Hello Mahonroy, generally speaking, castings, depending on the type, will tend to have a sort of a skin on them, this "skin" as I'll call it can have a number of different types of contaminants in it due to the casting process. First, if possible, carefully grind or machine the area that you are trying to attach the injector bungs, try to clean an area that leaves at least 1/2" to 5/8" of clean space around the bung. Place the bung in the spot that you are going to attach it and if possible clamp it or otherwise postion it so that it won't move. Preheating the area will most likely help in this case also, a very simple way to do this is to use an oxy-acetylene torch, this can be accomplished by lighting the torch with only the acetylene on and blackening the area that you are going to weld, next set the torch to a neutral flame and carefully heat the area until the black soot has burned off, once that has happened you can proceed with the welding, you need to do this immediately or else the aluminum will begin to oxidize and cause problems with your weld. You do need to pay attention to the condition of the tungsten, if you have dipped it in the puddle or hit it with the filler rod you should stop the welding process and break or grind the contaminated part off and prepare the end again, if you don't, a couple of things will occur, first it will be difficult to get a clean welding puddle to form without contaminants floating around in it, secondly it will require more amperage from your power source to attain the same welding energy as it did when the tungsten was clean. A lot of the time you will find that aluminum will weld better with pure tungsten, however, it requires a larger diameter to withstand various amperage loads than a lot of the alloyed tungstens do in the same situation, also, a pure tungsten will ball on it's own when you subject it to elevated amperages. As far as pointing the tungsten goes or balling the end, you will find lots of preferrences among welders out there, as far as which is correct, I say experiment a bit and decide for yourself. With the situation that you have described in this post I might make this suggestion, grind the tungsten to a point and then blunt the end to about a 1/16", providing you're using an 1/8" tungsten, I normally taper the tungsten back at a rate equal to 3 times the diameter of the tungsten for most applications. As far as the filler goes, I am not familiar with the particular alloy that you have described here, however, I might suggest using a 4043 filler as it doesn't sound like there is a great strength issue here and it appears to be more one of sealing the bung, this may also prevent any hot cracking from appearing in the welds. If you are dealing with greases or oils in relation to the welding that you are doing, using acetone to clean the parts can be a great asset, also brushing the weld area with a stainless steel brush before welding will help. Hope this gives you something to go on and good luck. Regards, aevald