GFM Network News

How to rebuild a transmission, Part 1


We tackle a major repair on a typical 
three-speed gearbox in the Grainews shop

topic image
With a little research and a good service manual, rebuilding an older manual three-speed transmission can easily be done in the farm shop. Photo: Scott Garvey

With the chassis work completed on the latest vehicle restoration project in the Grainews Garage, it was time to take a detailed look at the drivetrain components. The most obvious problem was the gearshift lever on the three speed manual transmission was stuck. Hoping to find just a minor problem, we pulled the top cover off to take a look inside. What we found wasn’t good news. A full rebuild was in order.

While each transmission has its own unique design, most manual three speeds, like 
something you’d find in an older farm pickup truck, are very similar, as are the rebuild procedures. Our transmission has a top shift lever; some will have side shifter linkages instead. 

With a detailed shop manual and a little research, doing an on-farm transmission rebuild is definitely possible. Here’s a look at how the rebuild went on our Spicer, which should provide a good idea of what’s required on any three-speed rebuild.

Related: How to build a custom tool box, Part 1

Step 1: The problem

topic image
Photo: Scott Garvey

Here’s the scary sight. When the top cover was removed, we found about a litre of water in the bottom of the case and significant rusting on all the components. There was no oil in it and all the components were stuck in place.

Related: How to build a custom tool box, Part 2

Step 2: Preparing for disassembly

topic image
Photo: Scott Garvey

The first job was to separate the transmission from the attached transfer case. Of course not all transmissions will be attached to a transfer case, obviously this gearbox came off a four-wheel drive vehicle. The water on the work surface is what drained out of the case when it was separated.

Related: How to safely extricate a stuck machine

Step 3: Starting the teardown

topic image
Photo: Scott Garvey

The first step in disassembly was to remove the front bearing retainer. This is on the input shaft that connects to the clutch.

Related: How to repair a faulty fuel gauge

Step 4: Removing the countershaft

topic image
Photo: Scott Garvey

With the bearing retainer off, and a retaining plate on the rear of the transmission removed that holds the counter shaft in place, we used a brass drift to push out the countershaft that holds the cluster gear. Using the soft brass drift prevents damaging the shaft.

Related: How to use a metal shrinker tool

Step 5: Gear removal

topic image
Photo: Scott Garvey

Once the countershaft is pushed out the rear, the large cluster gear that rides on it can drop down to the bottom of the case and make room for removal of the input and main shafts. There is a lot packed into the small case, so disassembly has to take place in the correct order to get everything to squeeze out of the openings.

Here the main shaft is pulled out through the rear of the case. With the oil collector unbolted, the input shaft can be pulled out the front of the case. A few light taps with a soft-faced hammer was all it took to pop the shaft bearing free of the case.

The last step is to tap out the reverse idler gear shaft and remove that gear. That’s all there is to the tear down.

Related: VIDEO: MIG welding tips

Step 6: All the parts

topic image
Photo: Scott Garvey

Keeping parts in order and noting their orientation is essential to ensure everything goes back in properly. A cell phone camera is a great asset for this job. Photographing each gear and shaft assembly as it’s removed provides a clear record of the assemblies. It’s important to have a shop service manual as well, which should include an exploded view of all the components.

Once all the parts were out, they were laid in order on the workbench. Fasteners and other small parts were bagged and recorded.

In the front of the image on the left is the input shaft with the oil collector in front of it. To the right is the still-assembled main shaft. On it from left to right is the synchronizer assembly, second gear and first-reverse gear. Behind those parts is the countershaft and large cluster gear that rides on it. The reverse idler gear and shaft is behind the countershaft.

Related: How to find the weak spots in used sprayers

Step 7: Cleaning the case

topic image
Photo: Scott Garvey

Next, we turned our attention to cleaning the transmission case to make sure it was still in good condition and ready for a rebuild.

Related: How to diagnose and repair a problem leak

Step 8: Checking the parts

topic image
Photo: Scott Garvey

With the disassembly complete, it was time to give all the parts a thorough inspection. While there wasn’t much wear on most of the gears and components, there was ample damage from exposure to water. 

This close-up view of part of the cluster gear — which sits at the lowest point inside the case — reveals significant corrosion. The part of the gear that was sitting in water was severely pitted, so the gear was too damaged for re-use.

Fortunately, just a new cluster gear and synchronizer assembly made up the entire replacement parts list. All the rest of the components cleaned up reasonably well, but it took several hours of elbow grease to get that job done.

Those parts along with a minor rebuild kit that includes new gaskets and bearings were ordered. When they arrive, we’ll start the reassembly process.

Related: How to check for a faulty relay


Stories from our other publications