Hylomar — the workshop’s secret weapon

Hylomar is a non-setting jointing compound. That is the whole point, and what sets it apart from almost everything else on the shelf. Silicone cures to rubber. Anaerobic gasket cures to plastic. Hylomar stays as a tacky, waxy film — today, in a year, and in ten years.

It seals by filling the imperfections between two faces. No machined face is perfectly flat; there are small scratches, pores and tool marks where oil and water find a way. Hylomar settles into them. The faces still meet metal to metal, so dimensions and clearances do not change. That is why it can be used on cylinder heads and bearing caps where a thick gasket would give the wrong clearance.

And because it never sets, it reseals when the joint moves. A cover that vibrates, a flange that grows in heat and shrinks in cold — Hylomar follows, instead of cracking the way a cured gasket can. That is why it was developed for aero engines, where everything moves and nothing may leak.

The other side of the same coin: Hylomar does not glue. It does not hold two parts together, and it does not fill large gaps. It is a gasket, not an adhesive.

Universal Blue is the original from the 1960s. It comes in three grades (light, medium, heavy) to suit how rough the surface is, and it is what most people mean when they say "Hylomar". Solvent-based: you apply it, let the solvent flash off, and assemble. Resists oil, fuel, coolant and combustion gases, from −50 to +250 °C.

Hylomar M is the same idea, but tackier and with acetone as the solvent instead of a chlorinated one. It was made for racing, where covers come off and on often and the joint has to hold under extreme vibration. Choose M when you want the gasket to stay put while you assemble, or when the joint sees a lot of movement. Same temperature range as Blue.

Advanced Formulation (AF) is solvent-free. No flash-off, no smell, no waiting — you assemble at once, or leave the part as long as you like. The consistency is like thick honey and does not change. It has become the default on gearboxes and drivelines, because it resists the gear oils that attack silicone. But being thicker and less tacky, it fills a little less than Blue and M: use it on faces that fit well, and with a gasket where there is one already. Hylomar’s own figures: −55 to +250 °C.

We also carry Aerograde PL32, which is Blue in its aviation form with Rolls-Royce approval, and Spezial, a solvent-free grade made to prevent filiform corrosion in joints. For an ordinary workshop the three above are enough.

Hylomar is the right choice where two flat, well-fitting faces must seal against liquid or gas, and where you may want to take them apart again:

Engine: rocker cover, timing cover, sump, water pump, thermostat housing, oil pump. On cylinder heads and liners it is used as a thin film on an existing gasket — "gasket dressing" — making the gasket seal better without changing the clearance.

Gearbox and driveline: here AF is the favourite. Gear oil is hard on silicone; Hylomar does not care.

Threads: drain plugs, sensors, fittings and bolts that pass through a wet gallery. Hylomar seals the thread without locking it — the bolt comes out again without heat.

Anything else that weeps: instrument covers, inspection hatches, pump housings. If it is two faces and a gasket that is not quite doing its job, Hylomar is the first thing to try.

1. Clean, dry faces. Old gasket, oil and old Hylomar have to go. HyloClean is made for the job; acetone works too. A greasy face is the most common reason "Hylomar did not work".

2. A thin film. An even, thin coat on one face is enough — Hylomar fills scratches, not space. More is not better; the surplus squeezes out and ends up in the oil.

3. Let the solvent flash off. This is the step people skip. Blue and M should sit open until the film has turned tacky and waxy — that takes a few minutes. Assemble with wet Hylomar and the solvent is trapped in the joint. AF has no such step: assemble at once.

4. Assemble and torque as usual. No waiting before the part can be used, no cure time. Surplus wipes off with acetone.

One trick: Hylomar shows up poor fit. Apply a thin film, put the parts together without torquing, and take them apart. Where the film has not made contact, the faces are apart — so you know where the problem is before you spend time chasing the leak.

We sell the whole Hylomar range, and they make more than the blue tube. That is because the blue one does not fit everywhere.

The gap is too big. Warped faces, a dent in a cover, a housing that is worn. Hylomar fills scratches, not millimetres. Then you want Hylotyte Red 100, which is semi-hardening: it fills gaps, seals instantly, and resists water, salt water and coolant. A classic on water pumps, gearbox housings and marine work. Hylomar does not list fuel among what it resists — there the blue is the safe choice.

A gasket has to be formed. Where there is no gasket to dress with Hylomar, and the face is large or irregular, a silicone (Hylosil) or an anaerobic gasket (Gasket 3400) is right. They cure into a gasket in the joint. The drawback is the reverse of Hylomar’s advantage: they must cure before the part is used, and they tear when you take the part apart.

The bolt must not come loose. Hylomar locks nothing. To secure a thread it is Hylogrip 2143 (threadlocker) or 5177 (pipe-thread sealant) — anaerobic, curing in the thread.

Above 250 °C. Exhaust is outside Hylomar’s range. There it is exhaust assembly paste that applies, made for 1000 °C.

Close the tube. Blue and M are solvent-based. An open tube dries out and becomes useless. AF does not dry, but close it anyway.

Ventilate. Universal Blue contains a chlorinated solvent that is harmful to breathe. Use it with ventilation and gloves. M has acetone — less harmful, but flammable. AF and Spezial have no solvents; Spezial carries no hazard label at all. Read the safety data sheet; it is on the product page.

Not on plastics that solvents attack. Blue and M can attack some plastics. Use AF where unsure.

Not where something must be glued. Hylomar holds nothing together. A cover that must stay on without bolts needs adhesive — not gasket compound.

Old Hylomar out before new goes in. New Hylomar on top of old, oily Hylomar does not seal. HyloClean dissolves it.