Filler Necks

Filler Necks: the coolant fill point that lets you run the cap you want

A filler neck is the threaded port where your radiator cap seats and where coolant goes in. On a cooling system that has been changed - a swapped or upgraded radiator, a relocated fill point, or a custom coolant routing - the stock neck often no longer fits or no longer holds the cap you need. That is where aftermarket filler necks come in: they give you a clean, sealing surface for the cap and a proper entry point to top off and bleed the system.

Some are vehicle-specific bolt-in or press-in parts; others, like the weld-on style, are meant to be added to a fabricated tank or pipe. Get the neck right and the cap holds pressure the way it should.

What ThreePiece stocks

This category currently holds 4 filler necks from 3 brands: Koyo, CSF and Moroso. Prices run from $73 to $139, which is the live range for what is in stock right now rather than a fixed offer. All four listed parts are aluminum.

On the vehicle-specific side, the fitment data covers Subaru, Mitsubishi and Scion - specifically the Subaru BRZ, Mitsubishi Evolution and Scion FR-S, across model years roughly 2008 through 2016. Two of the four products carry that named fitment; the others are more general. Product names also reference Toyota applications.

How to choose

The first split is application. If you are keeping a factory or direct-fit cooling layout, go with a vehicle-matched neck. The Koyo FN2316 for the Evolution X and the Koyo FN2320 are examples of that route, both sitting around $117.

If you want the neck and a high-pressure cap together, the CSF aluminum neck with high-pressure cap at $139 bundles both and sits at the top of the range here.

Building something custom? The Moroso weld-on billet aluminum neck at $72.99 is the low-cost, fabricator-friendly option - it is meant to be welded into your own tank or pipe, so it assumes you can do that work.

What else you need with it

A filler neck lives with the rest of the cooling system. If you are changing the neck because you changed the radiator, browse Radiators. For plumbing the connections, look at Fittings and Adapters. If the neck is going in as part of a wider engine refresh, our Gasket Kits and Head Gaskets categories are worth a look. All of these sit under the broader Engine Components section.

Fitment help and ordering

Not sure which neck matches your setup? Call fitment help at 813-535-5801 before you order, or reach us through contact us. Every order ships with free shipping, and Affirm and Katapult financing is available at checkout.

Filler Necks: the coolant fill point that lets you run the cap you want

A filler neck is the threaded port where your radiator cap seats and where coolant goes in. On a cooling system that has been changed - a swapped or upgraded radiator, a relocated fill point, or a custom coolant routing - the stock neck often no longer fits or no longer holds the cap you need. That is where aftermarket filler necks come in: they give you a clean, sealing surface for the cap and a proper entry point to top off and bleed the system.

Some are vehicle-specific bolt-in or press-in parts; others, like the weld-on style, are meant to be added to a fabricated tank or pipe. Get the neck right and the cap holds pressure the way it should.

What ThreePiece stocks

This category currently holds 4 filler necks from 3 brands: Koyo, CSF and Moroso. Prices run from $73 to $139, which is the live range for what is in stock right now rather than a fixed offer. All four listed parts are aluminum.

On the vehicle-specific side, the fitment data covers Subaru, Mitsubishi and Scion - specifically the Subaru BRZ, Mitsubishi Evolution and Scion FR-S, across model years roughly 2008 through 2016. Two of the four products carry that named fitment; the others are more general. Product names also reference Toyota applications.

How to choose

The first split is application. If you are keeping a factory or direct-fit cooling layout, go with a vehicle-matched neck. The Koyo FN2316 for the Evolution X and the Koyo FN2320 are examples of that route, both sitting around $117.

If you want the neck and a high-pressure cap together, the CSF aluminum neck with high-pressure cap at $139 bundles both and sits at the top of the range here.

Building something custom? The Moroso weld-on billet aluminum neck at $72.99 is the low-cost, fabricator-friendly option - it is meant to be welded into your own tank or pipe, so it assumes you can do that work.

What else you need with it

A filler neck lives with the rest of the cooling system. If you are changing the neck because you changed the radiator, browse Radiators. For plumbing the connections, look at Fittings and Adapters. If the neck is going in as part of a wider engine refresh, our Gasket Kits and Head Gaskets categories are worth a look. All of these sit under the broader Engine Components section.

Fitment help and ordering

Not sure which neck matches your setup? Call fitment help at 813-535-5801 before you order, or reach us through contact us. Every order ships with free shipping, and Affirm and Katapult financing is available at checkout.