The Novak Guide to
Rebuilding the Jeep T90 Transmission

The T90 can be one of the most enjoyably rebuilt transmissions. Its straightforward design, compact form and excellent parts availability make it a good introduction into the world of multi-speed gearbox reconditioning.

The Jeep® T90 transmission was widely used in Jeeps from 1945 through 1971 as the standard three-speed in Jeep CJ's, Pickups, Wagons, FC trucks, Dispatchers, & M38 vehicles.

Common Symptoms
Worn synchronizers will cause gear clash when shifting between gears.

Bearing noise, gear whine and fluid leakage are also factors encouraging a full rebuild of the T90.

A nearly notorious (but curable) problem with the T90 is its slipping out of second gear on decelleration. This is due to excessive wear in 2nd gear's integrated bronze journal bushing, allowing 2nd gear to tilt on the mainshaft - action that forces the disengagement of the gear's clutch splines from its synchronizer's sliding sleeve.

If the above problem has persisted over many miles of use, or if the T90 has experienced abusive shifting, the synchronizer sliding sleeve may also need replacement, as its internal splines edges may have worn themselves to a shape no longer conducive to keeping the transmission in gear.

Other factors that can contribute to slipping out of 2nd gear are a worn mainshaft bearing spacer ring, loose transfer case input gear retaining nut, worn pilot bushing, worn or bent shift forks, broken or sticking poppet balls and springs. Once again – these are contributing factors – usually the cause is second gear bushing wear as described above.

Before getting into the actual disassembly of the T90 transmission, a few words about separating the transmission from the transfer case, as well as removal of a Warn overdrive, should one happen to be used.

Preparation for the Rebuild

Rebuilding Tools & Supplies
You probably already have most of the tools required to build a transmission, including wrenches, ratches, screwdrivers, scrapers, hammers, etc. Additional tools and supplies that may be less common:

• A urethane or plastic mallet
• Bearing puller (helpful, but not required for a T90 rebuild)
• Pliers, snap ring, bevel (Eaton) style
• Large arbor press or hydraulic press
• Transmission assembly grease. Never use axle or conventional grease as it will not dissolve into the fluid and plug passages to the ruin of your transmission. Choose something tacky, which will allow you to stick parts together. We use Smart Blend Synthetic Assembly Lubricant #5500. Petroleum Jelly is a decent alternative. Store your assembly lube in the fridge to increase the tackiness for loading those pesky needle bearings. Your assembly grease will melt into the gear oil upon first operation of your fresh transmission.
• Gasket sealants: RTV (Permatex Black or Grey, The Right Stuff, Aviation Gasket Sealer, etc.)
• Safety glasses or goggles. Take a poppet ball, snap ring, solvent or case-hardened shaft / gear fragment in your eye and you’ll become a convert

Cleaning
You may choose to pressure wash the transmission prior to putting it on your bench. If you plan on doing a partial rebuild (upper end / mainshaft changeout) only, getting it as clean as possible prior to disassembly is important. If you do this, do not allow the transmission internals to be in contact with water for any real length of time.

If you are going to be solvent bathing your transmission and its disassembled components or hot tanking the case, you may forego the pressure wash. Scrape as much grease off the case as you can and then clean up the bench. Don’t worry too much about getting the outside of the case completely clean because it will be completely disassembled and cleaned. Your spouse’s dishwasher is actually a decent option, but we’ll advise that the interpersonal consequences could be quite heavy.

The Warn Overdrive

Removal of the Warn Overdrive (if so equipped)
The Warn overdrive must be removed from the transfer case before the transfer case can be separated from the transmission. This is accomplished as follows: shift overdrive into direct drive. Remove the shift linkage from the overdrive. Remove the 5 bolts that hold the overdrive to the transfer case (the 4 bolts on the back cover of the overdrive do not require removal). The overdrive housing can now be pulled straight back for removal. If necessary, the housing can be tapped back (while pulling on it by hand) with a plastic faced hammer or block of wood. Be careful! – the housing is aluminum and is easily damaged by hammering. When we remove a tight overdrive, we use an adapter that we made for our slide hammer that attaches to the overdrive via two of the rear cover bolts.

With the housing removed, the hub assembly is now accessible. Use a light to look down in the hole in the center of the hub assembly. You should see a square shaped flat wire lock ring in a groove. A thin pair of long nose pliers can be used to squeeze the ends of the lockring together for removal. With the lockring removed, use a regular 1/2" drive socket extension to remove the special hub retaining nut. This has a regular right hand (counter-clockwise to loosen) thread. When the hub nut threads disengage, the hub can be removed from the T90 output shaft.

The transfer case can now be separated from the T90 by removal of the 5 retaining bolts. One is on the front side near the front driveshaft yoke.

Remove these five bolts, and then the transfer case input gear. Since the input gear is larger in diameter than the Dana 18 input bore, the gear must be removed through the PTO port of the transfer case.

Removal of the Model 18 Transfer Case (without the Warn Overdrive)
Remove the five bolts on the sheet metal power take off cover on the rear of the Model 18 transfer case and remove the cover. Remove the cotter pin in the nut at the center of the input gear and remove the nut using 1-5/16" socket. Remove the gear from the T90 output shaft taking care not to drop the hub washer or nut into the transfer case. The transfer case can now be separated from the T90 by removal of the five retaining bolts. One bolt is on the front side near the front driveshaft yoke.

Removal of the Model 20 Transfer Case
The Model 20 transfer case is used in 1962-65 J-series with the T90. It is removed by taking out the 5 retaining bolts (one on the front side near the front driveshaft yoke) and separating the transfer case from the T90. The nut and gear are removed after the T90 has been separated from the Model 20 transfer case.

Transmission Disassembly

An exploded view the the T90 is available here.

A) The cane shift assembly (shift cover) should have been removed prior to removal of the T90 from the vehicle.

B) Remove the three socket screws and seal washers holding the front bearing retainer to the transmission. Remove the front retainer and gasket. Remove the snap ring from the input gear.

C) Remove the two socket screws from the recesses in the front of the case. These retain the oil collector inside the case. Note that these are 5/16-24 SAE thread and they have seal washers under their heads. Should the oil collector have been left out by a previous owner, it must be replaced as it plays an important role in proper lubricant distribution in the transmission. We normally stock these collectors as Jeep #640413.

D) Using a brass or soft steel drift, tap on the front end of the cluster shaft to free the lockplate. This is the metal piece in the slots at the rear of the cluster and reverse idler shafts. Remove this lockplate. Using the drift, tap the cluster shaft out towards the rear of the case, remove the shaft and drift, and let the cluster gear drop to the bottom of the transmission case.

E) Remove the rear bearing, rear bearing retainer, and rear bearing spacer from the mainshaft.

F) Remove the mainshaft and gears assembly out of the back of the case through the rear bearing retainer hole. The pilot roller bearings will faII out as this is done, but these and the cluster rollers are supplied new in the rebuild kit. IMPORTANT: Never mix old and new rollers. If a new roller or two is lost at reassembly, it is tempting to use the old ones for replacement. Don't do it. You might as well re-use all the old ones. We have extras if you need them.

G) Tap the input gear into the case far enough to allow removal of the oil collector. Remove the oil collector from the bottom of the case.

H) Tap the input gear into the case and remove through the rear hole. Remove snap ring and front bearing from shaft.

I) Using a drift against the front of the cluster idler shaft, drive it out of the rear of the case. Remove the cluster gear and thrust washers from the case. Remove the rollers, spacer rings and spacer sleeve from inside the cluster gear.

J) The reverse idler shaft and gear may be removed by driving the shaft just slightly into the case, using a soft drift. This will break it free. You can then drive the shaft rearward through the case. Note that it is not always necessary to remove the reverse idler gear during a rebuild. Bushing wear can be usually checked with the gear in place.

K) Complete the disassembly of the mainshaft by removing the snap ring that holds the synchro hub to the mainshaft. Remove the synchro hub, blocking ring and second gear.

This completes disassembly of the T90.

Parts Inspection

Wash all parts in solvent, paying particular attention to the teeth inside the synchronizer sliding sleeve, cluster gear bore, and the transmission case itself.

Look the case over very carefully for cracks between the bearing and cluster shaft bores. Dress the front and rear faces with a fine file to remove any burrs. Check the fit of the new cluster shaft in the front bore of the case. Occasionally, the cluster bore in the front of the case will be loose. About .001 maximum is all that should be tolerated before the case is replaced. Anything other than a press fit here be cause for an oil leak.

It is a very common misconception that the important place to look for wear on a synchro ring are the clutch teeth or "chevrons". While damage there is important to consider, it is the ring's tapered bore and its oil control grooves that do the hard work of synchronizing. Critical wear here is nearly indetectable. Given the very low cost of synchro rings, it makes no sense to not install new brass into your T90 while you have it open.

If the front of the cluster shaft should be a slip fit in this bore, silicone sealer can be used to form a ring of sealant on the face of the case just before it is bolted up to the adapter (or bellhousing). Note that the rear (slotted end) of the cluster shaft is slightly larger than the front and should not be used for checking this fit.

Slip the synchro sleeve on the hub and slide back and forth. Remove burrs with a hand stone until the sleeve slides freely.

Check the first and reverse sliding gear on the mainshaft to be sure it slides freely.

Check the condition of the bearing area on the front of the mainshaft. Quite often this will be worn. The diameter of the bearing area on a new mainshaft is .7625". Check the condition of the transfer case drive gear splines on the other end of the mainshaft. If the sides of the teeth are worn considerably, it will be impossible to keep the retaining nut (or overdrive hub) tight. Replacement of the mainshaft would be required to solve either the worn bearing area or worn splines condition.

Assembly

An exploded view the the T90 is available here.

A) If the reverse idler gear and shaft have been removed, install these in the case. Use assembly grease in the gear bore and face the slot in the reverse idler shaft towards the cluster shaft so the lock plate will slide in. Drive the reverse idler shaft in till the slot is about 1/32" to 1/64" from the rear face of the case.

B) Assemble the new rollers, spacer washers and spacer sleeve in the cluster gear bore using assembly grease to hold the rollers in the bore. Use grease to stick the thrust washers to the ends of the cluster gear. As discussed earlier, do not use grease such as the old style wheel bearing grease. That type of thick grease can actually prevent gear lube from reaching the rollers for some time after start-up and your transmission will crash and burn. A cluster bearing loading tool can be made from a piece of 3/4" diameter cold rolled steel shafting that has been cut to 7-1/16" long (with the ends "squared"). This will retain the bearings, spacers, and thrust washers in the cluster gear and simplify assembly of the transmission.

The T90 cluster gear, also called a counter gear

The sequence of assembly on the cluster gear bearings is:

• In the center is the split spacer sleeve
• On each end of this is a spacer ring
• On each end of this is a row of rollers
• One each end of this is another spacer ring
• Add another row of rollers
• Add the outer spacer rings
• Lastly are the face thrust washers

There are 88 rollers in the T90 cluster.

The cluster gear, when installed in the case, should have .012 to .018 end play. This is obtained by selective thickness of the rear (steel) thrust washer which is obtainable in thickness' of .0555" and .0625".

The cluster gear and thrust washers are laid in the bottom of the case. A piece of wire or string is guided under the gear and up over the sides of the case. This will be used to lift the cluster gear for installation of the cluster shaft. This must be done after the rest of the transmission is assembled.

C) Assemble the mainshaft and synchronizer assembly as follows: Put a new pawl spring (this is a circular wire spring with the ends bent outward) in each side of the synchro hub with one of the ends of each spring in opposite ends of the same slot. Put the 3 synchro pawls in the 3 slots in the hub with the "hollow" face down. One of the turned out ends of the springs should go into the hollow face of one of the pawls. Hold the pawls in the hub by circling the parts with the thumb and index finger.

Slide the clutch sleeve over the hub with long beveled edge towards the long end of the hub. Center the sleeve on the hub making sure the long end of both match and that the 3 pawls and springs are properly seated. install a bronze blocker ring on the short hub side of the synchro, engaging the 3 slots in the blocker ring with the 3 pawls in the synchro assembly.

Lube the second gear bore and install the gear on the mainshaft with the synchro taper facing forward. Install the synchronizer assembly on the mainshaft with the long taper/long hub side forward. (The bronze blocker ring should be towards the synchro taper on the second gear.)

Install the snap ring that retains the above described mainshaft parts.

Two Allen socket head bolts are used to secure the oil collector. We recommend using thread sealant on this bolt.

D) Place the oil collector in the bottom of the case, rotated so the top portion will allow installation of the input gear.

E) Place the oil slinger on the input gear with the outer offset away from the bearing. Press on the new front bearing. The side closest to the outer snap ring groove goes out. Take care to prevent grit or other contaminates from getting in the bearing. Install the widest snap ring available in this retaining groove. This snap ring comes in 5 widths. Install the gear and bearing assembly in the case. Install the large snap ring on the OD of the bearing.

Check to see that the retainer will go over the snap ring but do not install the retainer at this time. On occasion, a replacement snap ring may be found that will prevent the retainer from mating up flush with the face of the case. It may also overlap the 3 bolt holes for the retainer screws preventing these from engaging their threads properly. In the latter case, 3 flats can be ground on the snap ring for screw clearance. If the new snap ring is too large, re-use the old one.

F) Use assembly grease to retain the fourteen rollers in the input gear. Note that the last roller must be slid in endways.

G) Install the sliding first and reverse gear on the mainshaft with the shift fork groove towards the front of the transmission.

H) Install the front (2nd, 3rd) synchro blocker ring on the mainshaft taking care to engage the three slots in the ring with the three pawls on the synchro. This engagement must be maintained during the remaining assembly procedure.

I) Install the mainshaft assembly at a tilting angle in the case through the hole in the rear of the case.

J) Place the 5/16" thick bearing spacer on the mainshaft. If this spacer is less than .290" wide it should be replaced. This spacer is Jeep part number 643621.

K) Install the rear retainer and bearing (and bearing retainer snap ring on J-series truck T90). Check to see that the second to third blocker ring is in the proper location and is not stuck on the taper of the gear.

L) Using the piece of wire that was previously looped under the cluster gear, lift the cluster gear into position and install the cluster shaft. (This shaft must always be installed, as well as removed, from the rear of the transmission.) Align the lock plate slot opposite the slot on the reverse idler shaft and drive the cluster shaft in to within 1/32" to 1/64" of the slot.

M) Using a stiff piece of wire, extension magnet, or other suitable tool, position the oil collector to line up with the retaining screw holes. Install the socket screws (5/16-24 SAE) with a seal washer under their heads and tighten 10-12 foot pounds.

Special note on retainer oil seal. The felt oil seal washer that may be supplied in some T90 gasket sets is only used on stock installations that use the 15/16 diameter 4 cylinder input gear. Oil the inner felt surface lightly.

N) Using gasket sealer and a new gasket, install the front bearing retainer. Use a seal washer under each screw head and tighten 10-12 foot pounds. Note that the oil drain slot in the front retainer and gasket must line up with the drain hole in the case. Do not use so much gasket sealer that it squeezes into the drain hole and plugs it.

O) Check to see that the second to third blocker ring is in proper location. Also check to see that the blocker ring has not gotten jammed onto the input gear taper.

Check the operation of the transmission in all 3 gears and reverse. The input gear should turn easily by hand. It will be necessary to hold the mainshaft in while checking the transmission "in gear" to prevent the gear thrust from pushing the mainshaft rearward.

This T90 is assembled and ready to close up.

 

Thread a piece of wire through the cotter pin hole in the mainshaft and pull it tight through two of the retaining bolt holes on the rear face of the transmission case. This will prevent the mainshaft from moving rearward out of the case until it is assembled to the transfer case.

L) Install the lock plate in the reverse idler and cluster gear shaft slots and tap the two shafts in to retain the plate. Note that the plate must line up with the clearance slot in the transfer case. If it doesn't it could prevent the transfer case from mating at assembly, causing an oil leak or a broken casting.

M) Remove the mainshaft retaining wire, coat both the transmission and transfer case with gasket sealer, and assemble the transfer case to the transmission using a new gasket. Install the 5 retaining bolts. The bolts must be the proper length. Thread damage or a locked up transfer or transmission could result if they are too long or too short.

Three 1-1/8" long bolts and two 1" long bolts are used. These five bolts are threaded 3/8-16 N.C. Install the 1" long bolts in the lower left and front holes.

Transfer Case to Transmission Gasket
On occasion, we have inquiries related to continuous lubricant loss from the T90 into the transfer case. This can usually be traced to oil holes not lining up due to the use of an improper gasket or a different transmission or transfer case than was originally used. The oil passages between the two cases must match and not be blocked by a gasket or gasket sealer that has been squeezed into the oil passages.

Lack of an oil collector in the transmission can also contribute to this oil loss problem. Once again the felt oil seal is used only with 15/16" diameter input gears. Do not try to use this felt seal with any other gear.

Novak's T90 master rebuild kit, with a collection of the best parts we have found (or made) for the T90.

 

When filling your T90 with gear oil, we recommend that you select a conventional mineral oil or a para-synthetic in lieu of a full synthetic oil. Properly assembled manual gearboxes do not have the thermal strains seen by combustion engines or hypoid gears. Synthetic fluid in these gearboxes, while not harmful, is probably an economic waste. Hypoid gear oil is sulphurized higher than transmission oil and can be mildly corrosive to the non-ferrous alloys used for synchros, bushings and thrust washers in these transmissions. An 80W-90, API-GL5 or MT-1 rated fluid is very good. Some claim faster shifts from using a 50W engine oil in their transmission and we do not consider this to be contraindicated.