T90 transmission rebuild

1950 - 1952, M38, questions, discussions, regarding anything related to the M38.
Post Reply
Scott in NM
Sergeant Major of the Gee
Sergeant Major of the Gee
Posts: 322
Joined: Sun Sep 28, 2003 9:04 pm
Location: Tijeras, NM
Contact:

T90 transmission rebuild

Post by Scott in NM » Tue Dec 29, 2020 2:14 pm

M38 engine in for overhaul. Transmission could use a good going over while out. Two questions:
1. How difficult is it to overhaul the transmission? Overall, transmission works like a champ, only difficulty is downshifting from 3rd to 2nd (synchronizer gears worn?). Otherwise no complaints. It hasn't been touched in at least 30 years except for the periodic oil change. I have all the manuals and know how to read, so I'm pretty certain I can figure it out. I also have a great document from Novak Conversions on rebuilding this thing.
2. If self-overhaul is contraindicated, how much should it cost for me to take the transmission into a shop and have them overhaul it? I've not taken the cover off to see if the gears are good, but since everything was working, I'm assuming that they do not have to be replaced.

Inputs from those more experienced than I greatly appreciated!

Scott


Ken Perkins
G-Major
G-Major
Posts: 802
Joined: Fri Dec 13, 2002 10:26 pm
Location: LIVERMORE, CALIFORNIA

Re: T90 transmission rebuild

Post by Ken Perkins » Tue Dec 29, 2020 4:21 pm

you shouldn't have a problem...
1951 M38 #22936
1951 M100 DUNBAR
1953 m100 Dunbar
M416B1
2- M762
MVPA #11788
MVCC

JMZ421
Sergeant Major of the Gee
Sergeant Major of the Gee
Posts: 483
Joined: Sat Jun 02, 2012 2:21 pm
Location: N.E. Ohio

Re: T90 transmission rebuild

Post by JMZ421 » Tue Dec 29, 2020 4:37 pm

Scott,

I took apart a T90 from my A1 after one of those syncs seized up. No experience at all with T90. I just reseached here on the G503 and lots of pictures and put everything back together. I just changed the shaft and upper part, left the bottom part alone. I just remember having to deal with needle bearing and using thick grease to hold them in place. Got it done and no leaks from around the shaft. It helps not to be in a rush!! John

User avatar
dpcd67
G-General
G-General
Posts: 7947
Joined: Mon Jul 25, 2011 4:41 pm
Location: Iowa

Re: T90 transmission rebuild

Post by dpcd67 » Tue Dec 29, 2020 4:42 pm

The T90 is a far better transmission than the T84 and actually not hard to rebuild. Kits are being sold to do so. You can do it. I use vasoline to hold the lower needle bearings in place. Study the TM and you will have no problems.
How much will a shop charge to do it? Way more than I will pay.
U. S. Army 28 years.
Armor Branch

jnissen
G-Master Sergeant
G-Master Sergeant
Posts: 116
Joined: Sun Dec 16, 2018 11:43 pm
Location:
Contact:

Re: T90 transmission rebuild

Post by jnissen » Tue Dec 29, 2020 6:32 pm

Just use assembly lube to hold things in place and it’s easy. Plenty of good videos out there as well. Don’t forget to crack open the transfer case as well while your at it.


Sent from my iPad using Tapatalk
Jim Nissen
1955 M38A1
Patent Plate MD74784
Engine Plate MD85578
BS Hood Number 20974784

Scott in NM
Sergeant Major of the Gee
Sergeant Major of the Gee
Posts: 322
Joined: Sun Sep 28, 2003 9:04 pm
Location: Tijeras, NM
Contact:

Re: T90 transmission rebuild

Post by Scott in NM » Fri Jan 01, 2021 11:25 am

OK, started disassembly per the manual and several YouTube videos with the tranny sitting on the bench. Para 52 C, Loosen oil collector, it shows two cap screws that need to be removed. I removed the upper screw . I see the threaded end of the lower screw sticking out of the oil collector but can't seem to find the hole on the outside of the case. I've used 0000 steel wool and fine sandpaper to see if the hole could be buried under junk but nothing appears. Everything says to remove, but does the oil collector just swing out of the way? If not, I need help as I CAN'T FIND THE HOLE! (Hate it when that happens.)

Scott

jnissen
G-Master Sergeant
G-Master Sergeant
Posts: 116
Joined: Sun Dec 16, 2018 11:43 pm
Location:
Contact:

Re: T90 transmission rebuild

Post by jnissen » Sat Jan 02, 2021 10:43 am

If I remember correctly the two retaining bolts are recessed into pockets drilled in the front of the case. Should be as obvious as the nose on your face! Possible the bolt head broke off and your looking at the remainder.


Sent from my iPad using Tapatalk
Jim Nissen
1955 M38A1
Patent Plate MD74784
Engine Plate MD85578
BS Hood Number 20974784

Joe Gopan
Gee Addict
Posts: 49595
Joined: Sun Mar 02, 2008 2:37 pm
Location: Proving Ground

Re: T90 transmission rebuild

Post by Joe Gopan » Sat Jan 02, 2021 11:38 am

The Collector is attached with 2 each 5/16 NC x 3/4" " Socket Head (Allen Head to the peanut gallery) and 2 each 5/16" Copper washers.
2011 MVPA PIONEER AWARD - MVPA #1064
HONOR GRAD-WHEELED VEHICLE MECHANIC SCHOOL 1960 - US ARMY ORDNANCE SCHOOL(MACHINIST) ABERDEEN PG 1962 - O-1 BIRD DOG CREWCHIEF - 300,000+TROUBLE FREE M-38A1 MILES
LIFE MEMBER AM LEGION-40/8-DAV
7 MIL SPEC MAINTAINED MV'S
COL. BRUNO BROOKS (ARMY MOTORS) IS MY HERO

Scott in NM
Sergeant Major of the Gee
Sergeant Major of the Gee
Posts: 322
Joined: Sun Sep 28, 2003 9:04 pm
Location: Tijeras, NM
Contact:

Re: T90 transmission rebuild

Post by Scott in NM » Thu Jan 07, 2021 8:58 am

OK, rebuild continuing. Noticed that the input and output shafts prior to removal had about 1/8" of wobble when measured at the tips. The shaft bearings did not have any freeplay, but the bearings themselves were a little loose within the transmission case causing the movement. I thought these had to be pressed in and out, not able to be removed by hand. Is this correct? Or do the front and rear shafts lose the wobble when inserted into the transfer case and bell housing? Prior to this the transmission worked like a champ except for grinding when downshifting from 3rd to 2nd.

Scott

jnissen
G-Master Sergeant
G-Master Sergeant
Posts: 116
Joined: Sun Dec 16, 2018 11:43 pm
Location:
Contact:

Re: T90 transmission rebuild

Post by jnissen » Thu Jan 07, 2021 3:15 pm

The bearings press fit onto the shafts but yes you can remove the assembly by hand with bearing attached.

As for the side to side play... I’d replace the bearings out of caution as you will kick yourself if they start to growl shortly after you put it in service. Some side to side play is needed and 1/8” does not sound excessive. You may just be rocking the bearing in the case bore and see that much.

Consider a front sealed bearing with rubber plugs for the drains to the front. Common leaking location is that felt washer. We went this route and have been plenty happy with the performance so far.

More important is to inspect everything and replace anything pitted. Time and moisture are evil on old transmissions especially if the thing saw deep water at some point in its life.


Sent from my iPad using Tapatalk
Jim Nissen
1955 M38A1
Patent Plate MD74784
Engine Plate MD85578
BS Hood Number 20974784

Scott in NM
Sergeant Major of the Gee
Sergeant Major of the Gee
Posts: 322
Joined: Sun Sep 28, 2003 9:04 pm
Location: Tijeras, NM
Contact:

Re: T90 transmission rebuild

Post by Scott in NM » Thu Jan 07, 2021 4:05 pm

Thanks for the input. I'm replacing the bearings and anything else that comes in the rebuild kit. However, I must say that all internals look like they are brand new. Just that pesky grinding when downshifting from 3 to 2.

Scott


Post Reply

Return to “M38 Technical Knowledge Base”

Who is online

Users browsing this forum: No registered users and 4 guests