So I had along with my wife and dog a very pleasant catch up with my mate on his farm over the last week where we among other things, worked on the gearbox mount repair. He is extremely experienced in these type of crankcase weld repairs and modifications over many years. He reported that the cases weren't nice to weld due to probably a high content of zinc in the alloy very much the same as late model Ducati bevel cases. He started off in the mill by machining down the square lug section with a ball end cutter to the level of the case to give himself some space to get the torch in and a nice radius to weld up to. We then locally heated the case with a torch to 190 deg C. As soon as the welding was completed I held the case and he used a 1/4 inch arbour in a rivet gun and peened the entire weld while it was still soft. This stress relieves the area and reduces or eliminated any distortion to the gearbox end plate face and also shaped the welded area to a degree. He then used the die grinder and blended the welded area to the original lug. When I got home I did some further minor detail work with the die grinder as I had a very small cutter and then used some emery to further smooth it off. It did of course look a bit nicer with just the die grinder finish and if I weren't going to paint it and leave it in its natural finish I would bead blast it and it would be almost undetectable.
We checked with a machinists flat and the end plate face was totally undistorted but interestingly I had checked it on my granite surface plate before we started and found in the area from the cracked mount around to the fastener below the oil filler port you could geta .004" feeler gauge in so the rear plate face wasn't true to start with. Pete put it on the mill and levelled it up with a .0035" cut. The whole process took a couple of hours with me doing nothing but assisting with heating the case and holding it during the peening process. I guess it would be a $125 US repair at an estimate with the exchange rate as it currently is.
We both kind of feel that the cracks origin is probably more to do with the casting and cooling than anything else. Some will have a tiny intergranular crack develop during the cooling process due to the large local change in cross section and will then go on to develop a full blown leaking crack after an undetermined amount of heat/stress cycles. This case wasn't leaking and had no visible crack so it may have gone on for years without issue or the crack may never have propagated.
So cleaning painting and assembly to go. Might need to consult Pete Roper for a borrow of a few gearbox tools.
L/H mount radiused and smoothed.
This lug interface with the main body of the gearbox housing was also not radiused so I undercut the lug end and formed a radius.
The final repair