Coolant Filler Necks

Is your old filler neck worn or leaking, or does your modified radiator lack a place to add coolant? In this category you will find aluminium filler necks for specific car models as well as weld-on necks for your own cooling system.

What the filler neck does

The neck is where you add coolant and where the cap sits. The cap holds the pressure in the cooling system, and the pressure raises the boiling point of the coolant. If the sealing surface on the neck is warped, scratched or corroded, the system can lose pressure, which can increase the risk of overheating – especially under high load such as on a track day.

How to choose a filler neck

The first choice is between a ready-made neck for a specific car and a weld-on neck:

• Model-specific: e.g. Koyo aluminium necks for Toyota GR86/Subaru BRZ, Scion FR-S, Mitsubishi Evolution X and Honda Civic. Check the model years for the specific part number.
• Weld-on: e.g. Vibrant with 32 or 42 mm inner diameter for a radiator or pipe in your project. Welding is required, and the material must match what the neck is welded onto.

Also check that the cap fits the neck and that any overflow connection fits your hose. Mishimoto and Moroso are also among the brands in the category.

Installation and maintenance

A weld-on neck should typically sit at the highest point of the cooling system, so air can more easily collect and be bled out. After installation, it is a good idea to pressure test the system and bleed it thoroughly. If you are replacing cooling system parts anyway, it can make sense to look at thermostats and thermostat housings at the same time. You will find the whole range under thermostats, housings and filler necks.

Frequently asked questions

Should I choose a model-specific neck or a weld-on one?

A model-specific neck is made for a particular car and model year and is installed without modification. A weld-on neck is for custom solutions, e.g. a modified radiator, and must be welded in place by someone who can work with the material.

Why does my cooling system lose pressure at the cap?

It can be caused by a worn cap, but also by a damaged sealing surface on the neck. Look for scratches, dents and corrosion where the cap seal rests. A pressure test can show whether the leak is at the neck or somewhere else.

Will my original cap fit a new filler neck?

It depends on the neck. Check which cap type and size the specific part number is designed for before you order. The correct cap type and pressure rating are important for the cooling system to hold the pressure it is designed for.