goronimal

This mabe a dump question but I`m asking anyway,be gentle.Can you just rebuild the OD unit on the 518A trans.1st through drive work great,shift good no slipping.Just asking.......probably be better rebuilding tranny.Open for input.

BC847

I'm fixing to tear into mine and yes, the OD unit is serviced separately.

Be aware though, there's an 800 pound preload on the clutch-pack spring and it requires a special tool to compress the spring assembly so one can remove the C-clip so one can get to the discs and steels, etc.
There's also the transmission's governor located on the rear of the assemble right where the output shaft takes off. There's a couple of hydraulic tubes that are said to be a pain in the behind to get into proper place upon reassembly.

Further, if you remove the OD unit and don't plan on messing with it, there's a special tool (essentially an intermediate shaft with a handle) that is needed to hold all the discs internal splines as well as the drum splines in proper alignment for the purposes of reassembly.
Without this tool or its equivalent, it's my understanding the OD unit itself must be disassembled to a degree so as to allow reattachment to the 3-speed section.

Examples of the two referenced tools can be found here ~ http://www.transmissionpartsusa.com/...-000jt1318.htm

Hope this helps some.