I have never had to do this and have seen a lot of post on this and other forums about problems trying to keep the area clean and keep oil/fluids dripping out of the cracked portion and causing a lot of problems with weld quality and leak stoppage. I thought of something that should make a big difference in keeping the weld clean enough to weld. What about putting a shop vacuum hose on the crankcase vent to put a slight negative pressure on the inside to keep any fluid remaining contaminating the weld. If you try this and it helps please repost it so that this can help others. I saw a post on the Miller web site but for some reason I cannot post on that site. If you had an air vacuum unit this would be a lot safer than a non bypass cheap shop vacuum that could run fumes through the motor brushes.