Posted by Matt Wilson [108.162.221.211] on Wednesday, June 28, 2017 at 12:34:53 :
In Reply to: Varies with alumnum alloys. posted by Chris Case [173.245.48.192] on Wednesday, June 28, 2017 at 09:45:12 :
Pins and pistons are all brand-new, never used. Did some more digging last night. The alloy, according to info I could find, is FM132 eutectic alloy (not hyper-eutectic). After doing some research, it appears the eutectic and hyper-eutectic aluminum alloys do not expand much at all with increases in temperature, unlike other aluminum alloys, which tend to expand a lot, at least compared to steel and iron. What defines hyper-eutectic is silicon content greater than about 12% (up to as much as 18%), while eutectic is right around 11 - 12%. The silicon itself does not expand appreciably with temperature and more importantly, it literally insulates the aluminum from the heat generated by combustion and friction.
All of this might explain why the pins are loose in their bores at room temp - i.e., they won't expand much when going from room temp to operating temp, so they need to start off at clearances very close to where they are supposed to be during operation. Similarly, from what I've read, the piston-to-cylinder clearance needs to be smaller than what it was with the older, hypo-eutectic alloys, because those older alloys expanded much more than the eutectic or hyper-eutectic alloys.
One might ask what the benefit is with using eutectic vs. Hyper-eutectic in any piston. Well, apparently, the hyper-eutectic alloys have so much silicon that that it cannot all remain in solution in the aluminum (think about sugar dissolving in water, and more sugar can be dissolved at higher temperatures, until you add so much that the sugar can no longer be dissolved and just collects at the bottom of the water container). In hyper-eutectic alloys, the excess silicon congregates at the surfaces of the piston, where it helps with heat insulation and reduces scuffing, but also makes the alloy much more brittle and susceptible to damage from detonation or pre-ignition. Based on my reading, eutectic still offers most of these benefits, but without being so brittle. Very interesting stuff...
I certainly welcome any input from anyone who may have additional information.
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