727 transmission end gear question

[agoodmana]

Hello,

I have a '77 Scout II traveler, 345cid, 727 auto, 2-speed transfer case.

The low-reverse band broke awhile ago, and I rebuilt the transmission (my first rebuild). I did not check the low-reverse servo, and after starting it up, there was another crack, and reverse was gone again. Upon removing the pan (again), I could see that the low-reverse servo piston had shattered. I was a bit disappointed!!!

Over the last few days, I removed the drive shafts, the t-case, and the extension housing (tail housing?) and transmission pan, and got a new piston installed. Upon reassembling everything, the spacer(?) that fits into the oil seal in the t-case side of the extension housing fell out. I got it slid back in, and reinstalled the (bull?) gears and nut. I am now wondering if the spacer did not go all the way in, though... What should the end of the tail housing look like with the gears and nut installed? Does someone have a picture? The nut was just flush with the output shaft with no threads exposed, and I was able to get the transfer case on, but the transfer case seems to shift different.

I have not started it up yet, as I don't want to cause any damage if I made an error.

Thanks for any help!
-andy

 

[Michael Mayben]

Bummer!

But..."piston flop" in the rear servo is a kinda common malady. That is normally initially caused by a really "loose" low/reverse band and high engine rpm when the tranny is pulled into either d or r (such as with a cold engine/tranny with the carb on fast idle). Same condition causes the band anchor to break off like you found, that band material is cast iron.

Once it "flops" the first time, it May do it easier again! There are many aftermarket fixes for that including several different aftermarket piston setups. And even the transgo and B&M shift reprogramming kits include methodology for a fix that does not involve additional parts replacement.

But if the servo bore had any visible damage, it most likely will not "hold" an oem-type piston, that is when an aftermarket replacement item becomes a must in order to salvage the case/tranny.

Here's pic of what ya are looking for. Disregard the "blue" stuff, that is not rtv, but is a tranny assembly lube I use sometimes, it's applied to the lip of the two seals behind the bull gear to lube 'em to prevent an initial dry start rotation.

Also, you must use a gasket between the adapter and the transfer case, that establishes the very small running clearance between the bull gear and the output gear directly behind it. Do not ever use rtv at that location! If the bull gear contacts the rear output gear, extreme heat and noise/failure will result! The "distance piece" you are asking about is what establishes that clearance, though on occasion, I do "salvage" some of these parts by face grinding the end of the bull gear since those are very difficult to obtain. If yours shows any sign of contact with the output gear or signs of "heat/discoloration", that is something to deal with!

That one has a custom-fabbed ceramic bearing that has been "microblued" in the adapter housing, we have the same bearings in stock here at ihon but not the ceramic/microblue versions, that bearing is made-to-order and runs near $400!

About the only thing that would prevent proper bearing location over the output shaft would be if you removed the bearing and did not get it seated correctly in it's counterbore. For r&r of that bearing, I heat the adapter in the oven at 350f for an hour to both remove and install. Dealing with the snap ring is a real bitch also! If the bearing is not seated, you couldn't install the snap ring.

 

[agoodmana]

Excellent!

Thank you for the picture, that is what it looks like. I have and installed a new t-case to tail housing gasket, so that should be good.

The new piston I installed was of billet aluminum from a&a transmissions, and the bore seemed ok, so I hope that fixed it. Here is a link to what I installed:
727/518 billet aluminum low/reverse piston kit (1967-up) - a&a transmissions

Do I infer correctly from the reply that there is a transgo or B&M shift kit I should investigate that will make the recurrence of this problem less likely? In looking at shift kits, it seemed to me that they often advertise making shifts firmer, which I assumed would make parts more likely to break, not less.

Again, thank you for the picture. I hope to have the Scout running later today.

-andy

 

[Michael Mayben]

The a&a piston used a teflon ring set right, not the oem cast iron? If so, those are the only rings that can be used, the oem-type iron rings won't fit. I've used the a&a piston when needed and it will solve the issue permanently.

As for a shift kit, I always recommend the use, they can add greatly to both performance and durability because they correct the inherent" softness" which is due to overlap which can be directly attributed to long-term failure of the friction materials. For off-road use or towing apps, a shift reprogramming install is an absolute necessity for any "conventional" (non-electronic)slushbox.

Check this recent link, then follow the link in the link (is that a term??) to the install process for the transgo tf2 kit:

http://www.forums.IHPartsAmerica.com/transmission-tech/3646-installing-stored-727-a.html

 

[agoodmana]

Just to update, the Scout runs and the shift feel seems good. It has been a few years since it has had reverse, so this is great! Thanks again.

I did lose the spedo, seemingly I reinstalled the spedo gear in the incorrect position into the tail housing. (my Scout originally had a single speed t-case, so the spedo cable doesn't reach to the t-case.) I'll have to figure that out.

Still much more work...

I didn't really understand this comment, though: "the a&a piston used a teflon ring set right, not the oem cast iron?" it seemed to me that the a&a piston has a rubber seal on it, and the oem also had a rubber seal but the piston was made from sintered aluminum. Maybe I misunderstand something.

I included a picture of the broken piston and my Scout (no longer just parked in my driveway).

 

[Michael Mayben]

I was wrong about the a&a low/reverse servo piston, that one is a billet "copy" of the oem part and uses the oem seal system.

There are quite a few sources for these replacement pistons, some are aluminum, the one in this pic is steel and is the one I described that uses a teflon stabilizer secondary ring, along with a non-"standard" rubber lip seal. You can see there is no way that one will ever flop as it's got a much deeper section that contacts the bore for stabilization. The piston on the right in the pic is the oem part.

I'm not certain of the exact source of that piston, the reason I have not used it again is that the tranny it came out of was butchered in a previous repair attempt and who ever did that broke it's teflon ring and tore the "special" rubber lip seal. I could not wait for days/weeks to source replacements for those parts, so I put another brand servo piston in it's place.

Some folks think this is a chronic 904/727 issue...it is not! The original problem is caused by lack of maintenance (band adjustment), and too high an idle speed when the tranny is dropped into gear when cold!