TMR Customs: Shaving a GM Corporate 14-bolt differential
If you ever need to track someone’s 4x4 off-road, it’s easy if the rear diff under said rig is a stock GM “Corporate” 14-bolt: just look for the drag mark plowed down the center of the trail or the scrape across the rocks.
GM’s venerable (or should we say, vulnerable?) full-floating 14b (1973-1987), with its 10.5-inch ring gear and triple pinion bearings is as strong as it gets and easy to find laying around junk yards for a couple hundred bucks. The 8-lug rear end is an ideal upgrade for four wheelers in search of a beefy differential for their ½-ton pickup’s or custom 4x4s destined for serious off-road use or big power. The huge 3-1/2-inch axle tubes and wide range of aftermarket parts, plus the strength to handle 44-inch tires and a lot of torque are its calling cards.
Despite the full-float 14b’s robustness it does have a downside: a big centersection. This junkyard gem’s 13-inch diameter pumpkin hangs well below the axle tube, reducing valuable ground clearance. That means you have to run taller tires to make up for the loss of ground clearance over over a diff such as the Dana 44. The other option? “Shave” the housing.
The latter is exactly what we did with a Corporate 14b rescued from a local salvage yard to be used under the rear of a ’79 Chevy ½-ton shortbed 4x4 being rebuilt into a big-block mud runner and light-duty rock crawler.
“Shaving” is simply lopping off metal from the bottom of the differential and welding on a cap plate. The diff cover is modified in the same manner. Shaving a Corporate 14b will give the equivalent ground clearance as running three-inch taller tires without having to change gearing or the cash outlay of buying new treads.
There are several kits available to shave a 14b. We turned to TMR Customs “15-Bolt” conversion kit, which trims nearly two inches off the bottom the stock housing while maintaining its bullet-proof integrity. We also opted for TMR’s fabricated-steel rear diff cover, with the welded-on cap plate, for added protection from heavy hits during off-road forays.
Lopping off the bottom of the GM 14b axle housing in this manner will add about 1-3/4 inches of ground clearance, giving it clearance closer to that of a Dana 44. The process can be done in a number of ways depending on how much (or little) time and money you want to invest.
Some DIYers tackle the job with a Sawzall, porta-band, or cut-off-wheel to make the initial cut, followed by a lot of time with a grinder getting the angle just right to weld on the cap plate. This can take the better part of a day getting everything done. Not us.
TAKING THE EASY ROAD
We figured spending a long day undertaking such a tedious task could be better served doing other things related to the rear axle upgrade. So we took the clean, bare housing, along with the ring gear (the kit we used required it to be turned down to 10.050” diameter), to Northstar Fabrication, an industrial machine shop in Sutherlin, Oregon, to have them handle the milling and welding. What Northstar Fabrication did in a couple hours would have easily taken us eight using a hacksaw and grinder.
They carefully blocked and aligned our housing under a Cincinnati Milacron fly cutter as old as the housing, shaving it exactly to TMR Custom specs in a matter of a minutes under the skilled hands of machinist Mike Cozine. Mike also turned the ring gear, which required the use of a special ceramic insert in a lathe to quickly cut down the gear from 10.5” to 10.050” per TMR Customs specifications. Cost for the milling and welding: $300.
After Northstar’s James Denny welded the cap plate to the freshly shaved housing, we took the differential and TMR Customs’ diff cover across town to Double R Powder Coating where owner Jake Waddell put on the finishing touch to give the both a clean, crisp military look.
RE-INSTALLING THE GEARS
The 14b we shaved was still in good working order, so we re-used the same gear set and bearings to keep the cost down. When Mobile Diesel Service’s Matt Johnson disassembled it, he marked the pinion and bearing caps, and kept bearings separated for each side to ensure everything was put back in the same location as factory.
That made re-assembling the differential easy. “When you are re-using the same gear set and bearings it makes the rebuild go quickly,” Matt says. ”The only part of the ring-and-pinion re-installation that takes a couple minutes is setting the backlash, which I like to keep between .008”-.010” (specs call for .003”-.012”). Otherwise, it’s very straight-forward.”
Diff shaved and buttoned up gives us a bullet-proof Corporate 14-Bolt that maximizes both strength and ground clearance. It’s a good first step for anyone building a rock-solid trail runner.
TMR Customs complete “15-Bolt” shave kit for the GM Corporate 14-Bolt (14b) differential housing consists of fabricated steel cover with TMR logo, stainless steel bolts and cap plate. Two versions are available: with or without using a shaved ring gear. We chose the latter kit to maximize ground clearance.
Not to be spoilers, but here’s a shot of the finished product. Now let’s take a look at how it was done.
The stock 14b housing measures 13 inches from bottom to top. Shaving adds an extra 1-3/4-inch of ground clearance.
North Star Fabrication and Machine’s machinist Mike Cozine sets up our 14b under a Cincinatti Milacron milling machine to fly-cut the housing.
The most critical part of the milling process is making sure the angle of the cut in relation to the housing face aligns perfectly with TMC Customs’ cover. In this instance Mike found rotating the housing’s pinion down 5.5 degrees from square was the correct angle. (TMR Customs installation guide says 6-degrees.)
The milling machine’s spinning cutting head eats through the GM 14b housing like it was made from plastic. The shaved face is almost as smooth as glass.
Mike checks the fit of the TMR Customs fabricated differential cover as he’s nearing the last few thousandths of the milling process. Shaving the differential this way only takes a few minutes once everything is setup.
Here’s the fit after precision milling. It’s perfect, with no gaps between the cover, the housing and and the two-piece cap plate.
North Star fabricator James Denny recommends the razor-thin edge of the housing (created by milling) be ground down with a de-burring tool to provide a wider edge to better hold the weld.
Good pre-welding prep includes putting a wider face on the milled edge (arrow) so the inner cap plate weld is strong.
James and North Star project manager Seth Vincent (left) bolt the TMR differential cover to the prepped housing, which is then preheated to 400 degrees F before tack-welding the cap plate on. Pre-heating maximizes weld integrity between cast iron and steel.
Tack-welding the housing cap plate with the differential cover on ensures perfect alignment between the two-piece cap plate.
James pre-heats the housing and cap plate again to 400 degrees F. This type of attention to detail is what makes having a fabrication shop do the work worth the $300 to handle the milling and welding.
James uses an industrial wire-fed MIG welder to make quick work of welding the cap plate both inside and out.
Here the housing’s been shaved, the cap plate welded, and it’s ready to go.
The second part of the 15-Bolt conversion is turning the ring gear down from 10.5 inches to 10.05 inches per TMR Customs installation notes. North Star handled this as easily as they did the housing, using a special ceramic insert on the lathe (turning at 100rpm) to cut the hardened steel ring gear.
The tip of the special ceramic insert laying into the hardened steel of the GM 14b’s ring gear makes a fiery show as hot metal flies off the edge. Cuts are made in tiny increments, but the process still goes quickly.
The last stop for the conversion is at Double R Powdercoating where owner Jake Waddell (foreground) and shop manger Dakota Van Doran sprayed the housing Olive Drab green with flat black for the TMR logo to go with the truck’s future colors.
GM’s robust Corporate 14-Bolt housing measures 13 inches from top to bottom, which creates a lot of clearance issues off-road. Shaving 1-3/4 inches from the bottom solves those concerns.
Mobile Diesel Service’s tech Matt Johnson suggests all the 14b differential bearings be pre-lubed prior to assembly, including the drive pinion bearing that sits deep in the case.
Black RTV sealant is excellent for sealing the mating surfaces of the 14b. But keep it to a thin, even film for best results.
The old pinion shaft shim can be re-used if you re putting everything back together using the same gears and bearing as we did. Otherwise you’ll need to shim to specs depending on what new gear set is going in the housing.
Matt uses a hammer to gently tap the pinion into the drive pinion bearing because it’s a press fit. The pinion shaft has a slight taper to help it align properly into the bearing.
Matt suggests using an electric hot plate to bring the carrier bearings up to 190 degrees F before putting them on the differential case. A block of wood helps spread the energy across the bearing/race so it goes on square.
Tighten the left and right adjusting nuts with a long punch to bring gear backslash as close to .010” as you can before bolting on the bearing caps. Matt says this saves time in the re-assembly setup.
Matt installs the bearing caps (previously punch marked) and then tightens the driver’s side adjusting nut, while loosening the passenger’s side, until the ring gear backlash is between .003” and .012” per GM specifications for the 14-Bolt.
The backlash on our gear set is a little loose at .010, but still within factory margins. The margin will vary a few thousandths depending on where the lock nut holes for the keepers align.
Once the backlash is within factory spec, Matt puts a dab of Locktite on the bolt that secures the adjuster nut lock before torqueing it to 20 ft/lbs.
It’s always good to check the tooth pattern when re-assembling a differential. The gear mesh on our old ring and pinion looked fine and matched what is was before the unit was disassembled.
Matt puts a little muscle into our rebuild, tightening the carrier bearing bolts to 135 ft/lbs per specifications. He also inspects the ring gear bolts and lock washers prior to buttoning the case up to make sure none are loose or the washers broken.
Another dab of black RTV sealant round the face so we don’t have any oil leaks on the trail. Torque the 9/16” diff cover bolts to 35 ft/lbs.
Clean shaven. The TMR Customs 15-Bolt (the number of bolts around the diff cover) shave kit reduced the overall size of our 14b from 13 to 11-1/8 inches. That’s the equivalent in ground clearance to running three inch taller tires—at far less cost than purchasing the bigger tires and changing gearing to make up for the overall drive ratio change.