[Walt Fricke]

Well - assuming that some of the case 8mm studs weren't outfitted with aluminum washers, which got squashed but you didn't spot them and pry them off, which might account for you being able to split the case up to 19mm but not more -

How about relieving the frustration by taking off all the pistons you can get to. If you can get to one end of a wrist pin, you can get that pin out. A pain, and calls for some cleverness, but you can do it. And then remove the rods from those throws.

I assume that by mainshaft you mean crank shaft. You might be lucky and find that the bad rod journal is somewhere other than where things are hanging up, and then you can rotate the crank and pull the remaining pistons off after you have some of the rods out.

If not, you have done some work you will have to do anyway, eliminated at least some possibilities, gained a bit more access into the case innards so you can perhaps see what is going on (mirrors and lights will help), and will be better able to spray penetrating oil all over where you may need to following Adam's lead (assuming you don't have a 55 gallon drum of penetrant lying around).

Disjointed ideas: If you don't plan to reuse the chains, cut them (or just grind the heads off of two of the link rivets) and pull them out. I don't suppose they have fallen into the case and gotten jammed up somehow? And from one picture it looks like the chain on the left side is all nicely out.

Looks like you could remove the rings on that #3 piston. Even if the #3 rod is frozen you should get a "soft" stop if it is the side of the piston binding on the spigot, so maybe a ring is causing it to be a "hard" stop? If you don't plan to reuse the rings, all the easier to get them out.

There is seized and seized - how about putting two bolts back in the end of the crank and putting a crow bar or even longer cheater bar between them to get more leverage to see if you can't get it to move enough to remove more stuff?

But don't give in and pry on case mating surfaces. Once the sealant is broken, hammering isn't really needed, so my guess (and yours) is that brute force is not too likely to help as something mechanical is causing this.

Good luck.

Walt Fricke