Rebuilding a Sherman Combination Transmission

Copyright 2006 John Smith &

Due to their usefulness by adding several more gear selections, the popularity and demand for Sherman combination transmissions remain high among Ford "N" tractor owners. This high demand has brought lots of Shermans out of the woodwork from salvage yards and individuals parting out tractors to sell the high demand parts. Unfortunately, this has been a mixed blessing as many of these transmissions have been sitting and rusting for long periods of time or they are badly worn. The Shermans were very well made and extremely durable, but they wear out just like the rest of the tractor. They're 50-60 years old and it would be unreasonable to expect them to be in perfect condition without being rebuilt. It's entirely possible to get a good used Sherman from a salvage yard or from eBay, but it's also possible you'll get a junker and lots of people have done just that. Statements from sellers like "I took the top off and all the gears look good" or "I drove the tractor and it shifted fine" are not nearly enough to assure you're getting a good one. We've heard from dozens of people who have installed a used Sherman and then ask us what could be wrong with their bargain transmission when it pops out of gear, whines, growls, or doesn't shift properly. That's one of the reasons for the creation of this webpage, to help Sherman owners evaluate and repair their transmissions. Splitting the tractor to install a Sherman is too much work to have to do it all over again when the Sherman isn't right.

There are a few variations on the Sherman combination transmission but the internals are all basically the same. The early models were painted gray and the earliest of those had no retaining set screws for the input shaft bearing. The next generation was painted red but there were no other changes. The third generation had the casting extended on the top cover to completely enclose the shift rails with Welch plugs in the end of the rail bores. They also had a seal added in the top cover to the shifter shaft. These transmissions were for the 9N-2N-8N-NAA and 600 thru 900 series tractors. The fourth generation had the input shaft and nose cone changed to fit the 601 thru 901 series tractors. These tractors used a 15 spline clutch and the earlier tractors used a 10 spline clutch. In the late '50's the last generation of the Shermans was produced by Ford rather than Sherman for the "01" series and the 2000-4000 tractors thru 1964. These transmissions can be identified by the Ford part numbers cast into them rather than the Sherman part numbers. They were not painted at all. The rear oil slinger was changed to a ribbed type rather than the previous fan type and the shifter forks were cast steel rather than forged. However, all the parts of the latest transmissions were backward compatible so they would interchange directly with the earlier transmissions. The one other Sherman combination available was the model made specifically for the Ferguson TO-20 and TO-30. It was painted a lighter gray, had a modified lower case for clearance of the Ferguson countershaft cap and a modified top cover. The Ford Sherman will NOT fit a Ferguson tractor but the Ferguson model Sherman WILL fit a Ford.

The Sherman combination being rebuilt in the following photos is a 2nd generation model. It was advertised as being "Very good condition, came from a running tractor, works great".

Before disassembly a quick look at the outside reveals major problems with the output shaft.

There is serious rust pitting in the back of the output gear. This area is actually the bearing cup or "race" for the front Timken bearing on the tractor's mainshaft. The bearing race surface must be smooth. Installing it like this will mean a growling noise and vibration that will soon destroy the bearing on the tractor's mainshaft. This pitted output shaft is junk and this has quickly become an expensive Sherman rebuild.

First we completely disassemble the Sherman. Disassembly is simple and straightforward. Remove 4 bolts and remove the top cover, remove 4 bolts and remove the front input shaft and nose cone, remove the snap ring and take out the rear shaft and gears, drive the lower shaft out the rear of the case and remove the cluster gear. When removing the input shaft from the nose cone be sure to back off the 3 setscrews (shown above) that keep the bearing in place before separating those two pieces.

It takes only a few minutes to have the Sherman all apart. Here it's disassembled and ready for a solvent bath.

Starting with the main case we find the output shaft bearing race (cup) is badly rusted and needs replaced.

The 2 notches on the inside of the case will let you use a punch to knock the old bearing race out.

Clean and inspect the case for any cracks or other flaws. All the Sherman combinations have their serial numbers on the front of the case below the countershaft bore. The earliest number we've seen was in the 3000 range and the latest on a Ford case was in the 75000 range. This one is serial number 36249.

The case has been cleaned, inspected, and a new bearing race installed.

Next we remove the old bearing and oil slinger from the output shaft.

If the rear bearing cup on the output shaft was ok we'd need to check out the rest of the shaft. Diameter #1 is where the step-down gear rides on the shaft. There should be no wear or scoring on this surface. If there is a problem here there will also be a problem in the bore on the step-down gear. Diameter #2 on the splines is where the step-up gear rides on the shaft. Wear here is fairly common and it's not good. A couple of thousandths wear is ok but any more requires a new shaft. Wear here also indicates there is likely a problem in the bore of the step-up gear. The step-up gear can become rippled inside and will also need to be replaced. Diameter #3 is the inside shaft for the needle bearing on the main shaft. It should have no wear or pitting. If it does, the needle bearing won't last long.

The new output shaft, slinger and bearing ready to assemble.

The bearing cup end of the output shaft should look like this or close to it. Very minor pitting can be acceptable if the pits are small enough that none of them span the length of a roller on the bearing. If there is any pit or flaw big enough that a roller can be affected by it, the bearing will be noisy and will soon fail. That could mean big trouble.

Pressing the new bearing onto the shaft. The slinger should be tightly locked under the bearing when you're done.

Drop on the rear thrust washer and the output shaft is ready to install. This washer should be .062-.065" thick.

Next, closely inspect the lower cluster gear. All the gear teeth should be in good shape with no abnormal wear. Fortunately, the cluster gear in this transmission is a good one.

The needle bearings for the countershaft run directly against the inside bores of the cluster. Inspect the bore on each end for any pitting or damage to the bearing surface. It should be smooth and free of pitting or any other flaws, just like this one. A rough surface here will cause growling noises and will soon destroy the lower bearings.

This is an example of a cluster gear that has 3 chipped teeth. It's junk. Chipped teeth on the cluster gear cannot normally be seen just by removing the top cover on the transmission. This is the type of hidden damge you do not want to find. when you disassemble your Sherman.

The bearing bores in this example cluster gear show some pitting along the length of the needle bearing area and some galling near the inside end. This is not good.

Many times when there is wear on the countershaft and/or cluster gear bearing diameters the bearings will come out in pieces. Note the galling on this countershaft front bearing diameter. This shaft is junk.

Measure the countershaft at the bearing diameters. It should be smooth and round and should be 1.000" in diameter. If the shaft is otherwise in good shape I will accept a size of .997" but no more than that. If it's worn too much the transmission will be noisy and the bearings will fail. This one is badly worn and measures .984". It's junk.

An example of a badly worn countershaft bearing diameter.

An example of galling on the rear end of a countershaft bearing diameter. More junk.

This needle bearing has rust and pitting on the rollers. The rollers are also starting to crumble on the left end. It's very rare to find any of these that are good enough to use over again. Replace 'em.

If the countershaft is good with no excessive wear or pitting it can and should be lightly polished before reinstalling.

The countershaft with bearings and thrust washers ready to install.

Install the 2 bearings and the center spacer into the cluster gear. Apply a good coating of assembly grease to all parts when installing.

"Gluing" the thrust washers to the cluster gear with assembly grease will keep the bearings inside in place while you put the cluster assembly into the case. The small gear goes toward the rear of the case. Note - the front and rear end thrust washers are different. The one with the smaller hole goes on the front end.

Be sure to have everything lined up as the countershaft goes in. The last 1/2" or so is a press fit.

Once the cluster gear is installed make sure it turns freely and check end play with a feeler gauge. It should be at least .005" and no more than .025". If it has too much end play you need new thrust washers.

Next, closely inspect the step-down gear. Make sure there are no chips or abnormal wear on the gear teeth and no wear or rough spots in the bore. Since this gear spins with a metal to metal contact on the output shaft it must be a good fit and have a smooth surface inside the bore. Then check the dentals on the front side where the sliding collar engages with the step-down gear. They should look just like the ones in this photo. Any abnormal wear or damage to the dentals will result in a transmission that jumps out of gear under a load. See the photos below of the sliding collars for examples of abnormal wear on the dentals.

Put assembly grease on the output shaft tapered bearing and thrust washer and in the bearing cup on the back of the case. Grease the bore on the step-down gear and place it into the case as shown.

Lower the case with the step-down gear down over the output shaft and use a wood block or other suitable item to support the bottom end so it sits flat on the bench as shown.

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