Guides / Design

Aluminum Threads Strip, and Usually It Is the Design

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

Aluminum threads fail more often than any other feature on a machined part, and almost always for one of three reasons: not enough engagement, too much torque, or too many assembly cycles.

None of them are metallurgy problems. They are design decisions.

Start with engagement depth

Aluminum has roughly a third of the shear strength of steel, so a thread in aluminum needs more length to carry the same load as the same thread in steel.

Material	Typical engagement
Steel	~1 x diameter
Aluminum 6061-T6	~1.5 to 2 x diameter
MIC-6 cast plate	~2 to 2.5 x diameter

Past about 2.5 diameters you gain very little, because thread load is not distributed evenly. The first two or three engaged threads carry most of it.

On the drawing, specify full thread depth, not hole depth. M6 x 1.0, 12 FULL THREAD is unambiguous. M6 x 15 DEEP is not, because the shop does not know how much of that is usable thread and how much is the tapered lead-in and drill point.

When to fit an insert

Coil inserts (the generic name for what most people call by a brand) are a spring of diamond section wire screwed into an oversize tapped hole. They spread load over more thread, so pull-out strength typically improves substantially, and the working thread becomes stainless rather than aluminum.

Use them when:

• The joint gets assembled and disassembled repeatedly. Service panels, test fixtures, tooling.
• The fastener is torqued near its limit.
• The base material is soft, such as MIC-6 cast plate or a 5000 series alloy.
• A thread has already stripped and you are repairing rather than scrapping.

Solid bushing inserts (keyed or self-locking types) are a stronger and more permanent option for heavily loaded joints, but they need a bigger hole and more surrounding material.

The practical cost question is simple: an insert adds a small amount per hole in parts and labour. Scrapping a machined housing costs everything that went into it. On anything that will be handled, inserts pay for themselves the first time somebody cross-threads a hole.

Design details that prevent failures

Leave material around the hole. A tapped hole needs roughly one thread diameter of material on every side. An M6 thread 4 mm from an edge will break out the wall.

Chamfer the entry. A small chamfer stops the first thread from being damaged and makes the fastener start straight. It also gives displaced material somewhere to go.

Do not bottom out. Tapped blind holes should be drilled deeper than the required thread. A fastener bottoming in a hole applies its full torque to the last thread and splits the boss.

Use coarse threads. Coarse threads have more material between them and strip at higher loads in soft material. Fine threads belong in steel.

Think about galvanic pairs. A stainless fastener in an aluminum part exposed to moisture will corrode the aluminum around it, which matters most in marine environments.

Anodizing and threads

Coating grows into the thread from both flanks, and on a small thread a hardcoat can stop the screw entering. There are three accepted answers and you should pick one on the drawing:

1. Mask the tapped holes. Standard practice, costs hand work.
2. Tap after anodizing. Clean threads, bare aluminum inside.
3. Fit inserts after anodizing. Neatest solution for parts that need both a full coating and a strong thread.

MASK ALL TAPPED HOLES on the print is seven characters that prevents a whole batch of rework.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

• Screw thread
• Threaded insert
• ISO metric screw thread

Frequently asked questions

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