Through the years the core holes have either been a problem or were imagined to be a problem. People have threaded them, put in core plugs with pins holding them, used the expanding type, staked, and probably a dozen other methods to block those holes.
The cup plug is most common. Let's imagine a 1 5/8" cup plug with 19 psi behind it. 39.4 lb is trying to push it out.
What's holding it in? Friction. The plug expands into the hole. The force is the modulus of elasticity times the percentage amount the plug is compressed while in the hole. The friction coefficient between plug and block is the other factor.
The friction between brass and cast iron is quite close to friction between zinc and cast iron. I use zinc because the steel plug is galvanized, coated with zinc.
The force that the plug exerts on the hole is about half for the brass than for the steel, if both materials are the same thickness and the same percentage deformed. That's because the spring constant for steel is about twice that of brass.
So steel might win if the safety factor was around two. If safety factor is about four or five, either would provide service equally well.
The one unknown is how whatever we put between plug and block affects the friction coefficient. I have to believe that Hi-Tack or a thick coat of paint would allow slippage faster than no coating at all. There are compounds that harden completely, like one form of Permatex and Indian Head shellac. I have used The Right Stuff on cup plugs when I had a little left over. When deprived of oxygen, it sets.
In the long run we know that steel plugs will rust after 15 or so years and that brass ones don't. We know that steel plugs should be in there a little tighter than brass, but does it matter? We know that engines asked to produce vast amounts of power will deform under load, so plugs are more likely to come out.
There seems to be no definite right answer, so I guess you use what has worked for you in the past. I really hate this answer but a better one escapes me.
R.
