get a short shaft to slip into the cluster as you extract the 1 it came without must be the length of the cluster and washers now with cluster resting on the bottom. this makes the oh very easy as you don't have to chase the needle bearings. you need a new synk row and likely 2ndd  gear. most isues in thes trans are rushing the shift without releasing the clutch enough ,proper free play on the clutch throw out bearing is critical

Hi  Fifty years ago speed shifting would cause this. Would replace second speed gear would be good for a while. Sync teeth would get round off, pop out gear.

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

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.  

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.

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

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. 


71% of original size (was 710x19) - Click to enlarge

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.

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 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?


71% of original size (was 710x19) - Click to enlarge