The problem with material selection I have is knowing what the range of properties needed for a particular applicationwould be. How much temperature resistsance do you need for instance if making a piston head? What would be the normal operating temperature... 0 to 35deg C perhaps, but at compression the temperature will rise, & if it diesels then what happens to the material, will its edges soften & fail or alternatively will they cook & crumble after a few cycles. Then there's chemical resistance to the lubrication & a host of other things. So if I was to look at the properties ona selector guide I'd probably pick the toughest, most heat resistant, most chemical resistant one with the highest modulus...ie completely over the top & consequently pick the wrong one based on the false asuption that I was doing the right thing erring on the side of caution, but missing out on the benefits a more flexible shock resistant, beter wearing material might give. The selector guides might mention ellectric motor gears, hydraulic pump valves etc but translating that into relevant applications aint straightforward. So any known applications/ material grades would be interesting. Haven't looked at peek in any detail but there is a (for the unknowing) a bewildering array of engineering plastics available.