Here, you can see the slug in place. Cornett says it should be the tightest fit you can get and the same wall thickness as your main tubing. For example, this car uses 1 3/4 DOM mild steel tubing with 0.095-inch wall thickness. So the slug is 1 1/2 DOM mild steel with the same wall thickness. The wall thickness is important not only to add strength to the splice but also to ensure you do not burn through the metal when making your rosette welds. Finally, the slug should be long enough to extend past your holes on both sides by at least 1/2-inch. This slug was approximately four inches in length.