Differential Cover Buyer’s Guide – What You Need To Know
Looking for a new differential cover for your 4×4? This buyer’s guide has all the information you need.
When you’re in the market for a new differential cover, the first decision you have to make is whether to get an OEM or aftermarket diff cover. This buyer’s guide will help you reach an educated decision.
OEM Differential Covers: Pros and Cons
An OEM differential cover is the one that comes on a brand new vehicle. They’re made of stamped steel, and they’re shaped to make sure the gear lube flows across the gearset correctly. OEM diff covers do have some limitations:
- Some OEM differential covers are thin – just 1/16th of an inch. Thin covers are easily dented or punctured off-road, especially if you’re tackling rocky trails.
- The diff cover flanges are also generally thin. Rocks can easily peel the edge backwards.
- OE covers have been known to rust out and leak, depending on age and environment.
In a nutshell, OEM differential covers are well designed for use on road, but not off road. If you want to make sure your diff doesn’t dump gear oil in the middle of the wilderness, an upgraded cover is probably a good idea.
Off-Road Differential Cover Materials: Pros and Cons
Aftermarket differential covers are primarily made of five different materials. Each material has it’s own pro’s and cons. Here’s an overview:
Cast aluminum covers are lightweight and conduct heat well. Many have fins to aid in cooling the gear oil. Also, many allow for more gear oil to be added to the differential, as compared to stock.
With all this cooling, this type of cover can be great for trucks that tow on a regular basis. However, there are a couple of downsides to aluminum differential covers:
- Aluminum is not great for off road use. Cast aluminum can crack from impacts with rocks, and cooling fins can be broken off the same way.
- Aluminum has more than twice the coefficient of thermal expansion as cast iron. That means aluminum expands twice as much as cast iron when it heats up. So if an aluminum diff cover is bolted to a cast iron differential case, the mating surfaces of the two parts slide across each other as they heat up. There is, of course, a gasket between the two parts. So the gasket is subject to shear forces that will eventually cause it to degrade and seep or leak. This will happen sooner than it would if the gasket were between two more similar materials.
Part of the reason that aluminum is popular is all the styling options…it’s relatively easy for a manufacturer to mill a cool logo or pattern into an aluminum differential cover. Good looks are a selling feature, but it’s not what you need off-road.
Most aftermarket stamped or plate steel differential covers are thicker and heavier than the OEM covers they replace. They’re usually only 3/16″ thick plate steel, which is definitely better than some OEM covers, but:
- That’s not saying much and
- You don’t save a lot of money going with stamped steel
A stamped steel cover might be $40 or $50. A nodular iron or welded steel cover might be $100 or $150. For $100, it’s probably worth it to go for a stronger diff cover.
Welded steel covers are intended for off road use. They are often made of 1/4″ and 3/8″ thick bent and welded steel, which results in a very strong differential cover. Welded steel covers are heavy and somewhat expensive, but they are about as close to “bombproof” as you can get.
The downside to welded covers is that their shapes are simple, and they don’t guide oil to the gears as well as OE covers. This doesn’t matter very much at low speeds off road, but it can be a problem on a vehicle that does a lot of towing and/or high speed travel for long periods.
Nodular iron is a high strength form of cast iron. Because nodular iron covers are cast, they can have complex shapes that help guide oil, and casting also allows for other features.
- Several nodular iron differential covers on the market have horizontal oil fill holes, for example. Some have magnetic dipsticks, which make it a lot easier to see if you’ve got a problem, as opposed to draining the oil.
- Many nodular iron covers have recessed bolt holes to protect the bolt heads from trail damage.
- Some have ribs, which stiffen the cover, which in turn stiffens the differential housing.
The downside to nodular iron covers is that they don’t shed heat as fast as aluminum covers or even the OEM stamped steel covers. They are also heavy, and they’re at the top end of the price range (but still not expensive compared to a flatbed tow truck).
Chrome isn’t an option for a good off-road differential cover. Save it for your show truck!
What to Look for in a Aftermarket Differential Cover
For trucks that stay on paved roads, an OEM differential cover will probably be fine. They are designed by the manufacturer to ensure proper cooling, they have a lot of testing behind them, and they don’t cost anything because they’re already on your diff.
But if you’re worried about punctures or leaks, you need an aftermarket cover with the following features:
- Good oil flow. Compare the shape of the cover you are looking at with the OEM cover. Many aftermarket differential covers have poor flow characteristics.
- Good oil cooling. As shown in this video, Gayle Banks has proven that a flat or square backed differential cover has inferior cooling characteristics, even if the cover holds more gear lube.
- Recessed bolt heads and drain plugs. Recessed bolt holes protect the bolt heads from damage off-road, as does a recessed hex head drain plug. It’s a little thing that will save you a lot of headaches the next time you need to replace your gear oil or pull the cover off.
- Horizontal fill hole and dipstick. It’s not essential, but if you do your own maintenance or use your truck hard, these are really nice features.
Ultimately, it’s important to buy a differential cover based on performance rather than styling. Too many differential covers sell because they’ve got a eye-catching design or logo, and that just doesn’t make sense (at least not to us).