January 24, 2025

How to weld the impossible metal

Back in August of 2024 I hit some roadworks in my '64 Pontiac (CLICK HERE to watch the full 30min video on its back story) and it smashed the transmission pan on the Hughes Performance-built 4L80E four-speed auto. I got the car to Laurence at Brintech Customs so we could pull the trans pan and see if it could be fixed - being cast alloy it is a trick repair, as you'll see below.

Cast aluminium alloys are tricky to weld because they're nice and light thanks (in part) to their loose metal density. That refers to how the particles of metal sit together - in a pressure-forged item they're squeezed together to increase density and strength, but aluminium alloys are soft and cast parts don't have the same type of density. This means welding them requires very careful preparation and temperature control.

Laurence chucked the transmission pan into the hot wash to get rid of any left over transmission fluid. Cast parts have to be surgically clean before anyone can even think of welding them, as cast parts are often porous they can wind up having pulling oils and fluids into the metal which will contaminate the weld.

The cracks from the impact are clearly visible once the pan was off the car, but there was more damage. Thankfully it was easier to repair than we first feared.

I got the pan over to my mate Peter Kelly from Mallard Metalcraft, who is a guru at sticking metal together (as well as bending metal, drilling metal, or doing all manner of engineering with metal). He knew exactly what to do to seal up my pan, so come along as he shows us how to fix cast alloy. To start, the ends of the cracks are drilled to stop the cracks spreading further as the metal heats up, and the face is opened up to allow a fillet of weld to be laid into fresh, clean metal.

The metal around the crack needs to be heated so it doesn't distort, and so the welded-in material doesn't crack and fall out as the surrounding metal cools. Because of aluminium alloy's light material density it transmits heat very well, which is one reason it's often used on high-performance transmission pans, but this means you need some outside help to keep heat into the part and make it stable for welding.

Heat is still applied as Pete welds up the cracks with his TIG. This ensures nothing will crack as the alloy cools faster than the welded in material.

After the pan was left to cool naturally back to room temperature it was filled with water and an air blower was used to push the water into the welded areas. Nothing leaked so we were good to take it back and fit it up.

It went back on the car like a new pan, and has held fluid with zero leaks since then. While welding cast alloy isn't as simple as sticking two bits of stainless steel together, it can save some expensive parts from ending up in the scrap bin!