Anaerobic Sealant vs RTV: Which Suits Your Machine Flange?

This comparison covers formed-in-place gaskets on bolted machinery flanges such as gearbox, pump, compressor and engine covers. It does not cover standardized pipe flanges with cut gaskets, thread sealing, building joint sealants, or oxygen, chlorine and potable water service, where approvals or the maker's written confirmation decide.

Which conditions decide anaerobic sealant or RTV silicone on a flange?

Six conditions decide anaerobic sealant or RTV silicone on a machine flange: flange stiffness, the measured gap, the face material, the contact fluid, temperature and pressure, and how often the joint is opened. Anaerobic compounds are most often used on rigid metal flanges and cure to a tough thermoset plastic in the absence of oxygen, while silicone gasketing cures with atmospheric moisture and seals pressed or rolled sheet metal parts (Plant Engineering, 2000). Stiffness and gap usually settle the chemistry; the other conditions rule grades in or out within it.

Measure each condition first. Take stiffness and gap from the drawing or from a feeler gauge check at several points on the unclamped flange. Name the actual fluid on the inside of the joint, including flushing and cleaning fluids, with its service temperature. Use the peak pressure in service, not the nominal value, and count planned openings per year.

Then separate requirements from preferences. A requirement changes the answer: a stamped cover, a 0.4 mm gap, fuel on the inside. A preference such as bead color or a familiar tube size does not, and should not knock out a grade that meets every requirement. In Lubinpla, a product recommendation case fixes the flange specification and the contact fluid as requirements and keeps preferences that do not change the result out of the must-have list before any grade is compared.

How do anaerobic sealant and RTV silicone differ on a flange?

Anaerobic sealant and RTV silicone differ on a flange mainly in stiffness, gap, pressure and fluid resistance: the anaerobic cures hard between close metal faces, while the silicone cures to a rubber that stretches with the joint. The table below compares typical chemistry-level values from a 2000 trade article by a Loctite application engineer; read the grade's own technical data sheet (TDS) for the product in hand.

The pressure row decides many process flanges: Plant Engineering rates anaerobic gasketing for all pressures up to 34 MPa, against low pressures and a 1.75 MPa maximum for RTV silicone (Plant Engineering, 2000). Elongation explains the rest of the split. At 250 to 600%, silicone follows a flexing cover and joints with large differential thermal expansion; an anaerobic at 5 to 64% does not stretch that far, so it needs a rigid flange. An anaerobic joint gives metal-to-metal contact, so bolts, flange and gasket act as a unit and the joint has no compression set to retorque (Plant Engineering, 2000).

Anaerobic or RTV recommendation by flange condition

The anaerobic or RTV recommendation by flange condition follows one rule: the requirement that only one chemistry meets decides, and the grade's TDS confirms the value. The table below gives the recommended type for common machine flange conditions; read every row that applies, because one flange can match several.

The row that settles most cases is the first: a rigid machined metal flange with a measured gap inside the anaerobic grade's rating takes an anaerobic sealant. A stamped cover, a plastic face or a gap beyond that rating moves the choice to RTV silicone. When rows conflict, for example a stamped cover with fuel on the inside, neither type fits cleanly on the data above, so the answer is a different gasket design or the maker's written confirmation, not a thicker bead. For what a written recommendation to a buyer should carry beyond the chosen type, see What should a chemical product recommendation to a buyer include?.

Which gasket maker suits a flange gap above 0.25 mm?

RTV silicone suits a flange gap above 0.25 mm in most cases, because a typical anaerobic flange grade is rated only to 0.25 mm. One maker's data sheet for a general purpose anaerobic flange sealant states: "The product is designed for close fitting flanged parts with gaps up to 0.25 mm (0.01 in)." (Henkel LOCTITE 518 TDS, 2021). Incure places RTV silicone at larger, non-uniform gaps of 0.5 mm or more (Incure, July 2026).

Between 0.25 mm and about 0.5 mm, check two routes before defaulting to silicone. Some anaerobic grades carry a wider gap rating; Henkel lists a larger gap filling anaerobic product for close-fitting rigid flanges (Henkel Adhesives, accessed 2026). For the general grade, the same data sheet says applying activator to the surface improves cure speed where cure is too slow or large gaps are present (Henkel LOCTITE 518 TDS, 2021). The anaerobic route still needs the gap to sit inside the rating of the exact grade. Incure warns that forcing an anaerobic product onto a joint outside its gap tolerance leaves voids and leaks soon after assembly, and that a thicker bead does not fix the mismatch (Incure, July 2026).

Measure the gap with a feeler gauge at several points on the unclamped flange, as Incure suggests, and compare the largest reading, not the average, with the grade's rated gap.

Which gasket maker resists oil, coolant or fuel at the flange?

An anaerobic gasket maker resists fuel and aromatic solvents better than RTV silicone, while both types can handle engine and gear oils within the chosen grade's temperature limit. Plant Engineering rates anaerobics excellent against all automotive fluids and hot oil, and silicone fair against solvents with no fuel or aromatics, and fair to good against hot oil (Plant Engineering, 2000). Silicone grades made for oil sumps and timing covers exist: one oxime RTV data sheet lists "Excellent resistance to automotive engine oils" as a specific benefit and reports ageing in motor oil at 150 °C (Henkel LOCTITE SI 5910 TDS, 2019).

Coolant and water-glycol contact need the grade's own ageing data at the service temperature. Both data sheets cited here report strength after water and glycol exposure (Henkel LOCTITE SI 5910 TDS, 2019; Henkel LOCTITE 518 TDS, 2021), and the anaerobic sheet notes that its environmental table "is not a measure of sealing performance" (Henkel LOCTITE 518 TDS, 2021).

For a process fluid outside these lists, such as an acid, a solvent blend or a heat transfer fluid, ask the maker for immersion data at the service temperature before choosing either type, and record the answer with the grade name and the data sheet date. Neither of the two grades cited here should go on a joint carrying pure oxygen, chlorine or another strong oxidizer, as both data sheets advise (Henkel LOCTITE SI 5910 TDS, 2019; Henkel LOCTITE 518 TDS, 2021).

Does an anaerobic flange sealant work on a plastic or stamped sheet metal cover?

An anaerobic flange sealant is a poor fit for a plastic cover and for a flexible stamped sheet metal cover, and RTV silicone is the usual choice for both. One maker's anaerobic flange grade cures when confined in the absence of air between close fitting metal surfaces, and its data sheet says the product "is not normally recommended for use on plastics (particularly thermoplastic materials where stress cracking of the plastic could result)" (Henkel LOCTITE 518 TDS, 2021). Plant Engineering's table lists all metals, thermosets and crystalline thermoplastics as compatible substrates for anaerobics (Plant Engineering, 2000), so the sources differ; the grade's own TDS and a trial on the actual plastic decide.

Stamped sheet metal is hard to keep flat at the joint face, which is why such covers traditionally used thick, compressible cut gaskets such as cork composite (Plant Engineering, 2000). The cover also flexes in service, and an anaerobic film at 5 to 64% elongation is not made to follow it. Henkel offers anaerobic liquid gaskets for rigid machine flanges and silicone liquid gaskets for flexible or stamped flanges (Henkel Adhesives, accessed 2026). The exception is the structural stamping: Plant Engineering lists close-fitting machined castings and structural stampings as the flanges suited to anaerobic gaskets (Plant Engineering, 2000). A heavy, flat, stiff pressed part can take an anaerobic sealant; a thin cover that moves cannot.

Which gasket maker suits a flange that must come apart often?

For a flange that must come apart often, a low-strength RTV silicone is usually easier to separate than an anaerobic sealant, which locks the faces into metal-to-metal contact. Henkel describes its general purpose anaerobic flange grade as medium strength and its silicone grade for flexible flanges as low strength (Henkel Adhesives, accessed 2026). The silicone grade's data sheet gives a lap shear strength of 0.9 to 1.4 N/mm2 on mild steel at a 0.5 mm gap after 21 days at 23 °C (Henkel LOCTITE SI 5910 TDS, 2019).

Separation is only part of the cost of frequent opening. Every opening means removing old material from both faces and cleaning to bare, grease-free metal before the next bead, since both data sheets call for clean surfaces free from grease for best performance (Henkel LOCTITE SI 5910 TDS, 2019; Henkel LOCTITE 518 TDS, 2021). Count the time back to service too. The oxime grade's sheet suggests letting the bond cure, for example seven days, before heavy service loads (Henkel LOCTITE SI 5910 TDS, 2019), while the anaerobic grade resists low pressures immediately after assembly of flanges (Henkel LOCTITE 518 TDS, 2021).

A joint that opens every few weeks and must return to service within the same shift is likely better served by a reusable cut gasket or an O-ring groove than by either gasket maker.

Why do published gap limits for anaerobic gasket makers differ?

Published gap limits for anaerobic gasket makers differ because sources state different things: a grade's rated sealing gap, a chemistry-wide cure through depth, or a rounded rule of thumb. One maker's data sheet rates its general purpose anaerobic flange grade to 0.25 mm (Henkel LOCTITE 518 TDS, 2021), and one Incure page gives the same figure, "typically up to roughly 0.25 mm" (Incure, July 2026). Another Incure page says anaerobic gasket makers are "limited to gaps under roughly 0.5mm" (Incure, August 2026). Plant Engineering gives a cure through volume of 0 to 1.25 mm for anaerobics (Plant Engineering, 2000).

The figures answer different questions. A cure through figure most likely describes how thick a film can harden, while a rated gap is what the maker tested for sealing a flange. The 518 data sheet shows why the rated figure is the stricter one: uncured pressure resistance falls from 1.35 MPa at an induced gap of 0.05 mm to 0.1 MPa at 0.25 mm, and the rate of cure depends on the bondline gap (Henkel LOCTITE 518 TDS, 2021). Rounded web figures carry no test condition at all.

Use the rated gap of the exact anaerobic grade, from its current TDS, as the limit. Treat any wider figure from a general article as a reason to ask the maker for data at that gap.