Thanks for chiming in Old Magnet... Always like to hear your opinion... I've seen the locknstich done before but I have never done that technique myself so I'm not as familiarize as others on the forum.. But if old Magnet said it as far as I'm concerned "it is written in stone."

locknstitch is not the only source. Cast iron taper plugs have been used forever for that kind of repair.
goodson.com/collections/crack-detection-...s/crack-repair-plugs

Thanks for the continuing recommendations. Here is the outcome of my digging around today (as I also posted on Heavy Equipment Forum, where there are also a good many grey beards with useful advice):

The ongoing adventures of Hanna and the crack. I spoke today with a fellow who is supposed to be one of the best welders in the Territory at Mobile Maintenance (www.mobilemaintenance.ca/), showing him the photos and discussing options. He said that re-welding would be a crap shoot. It might work, but would just as likely it worse. He also noted how welding cast steel, unless done appropriately (which he explained at some length), would almost inevitably leave small imperfections that would lead to (in my case) leaks. From looking at the photo, he had no doubt that the weld was not appropriately done. Sigh.

He suggested going with an epoxy, but was unable to suggest any in particular. He also suggested grinding the exterior weld to flatten and make for a better surface to bond the epoxy. I was concerned about weakening the crack. He did not see that as an issue.

Which leads to the conversation I had with a representative for Belzona. I was asking especially about Belzona 1161. The rep said that Belzona would have been perfect IF it had not already been welded. The Belzona would not, he assured me, adhere to the weld. In time, it would just peel off. His suggestion was to check out a competitor: Defcon Tungsten metal epoxy. He also said that I should use a heat lamp to sweat out the oil in the cast steel pores. I.e., clean with brake cleaner, then put it under a heat lamp for 3-4 hours. Repeat at least a couple of times, leaving the heat lamp on overnight for the last time. Oh yeah, and to be sure to thoroughly rough up the previously applied epoxy on the inside of the diff.

Tomorrow I am going to call the technical support lines at Loctite, JB Weld, Permatex and Defcon and see what I can learn. I'll let you know what I find out.

I will also do some checking into the locknstitch method, which is totally new to me. As always, grateful for the advice and will let you know how things turn out. At this point I am concerned about not making things worse...

Have to say that I am very impressed by the Locknstitch. But maybe reserve it for next time if I can't fix (or effectively slow down) the leak... That said, I am going to give them a call tomorrow to see what I can learn from them.

Have to say that I am very impressed by the Locknstitch. But to use in my situation, in which a weld has been made, they would require that I cut out the cracked portion to at least 1/4" past the extent of the weld, replacing and stitching in a replacement piece of cast. So not a practical solution. So back to epoxies...

If your going to "goober it up with apoxy" incorporate a couple of layers of fiberglass window screen into the epoxy patch to help distribute the bonding.

i tried a product from my local Napa distributor to seal a gas tank on a 3/4T farm truck, been leaking for years and tried a few a different products with minimal success...till now....

-Tech Steel, Steel Reinforced Epoxy Putty....clean up the area to be applied, mix epoxy (wear protective gloves) and work fast, it sets up and hardens quickly....it worked! sealing oil should be a snap vs gasoline

Thanks, Old Magnet. What I am wondering about as I continue to ponder is to coat the inside, epoxied crack, with Permatex Gear Oil RTV Gasket Maker--a silicone gasket material designed for differentials and such. I would rough up the expoxy surface and sweat out as much oil as I could from the cast iron before applying the Permatex which, in theory at least, would remain slightly flexible to prevent cracking (as the epoxy has done, if you look closely at the photo). I am not trying to glue anything together, but to seal the leaks in the crack.

On the external crack, I would indeed goober up the weld with epoxy. And using a fiberglass screen might add some value. Have to think about that...

Onfoot, It is Devcon not Defcon. I have used it with great success. I agree with OM on the use of screen. I would smooth it off with a 60 grit flap wheel. That will also leave it with some grip. Then use the brake cleaner and a heat lamp to sweat the oils out. Once done you can smooth out the Devcon and a little paint no one will be the wiser. Devcon is a great product. The lock and stitch method will probably not work now since it has been welded already.

I'd be looking into "Glyptal" products for internal sealing/porosity sealing.
glyptal.com/glyptal_products.html