If you're not already familiar with the terms "aerobic," "anaerobic" and "RTV", you probably should be. These are the kinds of sealant that have replaced many cork and rubber gaskets on vehicle assemblies.
The terms refer to the curing properties of the sealant. Aerobic means that the sealant cures in the presence of air and can be used on flexible flanges and between machined parts. However, it should not be used where it might squeeze out and plug small passages. Parts must be assembled immediately or the sealant will harden.
RTV sealant is another name for a type of aerobic sealant, standing for Room Temperature Vulcanizing. Aerobic sealants are often identified as RTV silicone rubber compounds, under names such as GM, GE, Permatex® , Devcon® , Dow Corning, MOPAR® , FelPro® , Loctite® , or Versa Chem® .
Anaerobic sealants are those that cure in the absence of air. In other words, the sealant will not cure (harden) until the parts are assembled and the air is denied. Anaerobic sealants are for use between smooth, machined surfaces, but should not be used between flexible mounting flanges. They should also be applied sparingly in a continuous bead to a clean surface.
Uncured aerobic or RTV sealants can be wiped off with a rag. Cured sealants can be removed with a scraper, wire brush or common shop solvents.
| Fig. 1: Anaerobic sealant is available in several types
from a variety of manufacturers
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| Fig. 2: Epoxy systems are available for metal and plastics
and have different drying times
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| Fig. 3: RTV comes in various colors indicating specific
applications. Once again read the package.
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There are more thread sealants than can be counted. Add to these the several sealant tapes now on the market and the confusion can be great. Mechanics should be aware of the anaerobic sealant with Teflon® filler that can be used on all joints. (GM Truck has adopted it as universal sealant.) "Pipe Sealant with Teflon" is applied to threads. It creates an instant seal, but does not cure for 24 hours. This permits making changes if needed. Once hardened it prevents vibration-induced loosening.
Anaerobics: Clean surfaces with solvent and apply bead to one surface. Material will not begin to cure until parts are assembled. Sealing is effective in half an hour. Full cure is complete in 2 1⁄2 -10 hours depending upon temperature. Cold slows cure.
Aerobic or Silicone sealants: Clean and dry surfaces. Apply bead and let cure for two hours. To make a gasket that will cling to only one surface, apply bead to one surface and allow it to cure. Then apply grease to other surface, and assemble. Or, to make a gasket that will bond to both surfaces, apply and assemble. This will provide maximum blowout resistance. Material will cure to depth of 1⁄4 inch in 24 hours.
The basic guide in choosing a sealant is the size of the gap. Anaerobic materials are used only on smooth, rigid, machine-surfaced flanges which have a total gap less than .030 inch (.301mm). Silicones are used in parts that may flex (such as metal-stamping covers) and which have gaps that are more than .030 inch (.301mm) but not more than 0.25 inch (6.35mm). Both materials are impervious to the normal automotive fluids such as gas, oil, coolants and hydraulics. Anaerobics have a temperature range of --60-300°F (15-149°C), and silicones will handle--100- 450°F (38-232°C).
Anaerobics: Common applications for the anaerobic materials include fuel pumps, timing covers, oil pumps, water pumps, thermostat housings, oil filter adapters, manual transmission housings, differential covers and other rigid parts. Bear in mind that anaerobic materials add rigidity to the assembly because they help lock the surfaces.
Aerobic or Silicone sealants: Many silicone applications involve stamped metal housings such as oil pans, valve covers, and other parts such as intake manifolds, transmission covers, axle covers and rear main bearing seals.
Solvent release: Non-hardening sealants are used to repair cut gaskets on both rigid and flexible assemblies that operate at high temperatures up to 600°F (315 deg.C). On semi-permanent assemblies, the materials set quickly to bolster the conventional gasket. By remaining pliable, they permit easy removal later.
Hardening sealants dry fast and hard and are used on permanent assemblies to aid the conventional gasket, particularly when the flanges are damaged.
Most sealants also aid in assembly by holding the gasket in place during assembly. When such positioning problems are extremely difficult, a gasket adhesive can be used to hold the gasket in perfect alignment during assembly.
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