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Your vehicle is like a big puzzle. When it's put together, a car is a masterfully-crafted machine, but unlike your average jigsaw puzzle (hopefully), the pieces break or wear down over time and have to be replaced. Just as knowing the characteristics of the broken piece- where in the picture it is, the contours of the tabs and blanks, etc.- would be essential to find a replacement, the same thing goes for car parts. With fittings in particular, there are a lot of intricacies that need to be taken into account. Having a strong knowledge of dimensions, materials and finishes can help you pick the perfect part, so the 4LTL team is going to share the details of the "puzzle pieces" so you can make an educated decision on what part is the perfect fit for your automotive project.

There are 5 main fittings out there used for automotive projects: tube nuts, unions, bleeder screws, compression fittings, and adapters. Each is used for a different purpose in the vehicle. For example, bleeder screws are exclusively used on calipers, while compression fittings are used for lower-pressure lines. 

Tube Nuts are used to secure a variety of lines: the same tube nuts used to tighten a brake line together can also be threaded onto a fuel line. They come in a wide variety of materials, but the most important features are the tube and thread dimensions and the measurement system  (imperial or metric). Mechanically, tube nuts secure the flared end of a given line into the tube seat of a given assembly. Each of these seats has an inner shape which only accepts a certain form of flare, which is where we get the different types of flare names (SAE, ISO, Inverted, etc.) from. European parts tend to only accept bubble flares, and thus their tubing seats have a bowl-shaped end that the bubble flare butts into. SAE parts, however, have a conical shape at the base of the tubing seat, and so SAE flares are formed with 45-degree angle walls to accommodate. The tube nuts for each part they are meant to thread into are also shaped according to the flare they are securing. 

Tube Nuts Illustration

When shopping for tube nuts, it is important to know:

  1. The dimensions of the tubing being installed.
  2. The dimensions of the tube seating, in regards to what size and length the appropriate tube nut threads will need to be.

Unions are used to join lines together. Let's say that a pre-bent brake line package arrives; part of that package will be intended to snake back to the rear axle. Depending on the vehicle the package is designed for, the manufacturer might split that line in two if, for example, the firewall prevents a whole line from being pre-bent and installed in one go. In this case the factory will provide two lines with the expectation that the installing mechanic will join them with a union. Unions have been machined for all different types of lines: metric and imperial, 3/16" and 5/8", etc.. Most are made of brass, and can be threaded into by the matching tube nuts. Practically speaking, they can be considered double-ended tube seats.

Unions Illustration

When shopping for Unions, consider:

  1. The dimensions of the tubing being joined.
  2. The shape of the union's tube seat in relation to the flare being made.

Bleeder screws are screws used to bleed the fluid out of hydraulic parts for repair work. Each hydraulic part has one. They are machined with a little tube running through their length so that a mechanic can vent hydraulic fluid just by turning them slightly. Each has a significant thread and a hex that can be turned by a wrench. Most bleeder screws are made of stainless steel. Inside of the screw, a small ball is braced against the inlet of the bleeder screw to seal the reservoir of whatever part the bleeder screw is installed into.

Bleeder Screw Illustration

When buying a bleeder screw, pay attention to:

  1. The dimensions of the bleeder screw seat in your hydraulic part.
  2. The dimensions of the bleeder screw's hex.

Compression Unions are fittings designed to hold lines together merely with mechanical compression. Because of this, they are utterly unsuited to use with brake lines. DO NOT USE COMPRESSION FITTINGS to join brake lines. Compression fittings are designed for lower-pressure lines such as those used for fuel, compressed air, and water. These fittings are commonly made of brass, but they're complicated. Each compression fitting is made from five parts: two Compression Nuts, two Ferrules, and the Union. The ferrule serves as a seal, and is usually made from a soft metal like copper or brass. When joining lines with a compression fitting, the compression nut and ferrule are slipped on to the line, then inserted into the union. From there, the compression nut can be tightened down, and the line is secured into the union. This process is then repeated on the other side, finishing the union.

Compression Union Illustration

When scoping out compression fittings, bear in mind:

  1. The outer diameter of your tubing
  2. The tubing the Compression fitting is advertised to join.

Adapters are a legacy part which can convert the diameter of the lines you're fitting to. Adapters have a male and female end which fit and sit different diameters of tubing and tube nuts. These ends are usually designed for the same flare shape, but some are designed to work between flare types: one end of an adapter could take a bubble flare while the other side would interlock with an inverted port, for example. Way back when adapters were made widely available just in case mechanics or technicians found themselves in a place where they didn't have the right size of tube nuts available to get a job. These days we have the digital market to order from on-demand, so using adapters as a stop-gap solution is less common. They still have some utility for shifting line diameter and connecting between two different line flares, however. Most adapters are made from brass.

Adapter Illustration

When looking for Adapters, be aware of:

  1. The dimensions of the line you're fitting from and the line you're fitting to.
  2. The shape of the two ends of the adapter (inverted, bubble, etc.)
  3. The length of the male size of the adapter in relation to the tube seat it will be fitting into.


In addition to considering the range of fitting types, it's pertinent to consider their finishes. Most fittings come finished in one of three ways: Unplated, Black Oxide, and Zinc-plated. 

Unplated fittings are typically rust-resistant by token of the material they're machined from, being made from materials like brass or stainless steel. Straight steel fittings are liable to rust, so steer clear from them if at all possible. Fittings such as these are typically used for applications such as Unions, Adapters, and Bleeder Screws: fittings which won't have a lot riding on being removed at a later date.

Plated fittings are made from a ferrous material-- typically steel. The plating available is typically Black Oxide or a Zinc plating, both of which afford a durable finish to the part they're applied to. These sorts of plating are applied to parts which will have to be removed at a later date, but which have strong material qualities: tube nuts, mainly. 

4LTL works hard to maintain a rich catalogue of automotive information. If there is anything we have missed, feel free to send us a message and let us know! 


Did you enjoy reading our blog? If so, let us know by using promo code: BLOG10 at checkout and get 10% off of your order!

Best of luck on your next project,

The 4LTL Team

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