I recently aquired a RD-4 and the water jackets on the bottom side of the cylinders on the pony we're broken out. I needed to bring it up the hill so I can be close to the shop to swap it out. So I tried JB steel stick on the one side. I had not realized the other side was broken too. There's far less room there to try this stuff. I only needed it to get it started to drive it a shot distance so I was not intending on that to be a permanent fix. I have another 4 as a parts machine I had planned on swapping the pony from. However I inspected it and it also is broken in the same way. I'm assuming when you drain the water from these it holds a bit in this area. I have looked the rest of the cooling system over and I have not found any other damage from freezing. It's been repaired before. There's a old braze that can be seen. I plan to have it repaired the same way. Definitely want to run good coolant and prevent any other issues like this.

That's a pretty typical pony motor issue and brazing is the common fix. Problem is those blow outs are in areas where the castings a very thin and also accumulate debris. Tough to keep clean but needs to be done.

i wonder could mini welsh plugs be added to those areas to prevent that problem. most engines do have them as a designed failure point. it would be a simple job with a milling machine while you have it off the machine

Seems to be caused by years of use without antifreeze/inhibitor !!!
the lower passage and jacket filling up with scale and sludge which is still there when the engine is drained which then freezes ..
The solution , repair damage or better clean out Sludge to avoid it in the first place and run 50/50 anti freeze soft water mix ..