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

Selleys Instant Gasket is a tough R.T.V (Room Temperature Vulcanized) silicone which forms gaskets instantly around the tube: any size, any shape. It cures to a tough silicone rubber, and it replaces conventional cork gaskets. The manufacturer describes it as perfect for sealing out oil, dirt and dust.

The practical reason to reach for it is that you stop needing to source a gasket that fits. Instead of finding the right pre-cut part for a gearbox housing or an odd rocker cover, you lay a bead to the shape of the flange and let it cure in place. The trade-off is timing: you have ten minutes to get the parts together, so the job has to be set up before you open the tube.

Technical specification

Property	Typical result
Technology	100% acetoxy silicone
Colour	Black; also supplied in grey
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	370%
Movement capability (ASTM C719)	25%
Service temperature (ASTM C1246:2017)	−40°C to 205°C
Intermittent temperature resistance	Up to 260°C
VOC (USEPA Test Method 24)	43.68 g/L
Bead thickness	1.5–3mm
Packaging	100ml tube in black, 100ml tube in grey

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

What the key numbers mean in practice

The temperature ratings are two different things. The service temperature — the range the seal is designed to work in — is −40°C to 205°C. The 260°C figure is intermittent resistance: it survives excursions to that temperature, it is not rated to sit there. If your application runs continuously hotter than 205°C, this is the wrong product and the High Temperature version is what the range offers instead, with a 260°C service rating.

Skin time is 5 minutes and the assembly window is 10. That is the timing that decides whether the job works. The bead has to wet out against the second face, and a bead that has started to skin will not. Have the mating part, the bolts and the tools ready before you lay the sealant, particularly on a large flange where running the bead itself takes a minute or two.

The two-stage torque sequence is not optional. Hand-screw the bolts back until the sealant is just starting to squeeze out, wait 30 minutes, then torque properly. Going straight to full torque squeezes the bead out of the joint and leaves metal on metal where the gasket should be. The 30-minute set gives the silicone enough body to hold its thickness under clamping load.

Shore A hardness of 18 with 370% elongation at break describes a soft, very stretchy rubber. That is what lets a formed-in-place gasket conform to a flange that is not perfectly flat, and what lets it absorb the vibration of a running engine without tearing. The 25% movement capability is the joint movement the cured seal will tolerate.

Where to use it

The manufacturer lists it as perfect for use on:

• Gaskets
• Gearboxes
• Water pumps
• Oil pans
• Rocker covers
• Timing chain covers
• Differential and thermostat housings
• Lawn mowers
• Boats

It is resistant to water, oil and grease, and it is sensor safe for use in engines — worth knowing before you seal anything upstream of an oxygen sensor.

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 the joint is one you expect to open 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 at a 45° angle.
3. Squeeze the tube with steady pressure to form a 1.5–3mm thick bead onto one of the surfaces to be joined, around the perimeter of the part and around the bolt holes. Taking the bead around the inside of the bolt holes rather than outside them is what stops oil tracking out along a bolt shank.
4. Join the parts together within 10 minutes, hand-screwing 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 — refilling a sump or a gearbox — allow 90 minutes. Full cure is still 24 hours.

Limitations — where it should NOT be used

• This sealant did not adhere well to natural rubber, and has weak adhesion on EPDM.
• Not suitable for structural glazing and assembly applications.
• Not suitable for PE, PP or PTFE.

The EPDM point deserves attention because EPDM is common in automotive and appliance seals, and weak adhesion is the kind of failure that shows up weeks later rather than at assembly. PE, PP and PTFE are low-surface-energy plastics that silicone will not grip at all.

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 is single-use; a fresh bead laid over cured silicone will not seal reliably.

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. Let the sealant cure in the nozzle between jobs and pull the plug out before the next use rather than trying to clear it straight away.

Hazard classification

Selleys Auto Fix Instant Gasket Silicone R.T.V. Black is not classified as hazardous under the criteria of Safe Work Australia GHS 7. No signal word is assigned, no hazard statements apply, and the poison schedule is not applicable. 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 one component the documentation names is cyclotetrasiloxane, octamethyl- (CAS 556-67-2) at under 1% w/w. The full quantitative table is held in the safety data sheet and is not reproduced in the material available here, so nothing further is stated about it.

Setting that against its stablemates is the useful comparison. Within the same small range of auto sealants, this one and the High Temp Red version are not classified at all, while the neutral-cure Auto Fix Auto Sealant and Windscreen Sealant both carry H317 and H319 — skin sensitisation and serious eye irritation. Same shelf, same size tube, materially different precautions. The split follows the cure chemistry: the two acetoxy gasket makers are on one side of it and the two oxime neutral-cure sealants on the other.

Ordinary precautions are still worth observing. The product cures by reacting with moisture in the air, and acetoxy systems as a class release acetic acid while they do it — the vinegar smell — so work with the bonnet up or the door open rather than in a sealed space. Keep it out of your eyes, wash your hands before eating or smoking, and store the tube upright and capped in a cool, dry place away from heat and ignition sources.

Common questions about Selleys Auto Fix Instant Gasket

Does it really replace a cork gasket?

Yes. It is described as a tough R.T.V silicone rubber which replaces conventional cork gaskets, and it forms gaskets of any size or shape around the tube.

How long do I have to assemble the parts?

Ten minutes. The sealant skins in five, so set the job up before you start.

When can I torque the bolts?

Hand-screw them until the 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 hot can it run?

Service temperature is −40°C to 205°C, with intermittent resistance up to 260°C.

Is it safe around engine sensors?

Yes — it is sensor safe for use in engines.

How thick should the bead be?

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

How do I make a thicker gasket?

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

What if I want the joint to come apart easily later?

Wipe the mating surfaces with an oily cloth before applying, so the gasket does not adhere.

Will it stick to rubber seals?

It did not adhere well to natural rubber and has weak adhesion on EPDM.

Will it stick to plastic?

Not to PE, PP or PTFE.

Is it resistant to oil?

Yes — it is resistant to water, oil and grease.

Can I use it on a lawn mower or an outboard?

Lawn mowers and boats are both among the listed uses, along with gearboxes, water pumps, oil pans, rocker covers, timing chain covers, and differential and thermostat housings.

What surface preparation does it need?

Remove the 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.

Does it matter whether I use black or grey?

Both are supplied as 100ml tubes of the same 100% acetoxy silicone. The technical data sheet quotes its figures for the black.

What colours and sizes are available?

100ml tubes in black and in grey.

How do I clean up?

Mineral turpentine, before the sealant cures.