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Don't assume that just because you don't understand something simple that others don't as well. The .827 is the pin wall thickness, the 2.5 is the length of the pin, and the DBL, well that sounds a lot like double right? You're a sharp fella though right, I'm sure you can figure it out from there. As for it's relevancy with these pistons in the for sale thread, it's nothing special, and is used in all of Ross' OTS 4G pistons /ubbthreads/images/graemlins/wink.gif
EDIT> If you still can't figure it out you can cheat here. Just pay close attention to the second sentence of the first link!