Hi GB. As the Lumiweld ads used to say, its not welding and it's not brazing. Here is a bit of theory. Strictly defined, welding is "fusion welding" where the parent metal is melted along with the welding rod to produce a homogeneous weld pool of molten metal which then freezes to give the joint. Brazing uses a joining metal of lower melting point. The parent metal is unchanged. Brazing and soldering are similar in this respect. Because the braze does not melt the parent metal this is the usual choice for thin sheet repairs such as mudguards etc. For the inexperienced brazing is easier to master and can be done with cheaper equipment.
Lumiweld is a halfway house. You heat the parent metal and melt the Lumiweld rod, which amalgamates itself to your nice clean damaged casting. Perfect...well not exactly. Look what happened to berger in a cold sober moment.
It is not a fusion weld as such, since the parent metal does not melt. Neither is it brazing, because it is absorbed into the parent metal rather than sitting on top as in a brazed joint.
So now we come to the tragic bit. It will only work faultlessly on good uncontaminated clean metal, which we don't have. You have done everything to produce a good joint on your 60 year old oil contaminated unknown alloy precision die casting, and failed. In the end you take it to be welded by someone who knows. "Whats all this" You sheepishly explain what you have done. Alas, you have introduced another contaminant into your casting, the welder won't waste time on it because his weld will not be sound. You have a in a word, bollockska'd it. To have any chance of success the Lumiweld impregnated metal will have to be cut away, back to uncontaminated alloy.
So, only use Lumiweld on clean, new uncontaminated metal. It will work just fine. On a valuable casting, leave it to those that know what they are doing. In the right setting it produced amazing results, but your chances of producing a good sound repair to an old crankcase are zilch.
How do I know this? Yes, I am yet another who tried and failed.
Keep Spannering. Swarfy