In a perfect world, the bearing spacer (between the bearings) would be made of HT 440C stainless steel or HT SAE 52-100 Chrome Steel, ground to bearing tolerances, just like the inner bearing race would be. 303 Stainless or 6-4 Titanium can be used as a spacer material. Aluminum can only be used as a spacer material in a skateboard bearing assembly when the wheel nut is firmly toruqed against the system. Aluminum can not handle the abuse of the loosly installed environment and will change dimensionally in time. Also, Aluminum spacers that show up in skate bearing kits are almost always too short and are, obviously, of the poorest quality.
The Oust Spacer is made of aluminum, but I have spoken to them and they have assured me that it is of a very high grade 7000 series alloy that will hold up. I am testing tham now, and they seem to be holding up. The key to having the oust aluminum spacers and washers hold up, would be ensuring that the axle nut was fully tightened, eliminating any sort of play between the bearing and the spacer. This in theory includes shimming the axle to 8mm. Do yourself a favor and get some of the Oust Spacers or ABEC11 spacers and a quality core wheel.
The spacer should usually have a length between 0.400" to 0.440" (skateboard spacing standard plus wheel core tolerance) or 0.300" to 0.340" (roller skate standard plus wheel core tolerance) . Each wheel must be measured individually to find the exact spacing and the spacer must be at least 0.005"-0.010" over this dimention. Some companies are far better than others when it comes to holding this dimension in the wheel. I find that most wheel bearing spacing runs between 0.405" and 0.420". It is always better to space the bearings a little too much than too little. For Builins, it may be necessary to place a 0.005-0.040" spacer shim between the bearings and the spacer since Biltins are cut to exact .400" spacing (duh...Chris!!). This is true for the Oust spacer as well (duh..Karl!!) and it is that way for Powell as well (George, even you?). This step prevents the bearing from running in a higher friction, overly preloaded state. Oust is currently the only manufacturer besides Builtin/ABEC11 that is seriously considering bearing alignment as well as the bearing itself. It is just too bad that they took such a nieve approch and did not bother to measure the wheels on the market.
Myself and my sponsored riders are currently using custom made 4130 bearing spacers. I personally machine them and fit them to each wheel. For me, only the best will do.
The axle nut should be tightened firmly. The only detectable play should be coming from the bearings themselves, not the fit over the axle. For all but Biltins, an 8mm x 11.5mm washer should be placed on either side of the wheel assembly.Selecting a wheel with a high quality core insert will assist in keeping the outer races of the bearings aligned (faster) as much as current skate technology allows. Proper mounting of the outer races requires machined metal cores with fixing rings to lock or preload the outer rings in position. This practice is well beyond the reach of common skate wheel price points. Even in cycling, I know of only one company mounting hub bearings like this , Hadley, a very rare case. Motorcycles manufactures do not even bother with this, but both of these examples do use precision press fit bearings in metal hubs.