The real problem with a flat-back differential cover is fluid control. As the ring gear throws oil rearward, a square back wall makes the lubricant slam into the cover, turn hard, hit the top, and fall off to the sides. That does work to the oil for no benefit. It heats the fluid, aerates it, and reduces film strength where the ring and pinion need protection most. It also hurts oil delivery to the pinion and front pinion bearing instead of helping it. In this test, adding more oil only made the churning and aeration worse. More capacity by itself is not the answer if the cover shape is wrong.
Gale Banks returns to the differential cover test series to answer a central question: whether flat-back diff covers actually deliver lubricant to the ring-and-pinion mesh and the front pinion bearing. In the previous test, a plastic curved-back cover behaved somewhat like the stock cover, but a flat area at the top caused oil carried upward by the ring gear to strike that surface and spill off to the sides. That raised a larger concern about fully flat-back aftermarket covers. If the ring gear drives lubricant straight into a flat wall and then into a square upper corner, Banks wanted to know whether any meaningful oil flow still reaches the pinion interface or the pinion bearings.
Because the inside of an opaque aluminum cover cannot be observed directly, Banks modified a Mag-Hytec cover for visual testing. Since that cover had no meaningful internal features to preserve, the machine shop milled away the rear wall down to the point where the casting radius begins. The result was a cutaway cover that duplicated the internal shape of the original Mag-Hytec while allowing direct observation on the truck. Additional LED lighting was installed inside the housing area so the camera could clearly capture lubricant movement around the ring gear, reluctor wheel, and cover interior.
Banks also highlighted a common aftermarket design philosophy: pairing a flat-back cover with a higher static oil fill level. On this Ram axle, the stock configuration took 3.9 quarts. The tested cover, even at its low fill mark, sat about 3/8 inch higher than stock and required 6 quarts, or more than 2 additional quarts. At that level, the oil extended into the axle tubes. Banks questioned the assumption that simply adding more lubricant is inherently beneficial. His view was that the original axle designers understood durability extremely well, and while aftermarket parts can improve on stock in principle, he was not yet convinced that these covers actually do so. All tests used 75W-90 synthetic gear oil.
The test began at 5 mph. At that speed, both the reluctor wheel and the ring gear picked up lubricant and carried it upward. Some fluid motion was also created by flats on the differential assembly, which paddled the oil because the static level was so high. Banks contrasted this with the stock setup, where the dynamic running oil level drops once the axle is in motion. The intended lubrication path is for the ring gear to carry oil into the ring-and-pinion contact zone, where sliding friction and load are highest, and then sling lubricant onward to the front and rear pinion bearings. A successful cover design should support that dynamic flow rather than interfere with it.
At 10 mph, the dynamic oil level had already dropped, but Banks observed that a substantial amount of lubricant was being painted onto the inside of the cover and then falling away rather than continuing over the top toward the pinion area. By 15 mph, the problem became more obvious. Instead of following a curved surface that would guide the oil with the rotating ring gear, the lubricant was being driven directly into the flat rear wall of the cover. According to Banks, that impact does unnecessary work on the fluid, which increases heat and heavily aerates the oil. He emphasized that aeration reduces lubrication quality at the ring-and-pinion interface. At the same time, oil was being thrown high up inside the cover and then dropping off to the sides rather than being directed where it was needed.
At 20 mph, Banks said the behavior matched his concern about flat-back designs. Most of the lubricant that should have continued toward the pinion was instead being diverted to both sides after striking the flat surfaces. He also noted small eddy currents forming inside the cover. At 30 mph, the side spill had become a pronounced waterfall effect. At 50 mph, the internal activity was even more dramatic, with substantial visible agitation and aeration in the rear of the housing. Banks did not claim that no oil reached the pinion at all, but he made clear that a large amount of fluid energy was being wasted in chaotic motion at the back of the cover rather than being used to support the intended lubrication path.
After returning the axle to idle and letting the motion settle, Banks summarized the mechanism. With a square-back cover and a fill level more than 2 quarts above stock, lubricant is driven into the rear wall, forced to make a 90-degree directional change upward, then strikes another flat surface near the top and spills off to the sides. In his view, that sequence tends to starve the pinion and pinion bearings relative to a better-guided flow path. More importantly, all of that redirection represents viscous loss: energy spent shearing and accelerating the oil for no useful lubrication benefit. That energy becomes heat in the fluid.
Banks argued that this extra work on the lubricant creates several penalties. First, heating the oil degrades the fluid and lowers viscosity. Second, it costs fuel economy because the axle is spending power churning oil unnecessarily. Third, aeration introduces air into the lubricant, which weakens film strength. That matters both in rolling contacts such as bearings and, even more critically, in the sliding, heavily loaded ring-and-pinion mesh. In his explanation, both elevated temperature and aeration reduce the oil film's ability to protect those surfaces. He therefore described the flat-back concept, especially when combined with an even higher fill recommendation, as counterproductive.
Banks acknowledged one legitimate advantage of aluminum flat-back covers with external fins: they should transfer heat better than a stock cover because aluminum conducts heat well and fins increase external surface area. However, he argued that this benefit may be undermined if the cover geometry creates excessive churning and generates a large amount of heat in the first place. The next phase of the series, which he identified as part three, would move from visual flow testing to dyno testing. The plan was to run the axle to 300 degrees while applying 250 horsepower to the road surface, then compare how quickly each design reached temperature and how effectively each one cooled the lubricant relative to stock.