Yes,
841 = 4 speed w/ transmission PTO
851 = 5 speed w/ Transmission PTO
861 = 5 speed w/ Live PTO
871 = 10 speed SOS with Independent PTO
881 = 10 speed SOS with dual range Independent PTO
 
and..

811 - Select-O-Speed transmission, no PTO
821 - Four speed transmission, no PTO or 3 point lift
831 - Four speed transmission, no PTO
 
Just to fill in a bit more the series is 801 but as pointed out there are a number of models in the 801 series and the 871 is just one of them so yes all 871s are SOS transmission tractors
 

But be advised that it could have either the original over-running problematic SOS or the 2nd generation improved very reliable replacement. Most of the early ones got replaced.
 
Thanks to all for the info. Showcrop, How do I tell if is the
replacement? Also where is the serial number that will give me the
year of this machine?
 
Look for the starter. Just behind the starter it 2 bolt holes. Above those holes is a flat area that should have both the model and serial number. Should look sort of like this
8 7 1
^%&*^$
Top number may have diamond before and after and the lower number is the serial number
 
(quoted from post at 09:59:39 10/25/14) Thanks to all for the info. Showcrop, How do I tell if is the
replacement? Also where is the serial number that will give me the
year of this machine?

The original over-running SOS would free wheel down hill in three of the forward speeds, I don't recall off hand which ones. The best way to tell is the casting number on the side of the trannie but I would have to look that up. I am pretty sure that the newer one starts with CONN. Another real quick and easy one is the shape of the trans. cover. The original has a rounded hump that runs all the way across. The second gen. hump is truncated on the right side. It has a flat slant down to the right. Also the second gen has a drain plug on the bottom whereas the original did not. You may not find a serial number where Old told you to look. Some of the replacement trannies never got them stamped in.
 
Found the number stamped (75524) any idea of the year?. This
thing does free wheel is several gears, they are marked with
slashes over the speed numbers. I know 5th and 6th. Also I can't
find a drain plug.
 
(quoted from post at 17:09:51 10/25/14) Found the number stamped (75524) any idea of the year?. This
thing does free wheel is several gears, they are marked with
slashes over the speed numbers. I know 5th and 6th. Also I can't
find a drain plug.

Well, that means that you have an original SOS. There are not many around, so you have a rare one. The serial number is for a 1959, and it was built at a bout halfway through the year's production. If your SOS is working correctly now, most likely it will continue to for many years if not used for a lot of hours. To keep it working properly you should have to do little more than change the oil in it and adjust the bands every 500 hours or so. The need for oil changes depends as much on climate as hours of use. I changed the oil in mine right after I got it, then I was trouble shooting a problem with PTO engagement about ten years later, and drained the oil, and even though it was way short of the hours indicated for an oil change, the oil was pretty bad, and I blame it on condensation, and lack of running time to drive it off. If you don't already you will want to get an owners manual, as well as a shop manual. An IT manual off the rack at your CNH dealer should suffice. You need the shop manual in order to remove the hood or the top cover of the transmission. You will want to change the internal filter but you can't as they are NLA. A member on these forums is working with two manufacturers to get them made again. I got one of the last few a year and a half ago. There are a number of members on these forums who who are very knowledgeable on the SOS and can even do a major overhaul of it if needed. Good luck with it!
 
Thank you much for the info. I have owned this machine for a long
time and it has always functioned correctly. I have been constantly
afraid that it would fail and have heard that it is really difficult to
repair. I recently added fluid to the tranny, first time ever but had
not checked it in a long time. You said it would be good to change
the fluid but how do you drain it with no drain plug? About all the
tractor does is pull a finish mower over about 4 acres of grass and
occasionally plow a little snow.
 
(quoted from post at 07:27:45 10/27/14) Thank you much for the info. I have owned this machine for a long
time and it has always functioned correctly. I have been constantly
afraid that it would fail and have heard that it is really difficult to
repair. I recently added fluid to the tranny, first time ever but had
not checked it in a long time. You said it would be good to change
the fluid but how do you drain it with no drain plug? About all the
tractor does is pull a finish mower over about 4 acres of grass and
occasionally plow a little snow.

I am not positive but I believe that there is a screen on the left side at the bottom. You remove a probably 1/2 head screw and the thin steel strap in order to pull out the plug that holds the screen, and the oil will drain out that hole. Drive it, shifting a lot for a half hour or so to get particles suspended in the oil.
 

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