T84 shift rail thimble/cup keeps popping off?

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Chuck Lutz
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T84 shift rail thimble/cup keeps popping off?

Post by Chuck Lutz » Sun Jul 03, 2016 3:50 pm

Having the thimble or cup pop off the front of the T84 case and either get destroyed by the clutch or just fall into the bellhousing has been reported many times. Today one showed up that had been installed on a rebuilt GPW case....it popped off so we scrounged an original "F" marked on because of the possibility a repop one was metric or just plain too small. In both instances black RTV was used on the outside of the cup and wasn't introduced inside for the shift rail to compress and knock out...

When it came back this time...I devised a little experiment:
Put the T84 in neutral.
Tipped the T84 & T/C foward on the bench and filled up the cup with gear oil.
Tapped the cup back in with a hammer and no sealant.
Shifted from 2nd to 3rd and BAM! the cup flew twenty-five feet across the yard!

Hydraulics!

If you check out a bare case, there is a notched/groove running through the case where the shift rail rides...this allows any leakage of gear oil around the shift rail to migrate back or...to be forced back into the case. I suspect that the groove on the case where the shift rail rides is somehow plugged with either some sealant, paint or whatever and without a way for the gear oil to migrate or to be forced back into the case.....the action of shifting compressed the gear oil and it overcame the seating of the cup and sealant.

Lesson learned:

Be SURE that those two notches/grooves in the front of the T84 case for the shift rails are perfectly CLEAN before you reassemble the T84! You can probably check with a wire by inserting it to see if it is clear if the trans is already assembled.....

2nd & 3rd shift rail: The notch is at the 3 o'clock position when looking at the front of the T84 case.
1st & Rev: at the 9 o'clock position.

The other solution is of course the French Army zig-zag bracket attached to the top bolt on the front bearing retainer...just make sure you test fit the bellhousing to see it clears the bracket and fits up snug to the T84 case!
French Shift Rail Cup Retainer - gustl.jpg
French Shift Rail Cup Retainer - gustl.jpg (106.71 KiB) Viewed 1568 times
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Bantam T3-C 1947


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Re: T84 shift rail thimble/cup keeps popping off?

Post by Joe Gopan » Sun Jul 03, 2016 4:12 pm

Sounds like you are on to something. Sometimes it can be caused by aggressive speed shifting.
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Re: T84 shift rail thimble/cup keeps popping off?

Post by artificer » Mon Jul 04, 2016 2:33 am

Why not solve this problem that has always been there!
How was this done on later models of the same T84 transmissions with case mounted shift rails?
Different manufacturers used various models of this tranny that evolved into the T90 with rails in the tower.
The cap idea was no good, so an 'O' ring groove on/in either the housing or on the selector shaft would possibly solve this forever.
Ask Gindi, a master on repairing/fixing/modifying these transmissions & components for everyday use.
He obviously has resolved this always present problem, on his re-builds as would most practicing mechanics presented with a continually occurring similar issue.
His pictorials are not to miss.
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TO DIAGNOSE, TROUBLESHOOT OR FAULT FIND ANY AUTO SYSTEM....
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.

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Re: T84 shift rail thimble/cup keeps popping off?

Post by dinof » Mon Jul 04, 2016 5:33 am

For good measure, and to make sure this doesn't ever happen, I guess you could install the French made bracket. Me personally have never seen this offered by a supplier. I guess you could make one yourself. Either way, good tip Chuck for those that will be doing their tranny's soon.
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Re: T84 shift rail thimble/cup keeps popping off?

Post by Vic ALLAN » Mon Jul 04, 2016 5:53 am

I made a brass cup and a bracket about 40 years ago ......its still installed.......I didn't know at the time what should be there .....sure stopped my oil leak.
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Re: T84 shift rail thimble/cup keeps popping off?

Post by Chuck Lutz » Mon Jul 04, 2016 8:46 am

I think the cup and a little sealant on a clean case will prevent gear oil leaks....

However, if the cup isn't sealing then gear oil will leak past it.
I think that over 70 years of shifting has worn the CAST case rather than the STEEL shift rail so the opening may no longer be the same spec.
A loose or non-spec cup might leak gear oil.
A loose or non-spec cup might take flight also.
The "groove" in the case alongside the shift rail would let air or air/gear oil pass back into the case if it was not plugged up.
I think the French Army bracket solves the loose cup issue.
It may also help/solve the plugged relief groove but as a warning, guys should DEFINITELY check the case when assembling to see if the groove is plugged and be extra careful when installing the cup with sealant that the sealant does not...well...seal the groove also!
Chuck Lutz

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Bantam T3-C 1947

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Re: T84 shift rail thimble/cup keeps popping off?

Post by 19OlllllllO41 » Fri Jul 20, 2018 1:56 pm

Hey Guys,

Bringing back a very OLD thread here....

My shift rail cup popped off last week and I have new one on order. I am planning to try to install the new one through the access hole without removing the tranny. After taking a close look at it today, it looks as though removing the top bolt of the bearing retainer is possible through the access hole to slip on a "french army type bracket" as Chuck talks about above. Am I crazy for thinking this bracket can be added without removing the tranny? Anyone done it? Can I just remove that top bolt of the bearing retainer without it causing a problem?

Thanks in advance for any thoughts on this!
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Re: T84 shift rail thimble/cup keeps popping off?

Post by uglyjeep » Fri Jul 20, 2018 8:17 pm

Chuck Lutz wrote:
Sun Jul 03, 2016 3:50 pm
...

Hydraulics!

If you check out a bare case, there is a notched/groove running through the case where the shift rail rides...this allows any leakage of gear oil around the shift rail to migrate back or...to be forced back into the case. I suspect that the groove on the case where the shift rail rides is somehow plugged with either some sealant, paint or whatever and without a way for the gear oil to migrate or to be forced back into the case.....the action of shifting compressed the gear oil and it overcame the seating of the cup and sealant.

Lesson learned:

Be SURE that those two notches/grooves in the front of the T84 case for the shift rails are perfectly CLEAN before you reassemble the T84! You can probably check with a wire by inserting it to see if it is clear if the trans is already assembled.....

2nd & 3rd shift rail: The notch is at the 3 o'clock position when looking at the front of the T84 case.
1st & Rev: at the 9 o'clock position.

The other solution is of course the French Army zig-zag bracket attached to the top bolt on the front bearing retainer...just make sure you test fit the bellhousing to see it clears the bracket and fits up snug to the T84 case!
French Shift Rail Cup Retainer - gustl.jpg
Hello,

Here is another solution...
I have seen the inside of numerous transmissions and transfer cases, and sometimes there is a groove cut lengthwise at the end of shift rails, or a flat spot ground into the portion of the shift rails that extend into the case thimble caps to help prevent pressure build up that can pop these thimble caps out. It makes better sense to locate the groove in the shift rail, instead of in the case, since the thimbles are round and match the diameter of the shift rails nicely, so a pressure relief in the case wouldn't do much good once a round shift rail with no pressure relief groove enters the thimble cap.

Hope this helps,
Daniel

PS...my favorite sealant for installing the thimble caps into the cases is Loctite 640 sleeve retaining compound or Permatex 1 hardening formula, although I have also had success with non-hardening Permatex 2.

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Re: T84 shift rail thimble/cup keeps popping off?

Post by Joe Gopan » Sat Jul 21, 2018 3:52 am

I would try another shift rail, something is going too far forward.
2011 MVPA PIONEER AWARD - MVPA #1064
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Re: T84 shift rail thimble/cup keeps popping off?

Post by Chuck Lutz » Sun Jul 22, 2018 9:56 am

Use some sealant if you wish, but be advised that if you plug up the slot cut in the case to relieve the pressure built up by moving the shift rail into a sealed cap, something has to give....and as I posted way back when, our experiment proved that even a brand-new cup seated with a hammer into a T84 case on the bench if you plug up the slot, the cup WILL pop off. It may not do it to begin with as air is somewhat compressible but gear oil that gets in there is a lot less forgiving....

I can't find any pics of the front of a T84 that show where the slot is cut so you'll need to confirm where that is unless you remove the transmission and make the "French Connection" bracket to hold the cup in place.

It can be seated via the inspection hole but you need small hands and a lot of patience to get it started in there and to get a tool to seat it. Be sure to have telescoping magnet to retrieve it from the bottom of the bellhousing a few times as it is a difficult item to get in there TIGHT! A glancing blow will be sure to dislodge it so be prepared!


I would suggest you avoid the sealant and try to dry-fit the cup if the trans is still in the jeep. However if it is on the bench and you can CAREFULLY apply a little...LITTLE...sealant away from the slot prior to seating it, I would do that.
Chuck Lutz

GPW 17963 4/24/42 Chester, PA. USA 20113473 (USA est./Tom W.)
Bantam T3-C 1947

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Re: T84 shift rail thimble/cup keeps popping off?

Post by jkillelea » Sun Jul 22, 2018 10:39 am

Here are photos of those slots on the shift rails. Hope this helps.
John
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P1060403.JPG
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Re: T84 shift rail thimble/cup keeps popping off?

Post by jkillelea » Sun Jul 22, 2018 10:41 am

Correction. These are the slots in the front of the T84 case where the shift rails slide. These slots allow air and gear oil to relieve pressure buildup in the thimble cup.

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Re: T84 shift rail thimble/cup keeps popping off?

Post by 19OlllllllO41 » Mon Jul 23, 2018 8:23 am

Thanks for the replies gentlemen,

The pics of the slot are a big help, because I have not really been able to see them clearly with my tranny still in the jeep. From what I see it almost looks like the 2/3 shaft in my tranny has a groove in it, but that may just be from the bad view looking through a inspection mirror.

So two questions:

1. From what I gather (relating back to my original question), it is NOT possible to put the french style bracket on if the tranny is still in the jeep?

2. I may or may not be missing a 2/3 poppet ball and spring.... can this be something that is replaced WITH the tranny still in the jeep?

Thanks in advance!
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Re: T84 shift rail thimble/cup keeps popping off?

Post by jkillelea » Mon Jul 23, 2018 10:23 am

No. Not in my opinion. I finished rebuilding my tranny today and with the tranny on my bench it is a bit tricky to hold the poppet ball in place and slide the shifter rail on to it. It can pop out with a bit of trajectory, so cover the hole or wear goggles, or both. I slid the shift rail in through the back of the tranny case and held a screwdriver (through the front rail hole) to hold down the poppet ball. That spring and ball were more difficult to hold down than the first/reverse rail poppet ball and spring. I think you have to pull the tranny/t-case and separate them. If someone else has a different approach, hopefully they will chime in.
John

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Re: T84 shift rail thimble/cup keeps popping off?

Post by Chuck Lutz » Mon Jul 23, 2018 12:15 pm

1) from the photos above, you can see where the lots are on the front of the T84 case. If you CAN'T see if the slot is unobstructed then it may have some of the sealant closing it up towards the rear and that will probably defeat your clean installation.
2) If you can be sure it is cleaned out, then you also now know what 1/4 quadrant of the cup perimeter can NOT have any sealant attached to it.
3) the sealant isn't what holds the cup in place, it is sized to be driven home but that is not an easy task although it is possible. The sealant is merely a little added insurance. Unfortunately assemblers have often put enough sealant on the cup for multiple installations when just a little of it smeared around the 3/4 of the perimeter will be enough.
4) No, you can't install the "French Connection" bracket with the T84 in the jeep.
5) You should be able to "feel" the T84 poppet ball pop into position on the shift rail as you move the shift cane from 1st to Rev. Then do the same between 2nd and 3rd and you should still feel the poppet ball pop into the notch on the shift rail. Once you have understood what that feels like between 1st and Rev, if you don't feel that when moving between 2nd ad 3rd, and the shift cane just glides smoothly back and forth you are either missing the poppet ball or the spring is broken or missing or the ball gummed up down in the hole for it in the case/.
6) No, you can't install the poppet ball and spring with the T84 still connected to the bellhousing and the T/C.
Chuck Lutz

GPW 17963 4/24/42 Chester, PA. USA 20113473 (USA est./Tom W.)
Bantam T3-C 1947


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