holes to achieve spec. backlash. Tighten right adjuster so it just contacts bearings, then add about three holes of preload for new bearings, two holes of preload for used bearings. When the bearings have preload, backlash changes and may need to be re-set. Check it again! Get backlash close as possible, while also having bearing preload as close as possible. Torque bearing caps at 135 ft/lbs. I re-checked backlash after torquing caps, and had to loosen the caps, turn the right adjuster loose one hole, and tighten the left adjuster one hole. And then, just to be sure, I loosened the right adjuster, found where it just touched the bearing, and added the proper preload from scratch. In all, I played with the backlash and bearing preload multiple times before I was satisfied. In order to preserve the existing wear pattern on these "used" gears, I set my backlash just a little loose--about the same .014--.013 as before. At that point, the adjuster retainers are bolted into place.

Photo 22. Truetrac installed and adjusted for backlash and bearing preload.

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Seventh step. Lube hub bearings, torque axle shaft bolts to 110 ft/lbs. Install differential cover w/gasket, including brake tubing brackets to 30 ft/lbs. Add differential lube. I couldn't get the specified 7.2 pints into the axle, (ran out the fill plug) so I lifted the right side with the left side off the jack-stands, tire on the ground. Tilting the axle allowed me to get the proper amount in the housing. Two bolt holes on the new GM cover seem under-sized, the bolts won't go through them. Had to touch them with a die grinder and rotary file to open the holes. The only other significant difference is that the new cover has reinforcements on four of the bolt holes, so I used slightly-longer bolts on those four positions.

Photo 23. Rear cover installed.

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Installed driveshaft. Test-abused vehicle per differential manufacturer's instructions.

Drives fine, no gear whine on acceleration or coasting. Seems like a successful operation.

 

[Schurkey]

The National Weather Guessing Service (and my home thermometer) say that it's 29 degrees as I type this. Driveway is melting nicely due to solar gain. It may be below-freezing, but with bright sunshine the snow will still melt.

Plow truck has scraped the slush off the driveway three times in two days. The underlying ice then melts faster. Making progress.

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