I have recently found a 4-51 detroit in suprisingly good condition! This is quite a rare unit,it is NOT a 4-53 or 4-71 before you ask.They were a true 2-stroke without valves.It was found on what looks like a Massey-Harris frame with M-H sheet metal,but no ID from M-H.Have heard rumors that they experimented with them in the 50's.I am a oil-field heavy equip mech and am not sharp on my tractor and ag  history! Any info or history would help.I have the DDA parts book for the 51 series also.Eng model#43301,ser#4C3270. Thank you.