The original post indicated that this image came from a vintage BMX 'net source. I think old 3-piece BMX cranks usually had the ring mounted inside the spider, so the counterbore on the outside would make sense for BMX applications. My guess is that this is some sort of special crank for a Shimano-sponsored BMX rider. The fact that these DD arms have 9/16" holes suggests that the factory, or a source close to the factory, was involved in it's making. Others have suggested that the spider may have been modified to accept Pitch-10 rings, but that wouldn't make much sense for BMX use unless there some Pitch-10 single-speed freewheels (or Shimano 1-speed cassette cogs) were also made. My guess is that the mod was not for pitch 10, but for 110mm ("Sugino Maxy"), which I think was the BMX standard.
Tom Dalton Bethlehem, PA, USA
I seem to see a counterbore in the _outside_ of the chainring bolt holes, for a chainring bolt or nut to nestle in, with NO chainring mounted to the outside of the spider arms. A normal track crank will have the counterbore on the inside of the spider arms, and a road crank will have no counterbore at all.
I'm gonna guess this is a regular 1/2" pitch AX crank that has been modified with a smaller BCD and no ability to mount a ring on the outside, for some strange reason. Could this have been "factory"?
Or I could be seeing shadows - could the Original Poster (crank owner Rick) verify whether there is a counterbore?
I also notice the pedal axle holes are regular size, but the "meat" around the hole is so bulbous that the extra-large AX pedal hole could fit there - making me think these started as forgings for AX pedals, but had 9/16" holes drilled for some reason.
Mark Bulgier Seattle WA USA
Everyone is raving about the all-new Yahoo! Mail beta.