I have a issuse that am trying to get as much to know before I dig into the work.  My 3 spd OD which is a top loader in my 55 Ford seem to want to drop out of 2nd gear and go to natural at time.  So before I tear into it need as much insight as I can get to what to look at or for.  I know that under the cover there's a spring with a small ball bearing. Does someone know what this does?  Inside there are shifts that are grooved for the ball to slide into as the forks move to different positions.  Would this be a area that might cause the dropping out of 2nd gear issuse? 
I kind of working of what left of my memory when I was much younger.  Back then I would just rip into it but now at my age i would like to know what to look for, as those old task have become much harder to deal with.
You all have seem to have great input on other issuse I ask about for that I say great job and great people.  HELP 

I would first look for any issues external to the transmission, namely the bushings where the shift levers connect to the arms on the transmission. They wear out after awhile. Also check the shift lever adjustment at the lower end of the steering column. Place the lever at the top of the column in the neutral position, it should be horizontal or pretty close to horizontal. Then look at the tops of the 2 shift levers at the bottom of the column; they should be on the same plane relative to each other. If they aren't, disconnect them from the column and adjust the clevises.

https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.dennis-carpenter.com%2Ftrucks%2Ftransmission%2Fgear-shifter%2Fc5tz-7343-a-column-gear-shift-repair-kit&psig=AOvVaw0GxCq9G1i3Jvc4_GrLCC7m&ust=1612038691451000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCJDi_6n-we4CFQAAAAAdAAAAABAD

Internally, I would suspect the second gear synchronizer is getting worn out. or the 2nd gear bushing may have too much clearance on the main shaft which could be either gear bushing or shaft wear.

Here is a YouTube video on rebuilding the Borg-Warner T-86 with OD. 

https://www.youtube.com/watch?v=o2tFwBTrLRQ

I have the same transmission in my 1955 TBird.  It works great except for a small amount of noise if I shift fast into 2nd when the car is cold.  Once it warms up, it works with no complaints.  My R10 overdrive works too.   I love my T-86 with O/D. It's rare in a baby bird and original to my car. My rear gears are low and I can cruise on the highway at less than 2,000 rpm.  Back in the day, I broke so many three speeds trannys I bought one a month. Because of this, I baby my T-86.  How strong are the T-86 transmissions?

Those overdrives were not much stronger if any than the standard three speeds.  If you got the Mercury with the O.D. and/or the 312 engine in a ‘56/’57 Ford, then you likely got the T-85 as the standard equipment transmission.  Everyone else received the weaker and more prone to break weaker transmission.
 
Like you, I tore up my share of those 3 speed trannies and even broke one right down through the middle of the case dumping parts out on the ground.  The T-85 on the other hand was a completely different animal and pretty tough.  I have had one of those T-85 overdrive in my ’55 Customline now for 31 years and it's getting due for a freshening up.

Lorena, Texas (South of Waco)

I think they could grind into 2nd when brand new on a quick shift, just the nature of the beast. The T-86 was used behind 6 cylinders, flatheads and Y-blocks up through the 272. They can be used behind the 292 and 312 if you're careful. Don't light up the tires pulling out of the Dairy Queen parking lot.

Pretty much what I thought.  I've had T-85s before.  Very rugged transmissions.  The T-86 is a top loader, so I thought it might be a little stronger than the side plate passenger car tranny?  I have never dumped the clutch or speed shifted my T-86 and I don't plan to. These cars are old and should be respected if you don't want trouble.  You don't want to be pulling an O/D transmission out of a small TBird. Ask me how I know. 

Popping out of gear is the synchro. Might as well do an over all overhaul. 

The T-85 is a lot stronger by orders of magnitude. Borg Warner used it to develop the T-10 4 speed, possibly the 1st "rock crusher" transmission. The T-85 was used well into the FE era including the 427. MOPAR used them behind the hemi and Studebaker used them in supercharged applications, namely the Golden Hawk. It pretty much takes a deliberate act to break one.

Yes the detention balls position the shift shaft, but it’s the worn synchro that “poops” it of gear.  New bearings, bushings  synchro will give a decent transmission.
ys the T85 r11 combo is superior behind a souped up mill and would be a prudent swap, however if your 55’ is stock your T86 ought to provide your needs.  

Thanks for all your input.  Great people and great group to get help!