Industry
Threads, seals and corrosion protection for magnesium and aluminium castings
Light Alloy Die-Casting Why it matters
A die-casting is designed for the shape, not for the thread. Wall sections are thin, draft angles dictate where material can sit, and the alloy is soft enough that a thread cut into it wears quickly. Joining a casting therefore needs fasteners that either form their own thread in the alloy, reinforce the thread that is there, or seal the hole entirely.
Thread rolling tapping screws are the most direct answer: only a hole has to be drilled in the casting, and the screw is driven straight in, forming its own thread as it goes. That removes the tapping operation and improves both assembly efficiency and reliability. Where the thread must survive repeated service, a reinforcing insert takes over. And where the hole is a leak path rather than a thread, a pre-coated sealing screw closes it — the coating forms a seal at the head instead of relying on an O-ring or a nylon washer, so it is simpler to install and does not tear.
IP68
Sealing with pre-coated thread
>8.8
Insert strength grade
M0.6
Smallest catalogue thread
No tap
Thread formed directly in the casting
Field problems
These are the failure modes we see most often, and the engineering response that addresses each one. Problems are stated as they appear on the line, not as they appear in a textbook.
| Problem observed | Engineering response |
|---|---|
| Tapping a thread in a die-casting adds an operation and a rejection risk. | Thread rolling tapping screws need only a drilled hole — the screw forms the thread as it is driven, removing the tapping step and improving efficiency and reliability. |
| A thread in magnesium or aluminium strips after a few service cycles. | A reinforcing insert provides a steel or brass thread above grade 8.8, and can be replaced without scrapping the casting. |
| A fastener hole in a casting is a leak path once the enclosure must be sealed. | A pre-coated sealing screw forms its own seal at the head, so the sealing function moves from a separate O-ring or nylon washer to the fastener itself. That removes a part from the BOM and a failure mode from the line. |
| A steel bolt in a magnesium casting corrodes the casting. | An aluminium bolt reduces the potential difference at the interface, and an anti-galvanic adhesive on the head provides physical isolation where a steel fastener must be used. |
| Thin casting walls cannot carry a deep thread. | A thread-forming screw with a controlled minor diameter works within the wall the casting actually has, which is why the pilot hole and the screw are specified together rather than separately. |
| A casting is expensive to scrap, so a failed thread is a costly defect. | Inserts and thread-forming screws both move the wear away from the casting, so the replaceable part is the fastener rather than the housing. |
Specification basis
A starting specification, not a fixed menu. Most programmes adjust one or two of these after the first joint review.
Direct threading
Thread rolling tapping screws — drill only, no tapping operation
Thread reinforcement
Inserts for magnesium and aluminium, above grade 8.8
Sealing
Pre-coated sealing screws; head seal instead of a separate O-ring or washer
Corrosion protection
Aluminium bolts and anti-galvanic adhesive against galvanic corrosion
Panel joining
Clinching parts for thin die-castings and sheet metal, with grounding
Locking
Pre-coated thread locking compound
Finishes
Anodising, passivation, zinc, nickel, sealed coating
Recommended products
Chosen for the tolerance, finish and locking requirements typical of the assemblies described above.
Thread & Panel · Light alloy & plastic
Thread inserts give a lightweight structure a thread that survives repeated assembly — wire thread inserts, self-tapping inserts and key-locking inserts for plastic, aluminium and magnesium.
Micro Screws · M1.0 – M2.0
Self-tapping micro screws remove the tapping operation: the screw forms its own thread in a plain pilot hole in plastic or thin metal, so the housing needs no insert.
Thread & Panel · Lightweight joints
Cold-formed 7-series aluminium bolts that replace steel in magnesium and aluminium assemblies — removing galvanic corrosion and cutting fastener weight at the same time.
Thread & Panel · Sheet metal panels
Clinching nuts, studs and grounding terminals are pressed into sheet metal with a press, giving a permanent load-bearing thread in a panel that is often thinner than the thread itself.
Thread & Panel · One-side access
Rivet nuts and blind rivets install from the front face only, giving a threaded or permanent joint in a closed box, cabinet or duct where the back is unreachable.
Design notes
These decisions are cheap at concept stage and expensive after tooling is cut.
01
With a thread-forming screw the drawing controls the pilot hole and the screw controls the thread. That means the hole diameter and the wall thickness are the two dimensions that decide whether the joint works.
02
A thread formed directly in the casting is adequate for a joint assembled once. Any position that will be opened repeatedly needs an insert, because the casting cannot be re-threaded economically.
03
A pre-coated thread puts the sealing function on the screw rather than on a separate washer. That is cheaper to assemble and removes a part that can be installed incorrectly — but it only works if the coating and the mating surface are specified together.
04
Every steel fastener in a magnesium casting is a corrosion cell waiting for moisture. Specify aluminium bolts or anti-galvanic isolation at the drawing stage, not as a field fix.
Joint review
A drawing gives us geometry. The mating material, the service cycles and the cleaning chemistry decide whether the joint works. Send all three and we will review the whole stack.
Questions
Answers to the questions that come up most in this industry.
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The same fastener discipline applies across these industries, with different documentation and validation requirements.
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From a single prototype joint to a PPAP production launch, the same team reviews the drawing and the assembly context.
