OK, as I indicated here
I am posting my findings about good replacements for Megaminx screws and springs.
Why would you want to replace the screws and springs?
1.) As many know by now, the latest batch of Meffert's Megaminx puzzles (and now the Pyraminx Crystal) have had problems with the screws coming unscrewed from the core on counterclockwise turns. This is mostly a result of the Meffert's screw having the washer built in and attached to the screw, which means it has a wide area of friction on the center piece and a greater chance of that force unscrewing the screw from the core. Couple that with the notoriously strong springs in the Meffert's Megaminx and you have a lot of people reporting this problem. Odd that we haven't heard much about older Meffert's megaminxes with this problem. Perhaps different screws, or perhaps different cores? I don't have an older one, so I don't know. I can report, however, that my Meffert's DIY Megaminx hasn't had a single screw come unscrewed. Possibly this is because I immediately changed to softer springs.
2.) For anyone wanting to speed solve a Megaminx, it is highly recommended to replace the springs with softer versions. The strong springs add too much friction and it can move much, much better if they are replaced.
the.drizzle had mentioned (trace it back from the link above if you care) replacing the loose screw in his Pyraminx with a M3x20 metric screw, because he had them handy. I found it too big, and did some research on screws and washers in the non-metric sizes that are easier to find in the US.
[#4-40 flat 1", #4-40 pan 3/4", M3x20, Meffert's stock screw+washer, Meffert's stock spring, Chinese Megaminx spring, 0.016 gauge spring]
[#6 washer, M5 washer, M4 washer, #4 washer]
I found the #4-40 screw to be a tiny bit smaller than the M3, but larger than the Meffert's stock screw so perfect for a replacement. Length was harder with the 3/4" a bit short and the 1" a bit long. I went with longer for extra strength, although it required drilling the core a bit deeper. I used a #52 drill bit because it was what I had. Perhaps a bit small, but it worked fine and the next size up I had was too big.
[#52 drill bit in a large, screwdriver sized pin vise]
A major problem with the Meffert's Megaminx is the low height between the center piece and its cap. This really restricts the screw head and washer combination height, or it fills the space and you can't have any movement. At this point your springs are useless and your puzzle won't be as forgiving as you probably want.
The M3 pan head, with a washer, barely fit and the #4-40 pan had was not much better.
[#4-40 with #6 washer compared to the stock Meffert's]
I chose flat head (tapered on the bottom) to allow a lower height. The problem was finding a screw with a large enough hole but small enough outside diameter. The #4 washer was too small, and the #6 too large. The M5 washer was a great fit for a low height, but just a tiny bit to large to fit in the cap, which was a shame. the M4 was fairly low and fit the cap just fine. This allowed about 2mm movement, which is enough.
[#4-40 flat with M5 and M4 washers]
[M5 is too big, M4 is just fine]
A set of 8 screws was about $0.60 and a set of 5 washers was about $.50, although at another store I found them in a bin for $0.05 each.
I was looking for softer springs for my Meffert's and in the process of modifying a Chinese Megaminx (tiled, v2) I found its springs were much softer. In my Meffert's Megaminx DIY they make it very, very nice. So if you happen to be ordering from http://www.9sPuzzles.com
or any other site that caries these poor moving imitations, consider picking one up just to steal its springs.
But even at $7 for the clone, expensive shipping and wait times make this not an ideal solution. So I shopped around local hardware stores looking for suitable replacements. None of the springs were rated in foot pounds which was just as well because I had no proper way of knowing what I wanted. What I found was that springs with wire in the 0.016-0.020 gauge range were just about right. The Chinese Megaminx springs feel and look like the 0.016 gauge springs I found. I found them in 1 3/8" length, but cut in half with a bit of bending to flatten the end they were just about right at 11/16". Price was $2.99 for six, which cut in half gave me exactly the 12 springs I needed.
So if you have Meffert's Megaminx needing some fixups, hopefully you can save a lot of time at the hardware store.
P.S. Careful not to "screw up" your puzzle