What is Selleys Auto Fix Instant Gasket High Temperature Sealant, and where should you use it?

Selleys Instant Gasket High Temperature is a high-strength sealant designed for long-term reliability in high-heat applications. It can be applied in multiple spaces and on uneven surfaces to form a tough, high-pressure red seal that resists intermittent temperature up to 343°C. In service it is rated from −40°C to 260°C.

The one thing to establish before you choose it is what it is made of: this is a 100% acetoxy silicone. Acetoxy — acetic cure — is what gives it the heat performance, and it is also why the manufacturer states plainly that it is not for use on painted surfaces. If your job involves paintwork, this is the wrong product from the auto sealant range, and no amount of care in application changes that.

Technical specification

Property	Typical result
Technology	100% acetoxy silicone
Colour	Red
Density	1.02 g/ml
Application temperature	−20°C to 50°C
Skin time	5 minutes
Set time	30 minutes
Full cure	24 hours
Shore A hardness (ASTM C661)	18
Elongation at break	300%
Movement capability (ASTM C719)	25%
Service temperature (ASTM C1246:2017)	−40°C to 260°C
Intermittent temperature resistance	Up to 343°C
VOC (USEPA Test Method 24)	43.68 g/L
Bead thickness	1.5–3mm
Packaging	100ml tube, red

These are indicative data only, not intended for the preparation of specifications.

What the key numbers mean in practice

Continuous versus intermittent temperature is the distinction that matters most. The service temperature is −40°C to 260°C — that is the range the seal is built to live in. The 343°C figure is intermittent resistance: short excursions, not a working condition. A joint that sits at 300°C all day is outside what this product is rated for, even though it survives a spike to 343°C.

The 10-minute assembly window is the tightest constraint on the job. The sealant skins in 5 minutes and you must join the parts within 10 minutes. That means the bead should be the last thing you do before assembly, with the mating part, the bolts and the tools already to hand. Applying a bead around a large flange and then hunting for a socket will put you outside the window, and a bead that has begun skinning will not wet out against the second face.

The torque sequence is deliberately two-stage. You hand-screw the bolts back until the sealant is just starting to squeeze out, allow 30 minutes, and only then tighten to the desired torque. Going straight to full torque squeezes almost all the sealant out of the joint and leaves you with metal-to-metal contact where the gasket should be. The 30-minute wait lets the bead set enough to hold its thickness under load.

Shore A hardness of 18 is very soft, and elongation at break of 300% is high. Together they describe a gasket that conforms to an uneven flange rather than resisting it — which is exactly the point of a formed-in-place gasket, and why the manufacturer says it can be applied on uneven surfaces. The 25% movement capability means the cured seal tolerates the joint opening or closing by a quarter of its width, which is what absorbs vibration and thermal cycling in a running engine.

Where to use it

The manufacturer lists it as perfect for use on:

• Gaskets
• Mufflers
• Oil pans
• Valve covers
• Tail pipes
• Exhaust pipe connectors
• Dishwashers
• Refrigerators
• Oven doors
• Plumbing
• Heating and air conditioning
• Flanges of all kinds

It is also described as a crack, joint and seam sealant, suited to high torque and vibration usage, and sensor safe — which matters in a modern engine bay where oxygen sensors sit downstream of anything you seal.

How to apply it

1. Remove old gasket material and thoroughly clean the surfaces with mineral turpentine, then dry them. The surface must be left with no dirt and no moisture. If you do not want the gasket to adhere — because you expect to disassemble it again — wipe the mating surfaces with an oily cloth.
2. Remove the cap, unscrew the nozzle and remove the seal. Replace the nozzle and cut it at a 45° angle.
3. Squeeze the tube with steady pressure to form a 1.5–3mm thick bead. Apply it onto one of the surfaces to be joined, around the perimeter of the part and around the bolt holes. Running the bead inside the bolt holes rather than outside them is what keeps oil from tracking out along a bolt.
4. Join the parts together within 10 minutes and hand-screw the bolts back onto the gasket until the sealant is just starting to squeeze out. Allow to dry for 30 minutes.
5. Tighten the bolts to the desired torque. If a thicker gasket is required, assemble but do not tighten the bolts for 24 hours. The material cures in 24 hours.
6. Clean up before the sealant cures, with mineral turpentine.
7. After use, replace the cap. Allow the sealant to cure in the nozzle and remove that plug before the next use.

Before filling with liquid — oil into a sump, coolant into a housing — allow 90 minutes. Full cure is still 24 hours.

Limitations — where it should NOT be used

• Not for use on painted surfaces.
• Not suitable for structural glazing and assembly applications.
• Not for permanent submersion in water.
• Not suitable for PE, PP or PTFE.

The paint restriction is the one to plan around, because a flange is often adjacent to painted metal. Mask what you cannot afford to have the sealant contact, and be aware that this product is a different chemistry from the neutral-cure sealants in the same range — the general-purpose Auto Fix Auto Sealant is the neutral cure product that lists paint surfaces among its uses.

PE, PP and PTFE are low-surface-energy plastics that silicone will not grip. "Not for permanent submersion" rules out anything that lives underwater; a joint that gets wet and dries is a different case from one that stays immersed.

Disassembly, clean up and storage

If disassembly is required, pry the two flanges apart and remove the old sealant before reassembly. A formed-in-place gasket cannot be reused, and a new bead laid over old cured silicone will not seal.

Clean up before the sealant cures, with mineral turpentine. Store in a dry, cool place below 30°C, and use within 24 months from the date of production.

Hazard classification

Selleys Auto Fix Instant Gasket High Temp Red is not classified as hazardous under the criteria of Safe Work Australia GHS 7. No signal word is assigned, no hazard statements apply, and no poison schedule applies. It is not classified as Dangerous Goods under the Australian Code for the Transport of Dangerous Goods by Road & Rail or New Zealand NZS5433.

The composition section is almost empty, which is the reason. The only entity named is cyclotetrasiloxane, octamethyl- (CAS 556-67-2) at under 1% w/w, with the balance made up of ingredients determined to be non-hazardous or below reporting limits.

That is a genuinely unusual result for an engine-bay product, and it is worth setting against the restriction the page has already given you. The reason this sealant is not for painted surfaces is not toxicity — it is that acetoxy cure chemistry, as a class, releases acetic acid as it crosslinks, which is what attacks susceptible finishes and is the source of the vinegar smell when you open the tube. That is the chemical reading of an acetoxy system rather than something the safety data sheet states. The hazard the product does carry is to the paintwork, not to you.

The ordinary precautions still apply. Work with the bonnet up rather than in a closed garage, keep it out of your eyes, and wash your hands before eating or smoking. Store it upright in a cool, dry place away from heat and ignition sources, with the cap on — a half-used tube of moisture-curing silicone that has been left open is usually a solid lump by the next job.

Common questions about Auto Fix Instant Gasket High Temperature Sealant

How hot can it go?

Service temperature is −40°C to 260°C, with intermittent temperature resistance up to 343°C. The 343°C figure is for short excursions, not continuous operation.

How long before I can torque the bolts?

Hand-screw them until sealant just starts to squeeze out, wait 30 minutes, then tighten to the desired torque.

How long before I can refill with oil?

Allow 90 minutes before filling with liquid. Full cure is 24 hours.

How thick should the bead be?

1.5 to 3mm, run around the perimeter of the part and around the bolt holes.

How long do I have to assemble after applying it?

Ten minutes. It skins in five, so have everything ready before you lay the bead.

Can I use it on painted surfaces?

No. It is not for use on painted surfaces.

Is it safe around oxygen sensors?

It is listed as sensor safe.

Can I make the gasket thicker?

Yes. Assemble the parts but do not tighten the bolts for 24 hours.

What if I want to take it apart again later?

If adhesion of the gasket is not required, wipe the mating surfaces with an oily cloth before applying. To disassemble, pry the flanges apart and remove the old sealant before reassembly.

Will it stick to plastic?

Not to PE, PP or PTFE.

Can it be submerged?

No, it is not suitable for permanent submersion in water.

What size and colour is it?

A 100ml tube, red.

How do I clean up?

Mineral turpentine, before the sealant cures.