Ford Top Loader Three Speed Transmission
Ford used a similar transmission case from 1932 to 1939 on the passenger cars, and up to 1950 on the light pickup trucks. The early castings were marked 18-7006 (1932-34) and 48-7006 (1935-36). In 1937 Ford introduced the 78-7006 casting. All interchange with previous years, but the 78-7006 case had a slightly enlarged (deeper) bell section to allow for the larger clutch and pressure plates. The 48-7006 case will also handle the 11" clutch, but the 18-7006 will not. The 78 series cases were reputed to be sturdier than previous ones. The shifter housings will interchange on all three cases, as will the clutch release shaft. The Ford parts books will refer to the case by its assembly part number of 7005. This is the bare case and the pressed in bushings for the clutch release shaft.
The picture below shows the 78 series case from the back, clearly showing the case part number in the casting. This case was bead blasted and primed, so the numbers are easy to see. They are recessed and part of the casting (not stamped in and not raised) so they are often covered with grease and dirt, and difficult to see on a used junkyard or swap meet transmission. In addition, the rear mount (also known as the rear bearing retainer) is bolted to the six-hole flat surface you see, and somewhat covers the "78" numbers even more.
Another way to identify these transmission is to check the serial number which is stamped into the top front section of the bell housing portion. The number will begin with a stamped "star" like an asterisk, followed by a series of numbers, and finally another star. Use this link to break down the serial number by year. If the case has no number stamped into the VIN area, it may have been a replacement transmission, and no number ever assigned to it.
The transmission cases shown above are stripped cast iron from used transmissions. The obvious difference is the longer reinforcing rib on the bottom of the case. I have not found the suffix letters (B or C or D or T) in the Ford parts books, but it would appear that the "T" case may have been brought out for the late 1940's (to 1950) light three speed transmissions. This case will interchange all the way back to 1932 Ford passenger cars and light trucks.
The comparison picture above shows the early (48 series) case and the later (78 series) case. In nearly all respects, they are the same, except for the depth of the gearbox. The thumbnail picture below shows this, but the important area is trough at the front of the case. This is where the large end of the cluster gear is positioned. If you are using a 29 tooth cluster gear in a 48 series case, you will have trouble fitting the gear set in the case because the cluster gear will sit up too high and interfere with the installation of the main drive gear under normal installation procedures. You can still use the 29 tooth cluster gear in a 48 series case, but the front ball bearing on the main drive gear will have to be installed AFTER the main drive gear and all the rest of the gears are installed. This requires pressing the bearing down into position on the input shaft and into the front of the case by use of a hydraulic press. With the 78 series case, all cluster gears can be fitted into place without any special procedure.
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