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| After 15 months and 22,000 miles of operation with the modified gears, I started to hear a whining noise in 5th gear. It got gradually louder over a period of weeks. I removed the cover and found a small quantity of fine metal particles clinging to the magnet I had added. There was no visible damage to the gears, but I could catch my fingernail on surface damage to some of the gear teeth.
Gears are made to tight tolerances, so minor imperfections cause uneven distribution of forces, and local overloading. Also, gear teeth are typically case hardened so damage exposes softer metal and the gear usually progresses rapidly to total failure.
This car is my daily driver so I did not want to risk total failure and debris circulation through the rest of the gearbox. I removed 5th gear from the input shaft, but the other gear was stuck on the splines of the output shaft! I tried heating the gear with a torch, using thermal expansion to enlarge it, but it was still stuck fast. There was no room to use a gear puller because the gear is flush against the transmission bulkhead, as shown in the photo below. 
I decided to drill and tap holes on the gear, and screw in bolts for the gear puller. I managed to drill one small hole, but enlarging it was difficult as I started to hit hardened metal near the splines and near the gear teeth. I tried to tap threads into the hole, but my tap got stripped trying to cut the hardened metal!
In desperation, I decided to cut the gear off. I used a Dremel tool, first with grindstone bits and then with small cutting wheels. The bits and cutting wheels wore out quickly, and I ended up going through 8 or 10 of each. I used rags and plastic to protect the rest of the gearbox from flying metal chips and abrasive. But as I cut through the gear, I realized that some debris went behind the gear, through the roller bearing behind it, and into the gearbox! Uh-oh...
I finally cut through the gear, and cleaned out the debris as much as possible. I flushed a lot of solvent through the bearing and out the drain plug of the gearbox, trying to get all the debris out without removing the transmission from the vehicle for disassembly and cleaning.
I reassembled the transmission with 5th gear from the Escort sedan, put in clean fluid, and went for a test drive. Then I drained and re-filled the transmission again, to hopefully remove the last of the debris. I have driven the car for 7 months and about 10,000 miles with no further problems.
I examined the failed 5th gears closely to diagnose what went wrong. I found spalling (pitting) on both gears, and it was more severe on the smaller gear. The spalling was not uniform around each gear, and it was on the edge far from the weld on the smaller gear. Therefore I don't think it was caused by failure of the heat treatment.
I suspect the failure was due to misalignment or distortion, probably from the heat of welding. Imperfectly aligned gears cause uneven loading, and local overloads. It looks like the welds are a continuous bead on each gear, so the gears could have distorted unevenly as one side was heated and the other was still "loose". Ideally the welder should have started with a series of "tack welds". Or maybe brazing or silver soldering would have been a better choice than welding.
Oh well, it was fun while it lasted! It was an educational project, but I don't plan to repeat it on this car. Junkyard parts have gotten more expensive for this car, and my machinist friend has retired so I can no longer get free machine shop work.
I am disappointed to lose my tall overdrive. But several people have expressed interest in my work, and have pointed me to web sites about other fuel economy modifications (see "Web Links" above). Now I'm inspired to try some aerodynamic modifications! If you find my web site worthwhile, please link to it from your web site.
- Tom Leone TGLEONE AT GMAIL DOT COM
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