Resealing a differential cover is one of those jobs where the wrong sealant turns a 20-minute task into a weekend of wiping up gear oil. The differential runs hot, the cover flexes under load, and 75W-90 gear oil is brutal on cheap sealants. You need a gasket maker that bonds to the stamped steel or aluminum cover, resists hot oil, and stays flexible enough to handle the constant heat cycling without cracking or shrinking away from the bolt holes.
We pulled the most trusted RTV silicone and anaerobic sealants buyers actually run on rear and front diffs, then judged them on oil resistance, cure time, gap-filling ability, and how cleanly they let you do the next service. Whether you skipped the paper gasket entirely or you are sealing a magnetic drain-plug cover, one of these seven will keep your axle dry.
| Photo | Product | Score | Buy |
|---|---|---|---|
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Permatex 81158 Ultra Black Maximum Oil Resistance RTV Silicone Gasket Maker Best Overall Black RTV silicone, sensor-safe, oil resistant to 500F intermittent, 0.25 in gap fill |
9.5 | 🛒 Check Price |
 |
Permatex 81160 Ultra Grey Rigid High-Torque RTV Silicone Gasket Maker Best for High Torque Grey rigid RTV, sensor-safe, high-torque retention, 0.25 in gap fill |
9.3 | 🛒 Check Price |
 |
Permatex 25223 The Right Stuff Gray Gasket Maker Best Instant Seal Gray elastomeric RTV, seals on contact, 90% torque retention after 24 hr |
9.1 | 🛒 Check Price |
 |
Loctite 5920 Copper High Performance RTV Silicone Gasket Maker Best Heat Resistance Copper RTV silicone, up to 700F intermittent, high oil and coolant resistance |
8.9 | 🛒 Check Price |
 |
Permatex 51813 Anaerobic Flange Sealant Best for Rigid Flanges Anaerobic gel, cures only between mated metal, machine-finished flanges, no gap fill |
8.7 | 🛒 Check Price |
 |
Loctite Ultra Black 5699 Gasket Maker RTV Silicone Best Value RTV Black RTV silicone, sensor-safe, high oil resistance, flexible cured bead |
8.5 | 🛒 Check Price |
 |
ACDelco 10-5044 Gray Silicone RTV Sealant Best OEM-Grade Gray RTV silicone, OEM-spec, oil and coolant resistant, flexible cure |
8.2 | 🛒 Check Price |
1. Permatex 81158 Ultra Black Maximum Oil Resistance RTV Silicone Gasket Maker: Best Overall
If you only buy one tube for a differential cover, make it the Permatex 81158 Ultra Black. It is purpose-built for high oil resistance, which is exactly the stress a diff cover sees as it wallows in hot gear oil for the life of the axle. The black RTV bonds well to both aluminum and stamped steel covers, and because it is sensor-safe and low-odor, you are not fighting acetic-acid fumes while you work. It bridges minor surface imperfections, so a cover that is slightly tweaked from over-torquing still seals.
The real test is the reseal a few years later, and this is where Ultra Black earns its reputation. It peels away in long ribbons instead of baking into a brittle crust you have to scrape, which saves your sealing surfaces from gouges. The honest weakness is patience: it skins quickly but wants a proper overnight cure before you add fluid, and rushing it is the number one reason people report a weep. Treat the cure time as part of the job and this is as close to foolproof as diff sealing gets.
- Low-odor, non-corrosive formula that is safe for oxygen sensors and gear-oil-wetted surfaces
- Retains flexibility through hard heat cycling so it will not crack around the cover bolts
- Fills irregularities up to a quarter inch, ideal for slightly warped stamped steel covers
Pros: Exceptional resistance to hot 75W-90 and limited-slip additives; Stays pliable for years, peels off cleanly at the next service; Trusted shop-standard that diff specialists reach for first
Cons: Needs a full cure before filling, so plan an overnight wait; The tube tip can be fiddly to lay a perfectly even bead
2. Permatex 81160 Ultra Grey Rigid High-Torque RTV Silicone Gasket Maker: Best for High Torque
The Ultra Grey 81160 is the right call when your differential cover is a heavy, rigid casting with bolts spaced close together. Its high-torque rigid formula is engineered to maintain clamp load on stiff flanges, which is precisely the condition on many aftermarket finned aluminum covers and beefy truck axles. It shares the same oil resistance and sensor-safe chemistry as the Ultra Black, so it shrugs off hot 75W-90 and limited-slip additives over long service intervals.
Where it asks for honesty is flexibility. Because it cures more rigid, it is the wrong choice for a thin, slightly warped stamped-steel cover that flexes as you torque it down, and on those it can be more prone to a hairline weep than a flexible black RTV. Match it to a rigid cover with a true sealing face and it is outstanding. Use it on a tweaked budget cover and you may wish you had reached for the Ultra Black instead.
- Rigid, high-torque formula designed for joints with low flex and tight bolt spacing
- Resists hot gear oil and common limited-slip friction modifiers
- Sensor-safe and non-corrosive, so no harm to nearby driveline components
Pros: Holds clamping torque on covers with closely spaced bolts; Strong oil resistance for long service intervals; Wide availability and consistent batch quality
Cons: Less forgiving on warped or flexing covers than a flexible RTV; Grey color shows any squeeze-out clearly on a painted cover
3. Permatex 25223 The Right Stuff Gray Gasket Maker: Best Instant Seal
The Right Stuff is the answer to the classic question, what do I use when the diff has to be back on the road tonight. This gray elastomeric gasket maker seals on contact and retains the bulk of its torque within a day, so unlike a standard RTV you do not have to leave the axle empty overnight before adding fluid. That makes it a genuine lifesaver for a stranded weekend job or a fleet vehicle that cannot sit. It is also extremely flexible, which suits axles that see a lot of vibration and twist.
The trade-off is twofold. First, it is a premium sealant and harder to justify for a basic cover you are in no rush to button up. Second, the aerosol-can version dispenses a lot of material very quickly, and it is genuinely easy to over-apply and end up with a mess inside the housing if you are heavy-handed. Use a thin, deliberate bead and respect how fast it comes out, and you get a near-instant, oil-tight seal that few other products can match.
- Seals instantly and lets you refill with gear oil right away in a pinch
- Highly flexible elastomeric rubber that absorbs cover flex and vibration
- Brush-top or aerosol-can applicator lays a controlled bead
Pros: Drive-away-ready sealing when you cannot wait overnight; Outstanding flexibility for vibration-prone axles; Excellent oil and chemical resistance
Cons: Premium product that is harder to justify for a simple job; The aerosol can dispenses fast and is easy to over-apply
4. Loctite 5920 Copper High Performance RTV Silicone Gasket Maker: Best Heat Resistance
Loctite 5920 Copper is the pick for the differential that runs genuinely hot, think heavy towing, off-road crawling, or a truck that spends its life loaded. The copper-filled RTV carries the highest intermittent temperature rating in this group, and it pairs that with solid resistance to gear oil and coolant. The cured bead stays flexible enough to follow the cover as it expands and contracts, and it fills minor surface defects so a less-than-perfect face still seals.
Be honest with yourself about whether you need that headroom. For a daily-driver rear end that never sees extreme heat, the extra temperature capability is overkill and you are paying for capacity you will never use. The copper color also shows clearly, so sloppy squeeze-out looks worse than it would with a black product. For a hardworking axle, though, this is the sealant that will not bake out and crack.
- Copper-filled RTV rated for the highest intermittent temperatures in this lineup
- Strong resistance to gear oil, coolant, and shop chemicals
- Flexible cured bead that tolerates thermal expansion of the cover
Pros: Best-in-class heat tolerance for heavily worked or towing axles; Reliable Loctite chemistry with consistent results; Good gap-filling on imperfect sealing faces
Cons: Higher temperature rating is overkill for many street diffs; Copper color is conspicuous if any squeezes out
5. Permatex 51813 Anaerobic Flange Sealant: Best for Rigid Flanges
Anaerobic flange sealant is a different animal from RTV, and on the right cover it is superb. The Permatex 51813 gel only cures where it is squeezed between two metal faces and starved of air, which gives you two big advantages on a differential. There is no skin-over countdown rushing you while you align the cover, and any excess that squeezes into the housing never cures, so it simply gets carried off by the gear oil instead of forming chunks that could block a passage or foul a magnet.
The catch is that it has effectively no gap-filling ability. It demands flat, true, well-machined flanges with good metal-to-metal contact, which describes many factory aluminum covers and housings but not a cheap stamped-steel cover or a face that has been warped by over-torquing. Check your flanges with a straightedge first. If they are flat, this gives a remarkably clean and reliable seal. If they are not, an RTV is the safer bet.
- Anaerobic gel that cures only in the absence of air, between the metal faces
- Will not cure as squeeze-out, so excess wipes away without contaminating the diff
- Excellent for tightly machined, rigid cover-to-housing flanges
Pros: No skin-over clock, so you can take your time positioning the cover; Clean inside the housing because uncured excess simply wipes off; Very strong seal on flat, true machined surfaces
Cons: Will not fill gaps, so it needs flat, well-machined flanges; Wrong choice for warped or stamped covers with poor face contact
6. Loctite Ultra Black 5699 Gasket Maker RTV Silicone: Best Value RTV
Loctite Ultra Black 5699 is the sensible everyman choice for a differential cover. It is a sensor-safe, low-odor black RTV with the high oil resistance the job demands, and it cures flexible so it copes with the relentless heat cycling between the housing and cover. It works equally well on a stamped-steel cover and a cast aluminum one, and the tube dispenses a clean, controllable bead that makes laying an even ring around the bolt pattern straightforward.
It is honest about what it is: a dependable, good-value RTV rather than a specialist showpiece. It does not seal on contact like The Right Stuff, so you still owe it an overnight cure before refilling, and its oil resistance, while strong, is not quite the standout that the Permatex Ultra Black sets. For most drivers resealing a rear end on a Saturday, none of that matters, and this gives a leak-free result without fuss.
- Sensor-safe, low-odor black RTV with strong gear-oil resistance
- Flexible cure handles the heat cycling a diff cover endures
- Easy-squeeze tube lays a manageable bead for clean work
Pros: Dependable oil-tight seal at a sensible value; Flexible enough for stamped and cast covers alike; Widely stocked Loctite quality
Cons: Needs an overnight cure before adding fluid; Performance is solid rather than class-leading
7. ACDelco 10-5044 Gray Silicone RTV Sealant: Best OEM-Grade
If you prefer to seal a factory differential the way the factory would, the ACDelco 10-5044 gray RTV is a comfortable pick. It is formulated to OEM sealing standards, so it behaves predictably on stock GM-style covers and housings, resisting gear oil and coolant across a normal service temperature range. The cured bead stays flexible to track the cover as it heats and cools, and for the routine reseal it simply does the job without drama.
It is not the sealant to grab if your cover face is rough or slightly warped, because its gap-filling capacity trails the maximum-oil-resistance RTVs at the top of this list. Its availability can also be less consistent than the household sealant names, so you may have to hunt for a tube. On a clean, true factory flange with a normal service interval, though, this OEM-grade option gives a tidy, trustworthy seal that fits the original-equipment mindset.
- OEM-grade gray RTV formulated to original-equipment sealing standards
- Resists gear oil and coolant across a wide service temperature range
- Flexible cured bead that follows cover and housing movement
Pros: Backed by a recognized OEM parts brand; Reliable, no-surprises seal on factory covers; Good oil resistance for normal service intervals
Cons: Less gap-fill capability than a dedicated maximum-oil RTV; Availability can be spottier than the big sealant brands
Frequently Asked Questions
Do I need a paper gasket and gasket maker, or just the sealant?
For most differential covers you do not need both, and mixing them often causes leaks. Modern practice is to clean both faces thoroughly, lay a continuous bead of RTV gasket maker around the bolt holes, and torque the cover down. The RTV forms the gasket itself. If your cover came with a reusable rubber or molded gasket, follow that design and use only a thin film of sealant where the maker recommends it. Stacking a thick RTV bead on top of a paper gasket creates an uneven, squishy joint that can weep, so pick one sealing method and do it properly.
How long should the gasket maker cure before I add gear oil?
For a standard RTV silicone, give it a full overnight cure, ideally 24 hours, before you fill the differential. RTV skins over in minutes but needs that longer window to cure all the way through the bead, and adding oil too soon is the most common reason a fresh reseal weeps. The exceptions are fast-seal products like The Right Stuff, which seal on contact and let you refill almost immediately. Anaerobic flange sealants also cure quickly between the clamped metal faces. When in doubt, wait the night out, since patience here saves you from doing the whole job twice.
Will the gasket maker hold up against limited-slip and hot gear oil?
Yes, as long as you choose an oil-resistant RTV rather than a general-purpose sealant. Products labeled for maximum oil resistance, such as Permatex Ultra Black and the copper Loctite, are specifically formulated to resist hot 75W-90 and the friction-modifier additives used in limited-slip differentials. A cheap, general silicone can soften or break down in that environment and start to leak. The friction modifier you add for a limited-slip clutch pack does not attack a proper oil-resistant gasket maker, so the seal and the additive coexist fine when you use the right product.
How thick should the bead of gasket maker be on a differential cover?
Aim for a continuous bead roughly an eighth of an inch thick, run on the inboard side of the bolt holes so the holes are encircled. Too thin and you risk gaps that weep, too thick and the excess squeezes into the housing where it can break loose and clog a passage or stick to the diff magnet. Make sure the bead is unbroken and loops fully around each bolt hole. Then torque the cover down evenly in a crisscross pattern to the axle specification so the sealant spreads into a thin, uniform film without being crushed out completely.
How do I remove the old gasket maker before resealing?
Surface prep decides whether the new seal lasts, so take your time here. Scrape off the old sealant with a plastic or brass scraper to avoid gouging the soft aluminum or steel face, then knock down stubborn residue. Wipe both the cover and the housing flange with a brake cleaner or acetone until the rag comes away clean, since any oil film will keep the new gasket maker from bonding. Let the solvent flash off completely and avoid touching the bare faces with greasy fingers before you apply the fresh bead. A clean, dry, oil-free surface is what makes a leak-free reseal possible.
Our Verdict
For nearly every differential cover, the Permatex 81158 Ultra Black is our top pick, since its maximum oil resistance, flexible cure, and clean peel-off at the next service make it the most foolproof sealant for the job. If your axle is a rigid, high-torque setup with closely spaced bolts, the Permatex 81160 Ultra Grey is the runner up worth reaching for. Buyers who simply cannot wait overnight should keep a can of The Right Stuff on hand for a near-instant seal, while a rigid, well-machined factory flange is a great home for the anaerobic Permatex 51813. Prep the faces clean, lay a proper bead, respect the cure time, and any of these seven will keep your differential dry.
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