Small Details Make the Difference

by Randy Lyman

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When ever someone tells me the they are the "best" at what they do, I usually have doubts. Sometimes it seems like everyone is claiming to be the best at what they do. I have found that very few people are willing to put in the time and effort that it takes to be the very best, or even to be their own "best".

Taking care of the details requires knowing which details are important, and having the discipline to check every detail on every job. Enough rambling. Here is a partial list of details that I have found to make a difference when assembling a differential.

I always inspect the carrier case closely to make sure there is nothing that may shorten the life of the rearend. I pay very close attention to the bearing hubs on the carrier case. If the old carrier bearing spins on the case, the new bearing will not fit tight and will also spin. Any wear due to a spinning carrier bearing will cause the backlash to open up and the ring & pinion gears will eventually break. I also look carefully at the side gear bores and thrust washer surfaces to make certain they are not worn or galled. And I make sure the cross pin shaft bores fit the cross pin tightly. Lastly, I check the ring gear flange with a dial indicator to make sure there is minimal or no run-out.

There are a lot of opinions as to how much carrier run-out is acceptable. I have seen as little as 0.005" cause problems and I have seen rearends work fine with 0.009" or more. The main thing that I look at is backlash variation. I like to keep the backlash variation under 0.005" if possible. In a perfect world no variation would be our goal. However, even with a straight case the backlash can vary 0.005" without any negative consequences, so this is an area where each assembler will have to decide how much they are willing to allow.

I also look closely at the spider gears and cross pin shaft. The spider pinion gear and side gear teeth should be without pits or chips, and should not show any signs of heavy wear at the base of the contact patch. The spider pinion gears should not have any heavy wear or galling on the inside where they contact the cross shaft. And the cross pin shaft should show no more than light, even wear on the surface that contacts spider pinion gear without scratches or galling.

The pinion yoke is another part that can cause a lot of problems if it is not in good shape. It is good idea to inspect the ears or shoulders that retain the universal joint. If there is any wear in these areas it is best to replace the yoke. I also inspect the seal surface closely. Small grooves or light rust can sometimes be polished away, but any irregular spots can cause the seal to leak if they do not polish out completely. Repair "redi sleeves" are available to press on the yoke and replace the seal surface of my popular models. These sleeves work fair in most cases but usually do not hold up as long as a new yoke.

The most often overlooked problem that I have found with yokes is the splines. On some models the splines seem to last forever, and on some they loosen up quickly. The splines should always fit tight and it should take a lot of pressure or pounding to drive the yoke down until it seats against the outer pinion bearing. One other area to inspect on the yoke is the surface that touches the outer pinion bearing or slinger. If this surface has light wear it can be filed down flat with a hand file as long as the surface is kept true and is not filed unevenly.

Up until now I rambled on about a few of the small details that I consider important to high quality rearend repair and this month I talk about more of the details that I find important to building a strong rearend.

When working on any rearend, I always pay very close attention to the axle shafts and axle bearings. Sealed ball bearing type axle bearings are usually very easy to diagnose and any roughness makes replacement the obvious choice. Flat roller bearing style axles are not as obvious to diagnose, and many times I have seen customers dismiss wear that soon after leads to larger problems. Any pits, rust, grooves, or odd wear on the bearing surface of the axle shaft will lead to more wear. Just the same as chuck-holes in the road always get larger and never seem to fill themselves in, axle shaft wear always gets worse. Large tires and towing heavy loads accelerates axle wear and make inspection and replacement extremely important.

Axle splines are another area that can easily be overlooked. The axle splines in most rearend designs hold up well and seldom wear much at all. There are a few designs, however, that tend to wear out the splines quickly. Nine inch Ford rearends are notorious for wearing out the axle spline wear, especially when they use a Trac-Lock. The Track-Lock uses a two piece side gear and clutch hub on the left side, which tend to spread the load to the splines less even and accelerates wear. Swapping the axles from left to right can sometimes work as a fix if the wear is not to extreme, but I usually recommend installing a new axle shaft.

Bent axle housings are another problem that is seldom noticed, and rarely fixed. A bent housing causes the axle splines to be worked harder that normal due to the angular contact with the side gear and the sliding action that happens to the splines each time the axle rotates. Bent housings can sometimes be spotted by sighting down the axle tubes after the axles and carrier case have been removed. This takes some practice, but once the technique is mastered it is easy to determine if the housing is bent.. Although I have seen people use an axle truss to pull the housing straight by tightening up the truss every week or so until it is pulled straight, I recommend removing it from the vehicle and taking it to a competent welder or machine shop. Even though it may not be the best way to straighten a housing, an axle truss is definitely a good idea for any situation where the vehicle is being overloaded or abused, and it will help keep the housing straight.

Carrier bearing adjusters are another detail that will effect the life of the rearend if the worn parts are not noticed and fixed or replaced. Often the carrier bearings will spin in the housing and wear a groove into the adjuster. If the wear is not too extreme, the adjuster can be saved by filing the contact surface flat again. Just make sure that the surface is filed flat and square, and not at an angle.

Clutch plates and clutch plate guides are also important to rearend life. Clutch guides are the small, half round, hard metal plates that keep the clutches from eating into the carrier case on many limited slip designs. These guides can wear to the point of breaking, and once they are broken they can get between the ring and pinion gears and wreak havoc. I have made the mistake of not replacing worn guides only to have them break and destroy an otherwise good ring and pinion set.

Do these details really matter? Next time you see someone broken down on the trail try not to laugh at them. Any one of us can be surprised by some small detail that jumps up and bites us when we least expect it, and it always seems to happen at the most inopportune time.

Gear Talk is written by Randy Lyman, founder and President of Randy's Ring & Pinion.

Originally published in  4WD & Sport Utility Magazine.

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