Originally Posted by
dragons_fire
....2. would it make a difference if the piston and cylinder were made form different materials. right now im using aluminum for everything?.....
Aluminum running against aluminum is not good, it can readily gall up and seize solid.
Whenever you have two materials running against each other as in a sleeve bearing or a piston and cylinder they nearly always should be different. There are two exceptions that I know about:
One is brass, brass will run against brass very successfully and this also applies to some of the bronzes. In fact it is possible to run some brasses and bronzes against themselves even without lubrication and this was done in old clocks.
The other is cast iron, ductile or nodular cast iron which is what machine tool ways are made out of. Although with cast iron you do not really have two identical materials you have a mixture of materials; it is the graphite inclusions in the cast iron that permit it to slide with suitable lubrication without galling. Dry cast iron running against itself may gall and seize.
In general however, if you are making a bearing or a piston and cylinder, the best choice is a harder or stronger material running against a softer or weaker material.
But the weaker material should not be aluminum, it can gall and seize with almost any other metal. Some aluminum alloys are okay such as the ones used for cheap electric motors where the shaft runs directly in the alloy end housing but these are specialized diecasting alloys.
Notice that I say aluminum can gall up; it is not guaranteed that it will. It is possible to have aluminum running against aluminum with good lubrication, particularly a lithium or graphite grease and it might work for a very long time at light loads. But it is courting sudden failure to do this.
An open mind is a virtue...so long as all the common sense has not leaked out.