Norman, BIG difference between cast aluminum and cast Zamac, or even steel and cast iron. I too found boring the aluminum headstock and tailstock castings for my Gingery lathe a piece of cake, by comparison. They were also a lot smaller radii than this one where I'm boring a clearance hole over 2" in diameter, not to mention the even larger recesses for roller bearings. That alone means more much more metal to take off than a tailstock, and I have a full half inch to go on the through bore -- not a close bore to start.
Both metals were sand castings, and I can't exactly explain why if the holes had been the same size this one would be so different in cutting ease than the other, but the fact remains, they are.
One noticeable difference to aluminum I've found is that the tool grind angles have to be much sharper with Zamac because it is both slippery and very very tough, and the necessarily fine edge needed to cut it doesn't last well. Zamac machines beautifully, but not necessarily easily, and I'm working with a home concocted Z-12, not commercial quality. Filing and hacksawing also are noticeably more difficult than with sand cast aluminum, and those dull quickly as well, on the same types of jobs I would do with aluminum.
This particular casting really wants a carbide tool with sharp angles and top relief, but I don't have anything suitable that I can use with this boring bar. So it's slow going with HSS. And frequent re-sharpening. I do think that when the skin is fully removed from the bore (it was quite irregular and there are still patches of as-cast skin) that I won't have to sharpen as frequently.
No problem, really, Norman,, I'm patient, and I'll get there. I hand scraped the ways over several days, and I don't see that it's a big deal to take it slow boring either. All part of the game...