Ford  Top Loader Three Speed Transmission

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Synchronizers 

Ford began using a mechanical synchronizer in all their three speed light duty transmissions beginning in 1932. The synchro adjusted the gear speeds between second and high gears when shifting to prevent gear clash. The diagrams below show the four basic types used in the passenger cars and light trucks up thru the mid 1950's. Each successive type was an improvement towards faster, smoother shifting. The preferred type would be either the 01A or the 51A or 8M synchronizer, which must be used with 1939 and later gearsets.

The first one shows (48-7124) is an updated evolution of the original synchro unit that was introduced in 1932 (which did not have a 7124 assembly part number in the parts books). This style was actually used for part of 1939 production and at least into 1940 production for the light trucks and commercial vehicles using the light duty transmission. This style used a hub with six steel balls and six matching springs, which rested in the hub (7108). These were surrounded by the 7106 sleeve, which had an axial groove machined into its center. As the sleeve was moved forward or backward by the shifter fork, it would center itself in the neutral position when midway between second and high gears. This synchro design was matched up with the appropriate gears and washers, including the particular main drive gear and mainshaft. Thus, this style is not interchangeable with the later gear sets. Likewise, it uses the 68-7222 or the 40-7222 shifter housings with the narrow front fork. When looking into a gear box with this style synchro, you will see that the shifter groove in the sleeve (7106) is more or less centered in the sleeve. What differentiates the two early synchros was the two outer sleeves used. The B-7106 sleeve measures 1.406" wide. The 48-7106 sleeve measures 1.345" wide. The sleeve must be matched up with the correct cluster gear being used. Note, unlike all the later style synchronizers shown below, the early type did NOT use a snap ring in front of the synchro hub.

The second style shown (91A-7124) was a single-year version brought out in 1939. It was the first synchro to use the three flat inserts in the hub, and the separate brass synchro rings (7107). This type is easy to identify when apart. The three inserts are very narrow. They are held outward by the two wire spring retainers. Note that the hub and sleeve are unique and not interchangeable with the later synchro types. Likewise, the brass rings have narrow slots for the narrow inserts, and are not interchangeable with the later styles.

The third style pictured (01A-7124-A) is a type seen very often. This was introduced in 1940 and utilizes the three steel balls, three small springs, and three steel inserts to provide the centering action of the synchro sleeve. This type is the lone style to use the 7069 spacers (brass washers of varying thickness) which helped set the clearances on the mainshaft. The brass synchro rings have the wide slots that also fit the later style units.

The last synchronizer shown below is the 51A-7124 and 8M-7124 type. The 51A and 8M types are virtually identical and are interchangeable. These were the synchros used when Ford changed the hub to eliminate the three balls and springs. The steel inserts are basically the same rectangular size but have a raised detent to engage the groove inside the sleeve. The inserts are held outward by the two wire spring retainers. The 8M-7124 was listed for the light duty three speed transmissions at least through 1962 (except for the T-86 overdrive transmissions of 1955-62). The 7069 brass spacer was not used with these type synchronizers.

The 91A, the 01A, the 51A, and the 8M synchro sleeves all have an offset shifter groove. This groove will always be positioned toward the rear in assembly of these transmissions. Toploader transmissions using these type synchronizers must have the 91A-7230 type shift fork in the shifter housing.

Refer to notes at the bottom of this page about servicing these units.

 

Service Notes:

The synchronizer types with the springs and balls require some care when servicing. Be cautious when moving the synchro sleeve in an open transmission. If you move the sleeve by hand rather than with the shifter lever, you may move it beyond the normal range which could allow the steel balls to pop out. The 01A type has the 3 balls and you can probably catch them, but the B or 48 type has six balls under spring tension, and you will almost certainly see one or more fly out before you can see where they went! It is best to leave the synchro sleeve in the neutral position, and to remove the complete synchro assembly as a single unit. I highly recommend that either style is disassembled inside of a box with sides high enough to capture any stray balls and springs, or disassembled inside a shop rag. The B or 48 types with the six balls and springs will normally require some pressure to separate the hub from the sleeve, or use a soft hammer to tap the hub out from the sleeve (using a rag to catch the loose pieces). 

To reassemble the B or 48 synchro, place the hub flat, and place a dab of grease in each of the six holes. Insert the small springs and balls into their greased pockets. Pick up the hub carefully and set it down into the sleeve, where the splines began to engage. The hub should slide into the sleeve to the point where the six balls are protruding from the sides and preventing the sleeve from moving any more. Now, place a 2" worm-drive hose clamp around the hub, which should cover all the balls. Tighten the clamp snugly which should force the balls back into the hub pockets. Take a piece of wood (a short 2" x 2") and tap the hub down into the sleeve. You'll be able to drive it down so the balls are now covered by the sleeve, but will have to remove the hose clamp before sliding the hub the rest of the way down.

The 01A synchro uses the three balls and springs, but also the steel inserts. Using some grease, apply it liberally into the pockets in the hub. Insert the three springs, add a bit more grease and dab the three steel balls into place. Finally, with a bit more grease, add the steel inserts to the openings on the hub. Now, slide the sleeve down onto the hub as far as it will go. You can set the assembly on a flat surface. One at a time. pull the steel insert up a bit and press down on the exposed ball (a small flat-bladed screwdriver works well). Slide that insert and ball down into the hub. Repeat this with the other two inserts and balls. Keep the assembly together and centered in order to retain the balls. Be careful when sliding the synchro assembly back onto the main shaft during transmission reassembly, to prevent the sleeve from moving far enough to expose the steel balls.

The 51A / 8M synchro uses the three insert plates with the bump in the middle. To install these, first assemble the hub into the sleeve (hub snout to the front - groove in sleeve to the rear). Slide the 3 inserts into their respective slots. Place the wire spring on one side of the assembly, and slip it under the rear edges of each synchro. Repeat this for the front side with the other wire spring. Make sure the ends of the springs extend beyond an insert, not ending in the middle of an insert.

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