How do I join two pieces of aluminum parts?

from the description of the parts you want to join its hard to give a definite answer as to how it can be done...if they are solid cylinders then the length of the cylinder matters... you could drill & dowel them & then use socket head cap screws to keep them together.. or if they are longer than the required length you could step the smaller one inside a cutout on the larger one.. welding could be a problem because they are elliptical not round.. so a problem to re machine them after welding... drilling & doweling the parts will make sure they line up in the same position each time but will not hold them together... you could also look into gluing them together... Lotus glue there aluminium car together & that seems to work well????

get a big block of aluminum and mill out a single piece problem solved.

Welding would be the strongest bond, unfortunately welding aluminum is almost impossible because it melts at such a low temperature. You may be able to find someone skilled enough to do it but don't get your hopes up. The next strongest is the dowels you speak of. To use dowels you have to drill holes just slightly larger than the dowel in the two cylinders that you want to mate, coat the holes with aluminum epoxy and insert the dowel into one cylinder, then press the other cylinder onto the dowels until the faces of the cylinders are tight. For a stronger bond coat the surfaces of the cylinders with epoxy as well. DO NOT USE CHEAP EPOXY, WITH EPOXY YOU GET WHAT YOU PAY FOR!

Weld them with a torch. That should work best for what you are doing, a propane torch should be fine. If you can't do it, go to the shop and ask them too. Without welding the bond will be weak and will cause a majority of the stress on the part to be in the dowels, which isn't safe.

TIG welding by a professional welder. Explain to them what you want and either they can or you are S. out of Luck

no- you have to weld them for stability. if you don't know how to, find someone who does.