pressure seal a cast iron ass'y - 100 psi 373f operating temp.

If this should be in welding/fab, my apologies and mod feel free to move.

I'm trying to build up something that is normally cast in one piece. For scale, the outside of the cylinder is 2.75" and the holes are about 1/2" (1/4 NTP)
I tried silver solder but don't have enough heat even with building an enclosure. And silver solder is a one shot deal, hard to fix if something goes wrong.

The fit between the two parts is about .005 clearance. They are held together by 2 SHCS's on the face at the top of the photo. Getting fasteners in the face where the holes are would be tricky.

I am now trying Deacon sealants, 770L and 770P. They have to be heat cured. The 770L is about like cold molasses out of the can, but when curing it seems to go liquid and run out of the joint. DEACON(R) 770-L Thermal Reactive Liquid Sealing Compound | Jet-Lube

The 770P DEACON(R) 770-P Thermal Reactive Paste Sealing Compound | Jet-Lube is fibrous, and like the name implies, a paste. So that was the next compound tested. When cured, it takes a knotted wire brush to get it off. I'm getting air at 125 blowing between the holes. I suspect I may have had an air bubble in the paste. I then dumped some of the liquid down one of the holes and put air to it to try to blow it into any gaps. It held cold, it's now curing.

Does anyone have a better solution or advice? (edit, this a a mockup. A gasket if thin enough might help, but I don't know of a gasket material that is thin and would take the service conditions. plus it would be difficult to compress with fasteners only on one side, or even coming in from 2 directions. )

First thing that comes to mind is an o-ring. No problem with the pressure or the offset. Just need to find the material suitable for 373°F and the fluid you are holding back.

Without seeing a detail drawing of the parts before assembly, or knowing what the final use is, I'd suggest that since this sounds like series production use a furnace brazing company to do the permanent assembly.

In quantity it should be reasonably priced and dead reliable. No gaskets, no putties, just give them the parts and good assembly instructions and you'd get back something as good (or better) than a cast-as-solid part.