Reading a Larry Shepard article about Mopar valvetrains: he mentions that the hydro rocker shaft is NOT round, and the mechanical shafts are ground round. Ever hear of this? "...The hydraulic rocker shaft is generally not used with the mechanical rockers. Either the T/A or the W2 shaft can be used... The T/A and W2 shafts were ground “round” and the hydraulic production shafts are not. Mechanical rockers need a round shaft.." I mic'd a set of hydros at .8740 every way I could, Summit lists the crane ductile shaft at .872 as well as all other non roller shafts. and I mic'd my 273's at .872x. Im not sure of my mic's accuracy but maybe someone in the know could tell me if there is in fact a diameter or cylindricity difference between the 2. I know the ductile iron fit on both with no binding as I modded a hydro shaft with additional oil ports to match the ductile iron rockers oil ports. I did have to knock down the hydros bolt holes of ANY flashing as it would stop the iron rockers progress down the shaft cold.