OK, I have done this to a couple of motors in the past and other "fixes" that "cure" the dreaded rear main leak. I find lots of caps that have the hole drilled in them for this mod. It seems to have been a common "cure" for a long time and is not new. The drilled hole makes for a problem when it does come time for a rebuild because it has to be plugged which is not hard to do but, it is hard to clear the plug afterward. If the plug is not sufficiently cleared the rear main will leak and the hole will have to be redone. I have never been able to redrill a hole that is completely hidden without missing. This, in my opinion, reduces the strength of the cap and makes it look like swiss cheese.
Rear main leaks can be caused by several things like too much clearance at the bearing, too much clearance at the thrust, plugged return channel in the cap and, broken babbitt, to name a few. The only real cure is to replace the bearings.The engine was designed to not leak except for a slight amount, that however, can be annoying to some people. I would not recommend this to those engines but for a quick fix to get by till the engine can be rebuilt, it is a way to slow oil leaks.
Another quick fix is to drill a 1/8" hole in the down tube above the rear main. This won't hurt the engine but, like the previously mentioned, is only a band-aid fix and won't compare to a proper bearing repair. The one thing better about the tube hole is, it is somewhat easier to repair by replacing the tube and, yes they can be replaced. The one thing this "fix" does is it lets the oil drain directly into the pan and leaves some in the tube below the hole for start-up if the bearing will hold any at all, If not nothing will help and dry start-ups will eventually cause catastrophic failure.
I agree also with Harold, that the groove should intersect the wells to do the job it is being done for. The way it is depicted in these pictures and the same way I have seen it done is just a mickey mouse fix and won't do much good. If anyone tries this, make the groove go all the way from one well to the other and intersect the hole in the middle. Just make sure you don't cut too deep and compromise the strength of the bearing material and cause a crack.
Hope this helps explain this "repair".
