Evidence grade: B1 — Trade practice (trade sources, evidenced teardowns)
Answer: TIG-welding cast aluminum is expert work: castings are porous and oil-soaked, the pores drag contamination into every pass, and the wrong filler cracks. Miller's own guidance for cast aluminum (typically A356 alloy) is 4043/4943 filler, constant re-cleaning, and frequent stop-starts [1]. If you don't have TIG equipment and the part isn't structural, pressurized, or hot, a proper epoxy repair with reinforcement is the realistic win.
Effort: Moderate · 1–2 hours of work plus full cure time · degreaser, abrasives, stainless brush (aluminum-dedicated), epoxy, clamps or backing material
Why TIG is hard on cast aluminum
Porosity. Cast aluminum engine parts are "more porous" than wrought aluminum, and that porosity "impacts how long you can weld before pulling oils and contaminate material into the heat-affected zone" [1]. Years of oil soak live inside those pores; the arc's heat draws it straight into the weld puddle.
The discipline this forces. Miller's prescription: "stop and start frequently in order to keep the area cooler or to re-clean the area between passes" [1]. You weld a short stretch, stop, clean again, continue. It is slow, skilled work — not a weekend-first-weld project.
Filler selection is unforgiving. Most cast aluminum engine components are A356 casting alloy. Miller specifies 4000-series filler (4043 or 4943) and warns that 5356 — a fine filler for other aluminum — is "more prone to cracking" in these high-temperature applications (anything above 150°F counts as high-temperature here) [1]. Wrong filler on a casting is a crack waiting to happen.
Bottom line: welding cast aluminum demands AC TIG equipment, the right filler on hand, and the judgment to manage contamination pass by pass. If any of those is missing, welding is not the brave option — it's the expensive way to make the crack worse.
When epoxy wins: the procedure
Epoxy wins when the part is non-structural, non-pressurized, and stays cool — brackets, covers, housings, non-load cosmetic castings. It does not win on cylinder heads, pressurized cases, or anything that sees real heat or load (see stop rules).
- Degrease like you mean it. Oil-soaked porosity is the #1 killer of epoxy on castings too. Solvent-wash repeatedly until a clean white rag stays clean. The same contamination Miller fights with the torch will fight your bond.
- Expose fresh metal. Grind or file the crack zone to bright metal, extending the repair ½–1" beyond the visible crack ends in every direction — cracks run farther than the eye sees. V-groove the crack itself so adhesive has volume to key into.
- Reinforce, don't just fill. A crack is a stress concentrator; filler alone re-cracks. Back the repair: a fiberglass cloth layer wetted into the epoxy, or a thin metal patch bonded over the area, spreads load past the crack. Build the repair beyond the damage, not just inside it.
- Clamp or fixture, then cure fully. Epoxy reaches handling strength in hours and working strength in days (see Repair Lab B10) — don't load it early.
- Drill the crack tips first if the crack is still growing. A small stop-hole at each visible tip arrests propagation before you bond over it — see Repair Lab A2.
⚠️ Stop rules
- Cylinder heads, pressurized cases, brake or suspension parts — STOP. Heat, pressure, and safety-critical loads are outside epoxy's envelope and outside DIY welding's envelope. Professional repair or replacement.
- Oil won't come out — STOP. If repeated degreasing still lifts oil from the pores, neither welding nor epoxy will hold. Replace the part.
- Crack is growing under load — STOP. A crack that lengthens between inspections is a fatigue problem (see Repair Lab A7's territory); bonding over it is decoration.
- No TIG skill and the part is structural — do not "practice" on it. Practice on scrap; repair the part with epoxy or hand it to a welder.
PPE and ventilation
- Respiratory protection is mandatory when grinding aluminum — fine aluminum dust is a lung irritant and combustible; wear a P100/N95 dust mask or respirator and keep dust away from ignition sources. Wet-grind or vacuum-extract where possible.
- Solvent degreasing: nitrile gloves, eye protection, ventilation — no open flame near solvent vapors.
- Epoxy handling: nitrile gloves, ventilation; treat uncured resin/hardener as skin sensitizers (see Repair Lab C4).
- If you take the TIG route instead: full welding PPE — auto-darkening helmet, flame-resistant clothing, welding gloves, and fume extraction. No shortcuts.
Honest limits
- An epoxy repair on cast aluminum restores shape and sealing, not the casting's original strength. Never rate it for the loads the intact part carried.
- Thermal cycling works against rigid epoxy on aluminum (different expansion rates) — see Repair Lab A6 on expansion mismatch; outdoor/heat-cycled repairs need skepticism.
- Cast aluminum varies wildly in alloy and contamination. What holds on a clean valve cover may not hold on a porous, oil-soaked transmission case.
- For cast iron's version of this question — weld vs. braze vs. glue — see Repair Lab A4.
Sources
- Miller Electric — "Tips for Engine Repair Welding on 4 Common Materials" (Andy Weyenberg, Apr 12, 2024). Cast aluminum engine components typically A356; use 4043/4943 filler (not 5356 for high-temp applications); castings are porous and pull oils/contaminants into the heat-affected zone — "very common to stop and start frequently in order to keep the area cooler or to re-clean the area between passes"; >150°F is a high-temperature application. URL: https://www.millerwelds.com/en-us/resources/knowledge-hub/automotive-welding/tips-for-engine-repair-welding-on-4-common-materials
A 1:1 metal-repair epoxy is the right category for non-structural cracked cast aluminum — oil-free prep decides the outcome, not the tube.
Repair Lab · Release 1 · Evidence last checked September 30, 2026 · A project of AOJEL