What is dielectric grease? Where to use it and where not

By Grease Monkey Direct, UK independent trade supplier of auto-electrical parts and workshop consumables. Last updated 13 November 2026

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Dielectric grease is a non-conductive silicone compound that seals moisture out of electrical connections, so it goes over the outside of a clamped battery terminal and inside spark plug boots, never between metal contact faces or on plug threads.

It is one of those products nearly everyone in a workshop has used and few people agree on. Some say smear it on battery terminals, others say keep it off anything that carries current, and the same arguments come up with spark plugs, where the difference between the boot and the threads decides whether you have protected the connection or caused a misfire.

The stakes are not trivial. The AA reports that 12-volt battery faults accounted for 24% of its winter breakdown callouts between December and February, and the state of the terminals is often part of the problem. If you want the wider picture on battery health, our data page on how long car batteries last covers typical lifespans and what shortens them.

What is dielectric grease?

Dielectric grease is a silicone compound that insulates and seals rather than conducts. Permatex describes its Dielectric Tune-Up Grease as a silicone dielectric compound that prevents voltage leakage around electrical connectors and stops spark plug boots fusing to the plug. The product is rated for use from -54°C to 204°C. In practice that means a thin layer keeps water, dirt and corrosion away from a connection while leaving the metal surfaces to do the conducting themselves.

Because it is an insulator, the grease itself blocks current rather than carrying it. DuPont's data sheet for MOLYKOTE 4 Electrical Insulating Compound, a comparable product, lists a typical dielectric strength of 338 volts per mil under ASTM D149 with a 50 mil gap, which is a measure of how much voltage the compound can hold back before it breaks down. That insulating behaviour is exactly why placement matters so much, a theme we look at across our guide to workshop greases.

Is dielectric grease conductive?

No, dielectric grease is an electrical insulator, and that is the whole point. The DuPont data sheet for MOLYKOTE 4 lists a typical dielectric strength of 338 V/mil (ASTM D149, 50 mil gap) and a typical volume resistivity of 4.62 x 10^13 ohm-cm at 23°C. MG Chemicals rates its 8462 silicone dielectric grease at 1,300 V/mil dielectric strength. Both figures point to a material that blocks current rather than carrying it.

That still leaves room for it on a connector, because the grease is not meant to sit between the metal contacts. When the connector is mated, the contacts press through the thin film and wipe it aside, so metal still touches metal and current flows as normal. The grease then seals around the joint, keeping out the moisture and corrosion that would cause voltage leakage or a poor connection over time.

Should you put dielectric grease on battery terminals?

Yes, but only on the outside of the finished joint, never between the clamp and the post. MG Chemicals advises applying the grease over the top of the lug and keeping it off any part where the clamps contact the battery, because it is not conductive and would sit in the current path. Where the clamp meets the post, MG Chemicals recommends electrically conductive grease instead if you want to improve current flow at that interface.

Permatex takes a broader view and lists battery terminals among the typical applications for its Dielectric Tune-Up Grease, alongside spark plug boots, distributor cap nipples, ignition coil connectors, headlamp connectors and trailer electrical connectors. There is no real conflict between the two: make a clean, tight metal-to-metal joint first, then seal the outside of it with the grease to keep corrosion out.

The order of work matters. Clean the post and clamp, tighten the joint, then coat the outside of the assembly, topping up any replacement battery terminals the same way. AAA notes that dirty, loose connections limit the flow of current and are a common cause of battery issues, so treat the grease as the last step, not a substitute for a sound connection. A battery terminal cleaner and protector spray can handle the cleaning and light protection stages before the grease goes on.

Does dielectric grease go on spark plug threads?

No, it never goes on spark plug threads. Permatex directs users to coat the inside of the spark plug boot with a thin layer and to coat the ceramic insulator of the plug. The threads themselves stay clean and dry so the plug can be tightened to its specified torque.

NGK Spark Plugs explains that its plugs are installed at the factory dry, without lubrication or anti-seize, because the trivalent plating acts as a release agent. NGK also warns that anti-seize can act as a lubricant and alter torque values by up to 20 percent, increasing the risk of thread breakage or metal shell stretch. Copper grease is a separate product with its own uses, and our guide covers what copper grease is used for if you are looking for thread protection on other jobs.

Torque accuracy is the reason the threads stay bare. NGK notes that metal shell stretch from over-tightening changes the heat rating of the spark plug and can result in serious engine damage caused by pre-ignition. Fitting plugs with a dedicated spark plug socket set helps you seat them squarely and reach the specified torque without slipping.

Where should you use dielectric grease, and where not?

Use the grease on spark plug boots, ignition coil connectors, headlamp connectors, trailer electrical connectors, rubber seals and other sealed joints. Permatex lists spark plug boots, distributor cap nipples, battery terminals, ignition coil connectors, headlamp connectors and trailer electrical connectors as typical applications. DuPont describes its MOLYKOTE 4 electrical insulating compound in similar terms, as a lubricant and seal for cable connectors, battery terminals, rubber seals and O-rings, and as a moisture-proof seal for ignition systems and sealed electrical connectors. That makes it a good match for sealed Superseal connectors and for trailer leads and plugs that spend their lives outside the vehicle.

Keep it off the places where it causes problems: never between contact faces, never on spark plug threads, and never on a surface that is going to be painted. On the last point, DuPont advises that silicone compounds should not be applied to surfaces that require painting or coating. For battery terminals, the rule from earlier in this guide still applies: clean and tighten the joint first, then seal over the outside of the clamped connection.

Dielectric grease vs silicone, copper and conductive grease

Dielectric grease is usually a silicone grease formulated for electrical work. "Silicone grease" on its own is a broader term, and it covers general-purpose lubricants that were never meant for connections. MG Chemicals sells its 8462 as a silicone dielectric grease, and DuPont describes MOLYKOTE 4 as an electrical insulating compound used on cable connectors, battery terminals, rubber seals and O-rings. The common workshop greases and sprays overlap enough to cause mix-ups, so the table below sets out where they differ.

One caution applies across the silicone family. DuPont advises against applying silicone compounds to any surface that will be painted or coated, so keep grease and spray off panels due for paint. For rubber care, an aerosol silicone spray is the quick option, while our workshop grease range covers the copper and anti-seize side of the job.

How to apply dielectric grease

Use it as a thin sealing film over a finished joint or inside a boot, never between the contact faces that carry the current. The steps below cover both battery terminals and spark plug boots.

1. Disconnect and clean. Take the battery lead off or pull the spark plug boot free, then clean away dirt, corrosion and old grease so you are sealing a sound surface.
2. Check the terminals or pins. Look for corrosion, bent pins, cracked boots or stretched clamps, and replace anything worn before you seal over it.
3. Mate the connection. Clamp the lead to the battery post and tighten it fully, or push the connector home, so the metal joint is made dry and tight first.
4. Apply a thin film. Spread a light coat over the outside of the clamped joint, or inside the boot where it slides over the insulator, keeping it off the contact faces.
5. Wipe away the excess. A thin film is all it takes. Heavy smears attract grit and can work loose.
6. Re-check after a few runs. Confirm the joint is still tight and the boot is seated, and top up the film if it has been displaced.

If you are replacing terminals or rebuilding part of the loom while you are in there, our guide to choosing electrical connectors covers the options in more detail.

Sources

1. Permatex, accessed 2026-11-02
2. DuPont (MOLYKOTE), accessed 2026-11-02
3. MG Chemicals, accessed 2026-11-02
4. MG Chemicals, accessed 2026-11-02
5. NGK Spark Plugs, accessed 2026-11-02
6. The AA, accessed 2026-11-02
7. AAA Oregon/Idaho, accessed 2026-11-02