This is one of the few cranksets that combines different metals. The arm section is made of Cro-Moly steel, has an elliptical shape and a hollow interior. The arm tapers from a 34.5mm width at the B/B spindle down to 22mm at the pedal spindle. The hollow, elliptical, tubular arms are drawn in their tapered shape by True Temper and then sent to Syncros. Syncros makes the Cro-moly pedal and spindle lug that is TIG welded to the bottom of the arm. The Cro-moly steel arms are powder painted in a matt black finish. The actual piece that seats the B/B spindle is made of aluminum. This aluminum piece is machined from 7075-T6 aluminum rod drilled and tapped with 22mm by 1mm threads for the cranks arm dust cap. The piece is also drilled through then broached to make the square sides two degree tapered spindle fitting. This aluminum piece has tapered sides to just squeeze into the outer B/B spindle lug under machine pressing. Both arms have this aluminum core within the steel outer B/B lug. The aluminum piece is considerably larger on the right crank because the machining process leaves an 88mm diameter disc for the inner chainring and the adapter spider to bolt to. So, again for the right arm the aluminum insert is made of one large piece of aluminum that is machined down to create the core insert with the adapter plate turned of the same billet piece. -Bike Pro