I would like add to this thread with my recent winch install on my M923A0. I installed everything but the winch, frame extensions and pto in about 10 hours. The pto itself took about 7 hours, with a transmission oil change also done at the same time.
There are definitely 2 different styles of pto that fit the M939 trucks. I have the earlier one, which has a long bolt that goes in the center top mounting hole of the pto, and is tightened from the outside of the pto.
The second PTO style has a SHORT bolt that goes in the top center mounting hole, and is tightened from the INSIDE of the pto, with the actuator cover off. The actuator cover is also moved into a top position, in order to clear the frame and allow removal of the cover without COMPLETE removal of the pto from the transmission in order to check backlash, or make repairs.
Here is how I installed my pto, and what I found that made it significantly easier, WITHOUT removing transmission mount bolts, engine mount bolts, or anything else crazy that would make it a 3 day job.
First remove your trans dipstick assembly, and both trans cooler lines. Now is a good time to change your trans oil and filter. Removing these item buys you some room to work. Install your (2) pto studs in the lower holes, be sure to get them threaded in all the way, keep in mind you are screwing an iron stud into an aluminum case. If you have the earlier style pto like I do, remove the pto actuator cover(4 bolts), and begin install. If you have the earlier pto, you will be installing the pto twice, once to check backlash, and the second time to complete install.
The way I found easiest to get the pto onto the trans with the 2 studs installed is to place the pto inside of the passenger side frame rail, at the very rear of the trans, near the transfer case, and move it forward slowly with the pto's output shaft pointed at the transmission. When you get the studs, swing the pto around and align it with the 2 studs. HERE is where I ran into clearance issues. I had to remove 2 bolts that secure the front toolbox mount to the frame. There are 4 of them on the inside of the frame, and when installed with the nut side towards the pto, they protrude approx. 1", and you desperately need that 1" to complete your mounting of the pto to the transmission.
Pic shows the bolts I removed to gain clearance from the outside of the truck. Once these 2 bolts were removed, the pto literally fell onto the truck.
It's that easy. Anybody could do it. So I wound up with 3 gaskets for shims, one thin and two thick, for a total of 0.050". For the final install, place the pto actuator and cover into the pto, but LEAVE the 4 mounting bolts out for now. Mount the pto onto the trans, be sure to get the really long bolt into the pto before placing it into the frame; this bolt goes in the very bottom center mount hole, it is too long to install after the pto is mounted onto the trans, unless you plan on drilling a hole into the frame. I suggest using a liberal amount of duct tape to hold the bolt in the pto while sliding it thru the frame.
I used blue medium threadlocker on all my fasteners to complete the install. A good thin 12pt. snap-on wrench will also make life easier while tightening the 6 bolts/studs that secure the pto.
I you have the earlier pto like I do, you will have to tilt the pto actuator cover up in order to install the pto actuator arm onto the pto cover. You will also need to know the indexing(clocking) position of the arm in order to get the throw of the cable correct. This is tedious at best. The reason for this is the bolt does not have room to thread into the cover, because of the frame.
I am guessing that the later style of pto alleviates this, because the position of its actuator cover appears to be on the very top, instead of tilted onto the side somewhat like my earlier style pto. This would mean that you could install the pto once, check the backlash, then simply install the actuator cover and actuator arm WITHOUT any grief or cursing.
Overall I found the pto install to be not that bad, and certainly not as painful as some make it out to be. I suspect that the reason opinions vary on this install is that there are big differences in A0/A1 and A2 trucks. Just guessing here, but installation on an A2 could be harder, due to the powertrain being located forward in the truck chassis several inches, and thus the pto not being directly below the in cab access door. Also guessing, but I bet that when the A2 trucks came into production, is also when the second later style of pto was produced because of unique clearance issues.
