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Choosing a Material for a Custom Auto Part
Eleven materials cover almost every machined car part. Compare them on weight, how they cut, where they get used and which finish they take.
Materials compared
Density is a datasheet value. The cutting column is relative: easy cuts fast and cheaply, slow means more machine time per part, so a higher price.
| Material | Density (g/cm³) | Typical auto use | Cutting | Finish options |
|---|---|---|---|---|
| Aluminum 6061-T6 | 2.70 | Brackets, covers, adapters, spacers | Easy and fast | Anodize, powder coat, bead blast |
| Aluminum 7075-T6 | 2.81 | Loaded brackets, pulleys, race hubs | Easy, a little harder on tools | Anodize (hard anodize is common). Does not weld. |
| Mild steel 1018 / 1045 | 7.85 | Weldable brackets, plates, tabs | Easy to medium | Zinc plate, black oxide, powder coat, paint |
| Alloy steel 4130 / 4140 / 4340 | 7.85 | Shafts, hubs, couplers, suspension parts | Medium, slower once heat treated | Heat treat, then black oxide or zinc plate |
| Stainless 303 / 304 / 316 | 8.0 | Exhaust hardware, fuel and brake fittings, underbody parts | Medium to slow. 303 cuts easiest. | Passivate, bead blast |
| Titanium Grade 5 | 4.43 | Race fasteners, brackets, retainers | Slow, hard on tools | Often left bare, or anodized for color |
| Brass C360 | 8.5 | Fittings, bushings, shift parts | Very easy | Left bare, or plated |
| Bronze, bearing grades | 7.6 to 8.9 | Bushings, bearing carriers, thrust washers | Easy to medium | Left bare |
| Copper C110 | 8.9 | EV busbars, grounding straps | Gummy, needs sharp tools | Tin or nickel plate |
| Delrin (POM) | 1.41 | Bushings, spacers, trim parts | Easy, moves with heat | None, cut to final size |
| PEEK | 1.30 | High temperature bushings, insulators | Medium, costly material | None, cut to final size |
Pick by what the part has to do
Start from the job, then check cost. Most custom auto parts end up in 6061, and the exceptions have a clear reason.
- Light and stiff6061 for most parts. 7075 when the part is loaded and will not be welded or left bare outside.
- Takes shock load4140 or 4340 steel, heat treated. Hubs, couplers, shafts and suspension parts that a bracket in aluminum would fatigue.
- Lives in salt or heatStainless for exhaust and underbody hardware. Aluminum loses strength at exhaust temperatures, so hot side parts are stainless, steel or titanium.
- Light and strongTitanium Grade 5 when weight is the whole point and the price per piece is acceptable. Race parts mostly.
- Slides or wearsBronze for a hard running surface, Delrin or PEEK for low friction and no corrosion. Leave clearance for plastics, they swell with heat and moisture.
- Carries currentCopper for the lowest resistance. Aluminum busbars weigh less and cost less, but need a bigger cross section for the same current.
Finishing by material
Finishing is usually done by a second vendor and adds cost and a few days. Anodizing and plating add a thin layer, so say which bores and faces are final size.
Aluminum
Anodizing for corrosion and wear, in clear or color. Powder coat for a thicker, cosmetic coat.
Carbon and alloy steel
Heat treat for strength, then black oxide, zinc plate or paint against rust. Steel rusts bare.
Stainless
Passivation restores the corrosion resistance that cutting removes at the surface.
Copper and brass
Tin or nickel plating stops tarnish and gives a better electrical contact.
Titanium
Usually bare. Anodizing gives color but not corrosion protection it does not already have.
Plastics
None. The part is cut to final size, so tolerance it with heat and moisture in mind.
What to say about material in the RFQ
Four lines remove most of the back and forth between you and the shop.
- Exact alloy and temperWrite 6061-T6 or 4140 pre-hard, not just aluminum or steel.
- Certificate neededSay if you need a mill certificate with the part. Not every shop supplies one unless asked.
- Substitutes allowedSay whether a close equivalent is fine when the stock you named is slow to find.
- FinishName the finish and which faces stay unfinished, such as a sealing face or a bearing bore.
Material guides
- Materials
6061 or 7075 Aluminum for Automotive Brackets and Mounts
The two aluminum alloys most billet car parts are cut from, compared on strength, fatigue, corrosion, welding and cost, and when each one is the wrong choice.
- Materials
4130 vs 4140 Chromoly for Machined Automotive Parts
4130 vs 4140 chromoly compared for machined automotive parts: carbon content, strength, weldability, heat treatment, stock forms and which suits which part.
- Materials
Anodizing and Finishes for Billet Car Parts Explained
Clear anodize, color anodize, hard coat, powder coat, plating and bead blast compared on cost, durability and how much each one changes a machined dimension.
Common questions
Something else? Email sales at autocncparts.com.
Which aluminum should I choose if I am not sure?
6061-T6. It cuts well, anodizes cleanly, welds and costs the least. Move to 7075-T6 only when the part is loaded and you know it will not be welded.
Why is titanium so much more expensive?
The bar costs more per kilo and it cuts several times slower than aluminum, so each part uses more machine time and wears more tools. Use it where weight is the reason.
Do shops supply a material certificate?
Many do when you ask for one in the RFQ. We do not check certificates ourselves, so ask the shop for the paperwork you need before you order.
Can I send my own material?
Some shops accept it, many prefer to buy their own because they know where it came from. Say so in the RFQ and we match with shops that agree.
Have a part in mind?
Tell us the material you have in mind, or what the part has to do, and we match shops that cut it. We reply within 24 business hours.