Hey, I looked on the
www.sk8kings.com site and it did'nt really give the info I wanted to answer your question about buying them. Here's my thoughts:
8mm axles DO improve traction because they get rid of some slop, sloppy wheel jiggle is one of the reasons you will lose traction. However, it's really only good if the axle is truly straight. If a bent stock axle is pushed out of a hanger, then the hole it came out of is likely bent also. If you make a perfectly straight axle out of 8mm rod and cut threads on the ends and push it into the hanger's bent hole(likely from being cast around a bent axle), now you have a perfect BENT 8mm axle following the bent hole. You might get lucky and have a truck that has a straight stock axle and leave you with a straight hole, but not likely by my limited experience.
You also need to have the ends of the hanger and the axle nuts exactly shaved off to be 90 degrees to the axle so those surfaces don't cause "angular binding" on the bearings. With the right spacers between the bearings and with 90 degree hanger/axle/nut/bearing interfaces, you should be able to tighten that axle nut down pretty snug and have a free spinning wheel, this is almost non existant on a stock(misaligned) truck. Wallgren made(makes?) "monkey nuts" which are axle locknuts turned on a lathe with a "built-in aligned "speedring". I think(?) Wallgren also is who started lathed hangers with a lathe cut "built-in alighned speedring" bearing seat. He's sorta dropped his website I think. You can probably get his attention on the "Machine Shop/Truck" forum.
*****If Sk8kings is putting in a straight 5/16" axle and machining the ends of the hangers 90 degrees to the axle and using good axle nuts that are 90 degrees to the axle, then you are pretty well set. That extra amount of 8mm advantage is real, but not as big of an issue as good alignment and 90 degree hanger/axle/locknut interfacing. I would email them and ask them about those issues.*******
What I do next, is I take a stock RT-X or S to a machinist, have them push out the old stock axle, make me a new 8mm axle that is longer than the stock axle so that there is clean unthreaded axle under the entire length of the bearings so I have to use some kind of spacer or speed rings on the OUTSIDE of the bearings between the outer bearing and the locknut. This leaves you with pristine axle and no threads under your bearings. This will eliminate slop a little more.
Then, I take my empty hanger home and my newly made 8mm axle. I clamp the hanger in a vise and use a drill bit just a smidge bigger than 8mm and OVERBORE the hole just slightlty enough to let the 8mm axle slide in and out unimpinged.
I then put JB Weld metal epoxy into the hanger/onto the axle and put the axle into the hanger with masking tape over the portions that will be exposed sticking out of the hanger when centered in place. This will fill the empty space.
Next bake the hanger in the oven at about 175-200 degrees F for about 1-1.5 hours so the JB Weld hardens harder and a LOT faster. Don't go too hot or you'll change the temper of the aluminum of the hanger or boil the JB Weld and get bubbles.
Next, if you have a lathe, or an electric drill and a wheel with bearings and some clamps to improvise a lathe, get that hanger spinning on a nice medium speed(not too fast or it's imbalance will make it wobble around), and use your cutters or a metal file to carefully bring that hanger face to 90 degrees to the axle without dinging the axle, you can likewise do some work on the axle nuts to make them exact 90 degrees to the axle. Now you're set.
Finally, put a good 15 degrees of Pos wedge on that RT-X in front, about 10-15 degrees negative wedge on that RT-S in back(with some harder bushings also). If you have kinpins long enough for FULL HEIGHT BUSHINGS TOP AND BOTTOM (cone on top, barrel on bottom for the rear truck, cone on top and barrel or another barrel on bottom on the front truck). Khiro makes bushing combinations that fit what I just described. If you can't fit full height bushings top and bottom (like the new Indy's won't allow it), you can go to the hardware store and get a longer grade 8 3/8" bolt to replace the stock kingpin.
I do use the Phase I Splitfire(uses the RT-X baseplate) as a T/S and hybrid rear truck more than any other truck and prefer it over all others. S'posedly Fluitt is going to make more Phase I's and put them out in the near future. They are worth it. But so are Radikals front and back(IF you like the back truck to feel like that, some riders do and some riders don't) so I would recommend trying out every kind of truck you see on other people's decks before putting that kind of money into it.
Hopefully that's more usable info than confusion-Good Luck, Paul