'51 M drive gears and front cover.

Is an M with serial # 266XXX (51) late enough to have the strengthened drive gears and front cover to safely run an add-on live hydraulic pump? Thank you.

Yes, the change to the front cover and cam gear took place at #228082.

Kirk: Thank you. You made my day!

IH did change the bolt pattern on the front cover and plate at engine serial # 228082, tractor serial # 238082. Bolt pattern matches the bolt pattern of hydraulic pumps IH used on other tractors. But they also made other changes to the covers after 1951 that may effect how you mount pump. If you use a pump with a snout on shaft in front of pump drive gear to mount a drive gear for a mechanical hour meter look at the following. After removing ignition drive, look in the hole for a oil fill screen at front. Flat plate with holes and mounted by 2 bolts. If you have that type cover the snout on pump may contact the front cover when the mounting surface of pump is just a little way from being flush. If this happens the snout can be shortened or removed if not needed. Another caution don't put a bolt to long in the lower inside pump mounting hole. End of bolt will go agianst cam gear if to long.
I have tried for a long time to determan when IH changed cam gears to 6760 DA and wish I could tell you exactly when. From what I do know it was after 1951. Don't know how much I would worry, lots of them were changed out over the years so you may have a later one. Also some have a pump and original gear still going.