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 this is a difficult 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.

Cast parts are cheaper to manufacture in large production runs, but depending on the casting technique, metal used, and surface coating, they can also be porous. If you need to repair a part which holds fuel or oil, like a transmission pan, that means you need to take extra steps to clean it so the weld will stick properly to the metal.

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, although this is true for any metal you need to weld.

Welding lightweight aluminium alloys takes an experienced welder on the tools, and while you could get away with MIG or stick welders, it will be easiest done with a high-quality TIG welder. The reason aluminium is tough to weld is because it is so light, and its loose metal density, which means it responds super-quickly to heat.

Obviously, welding is all heat-based, so controlling how the metal heats and cools is key to getting everything to stick together properly. Amateur welders who've tried welding aluminium often find they simply blow holes in the part they're trying to stick together, as their welder is set too hot and it just torches through the alloy.

The repair on my pan was going to be tricky, but not impossible if you know the process you need to undertake to properly repair cast alloy. The cracks in the pan 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 thanks to the excellent quality of the Hughes Performance transmission pan.

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. This is one trick that is all-important when welding cast materials as their lower metal density will see them move around as heat is introduced. The other important step is the face of the crack was opened up with a grinder to allow a fillet of weld to be laid into fresh, clean metal.

As mentioned above, the aluminium alloy around the cracks needs to be heated so it doesn't distort when the hot weld material is introduced, and so the filler rod from the TIG welder 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. Cast metal parts of all sorts, including iron and steel, need to have their temperature tightly controlled if you want your repair to work.

Heat is continually applied as Pete welds up the cracks with his TIG welder. This ensures nothing will crack as the alloy cools faster than the welded-in material, but he and Nick are careful to not introduce too much heat as this could cause the part to warp or soften.

This is one of the reasons why using a highly experienced welder to repair cast or aluminium parts is crucial as they know how far to heat the metal, and when to back off and let it cool.

Pete did one pass, let the metal cool and checked how the repair looked. He decided, in the interests of having the strongest repair possible that he'd not only weld the inside of the crack, but the outside as well.

Before the pan cooled completely he flipped it over, dressed the top side of the crack and laid weld down along the exterior. While we didn't theoretically need to go this far, as the weld along the inside of the pan should hold the transmission fluid perfectly well, it is a nice piece of security given the pan sits on the ground when the car is aired out.

The outside of the pan needed to be cleaned up with a dremel as the cooling fins meant Pete couldn't get an angle grinder in there to cut a V. A carbide burr was used to clean the metal out and Pete also checked for cracks in the cooling fins, as these could be welded up at the same time.

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!