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I've been away from the list for a couple years due to riding and repairing other bikes. Now I'm back around to addressing the tranny problem in my 79 Hybrid. I built this bike a few years ago from a wreck. When I took it out for a test ride, I discovered the 2nd gear slip problem, so I just parked it. I found a used tranny on eBay.
Last week I decided to do the in bike teardown, and discovered the expected rounding of the leading edge of the 5th gear dogs and to a lesser degree, the rounging edges of the 2nd gear slots, and the shifting fork was bent and gaulded. Another thing I DID NOT find (and something I haven't read about in other posts) is the circlip on the right end of the secondary shaft! The parts diagram clearly shows a circlip on shaft end next to the middle drive gear that rides between the gear and the case bearing. My bike did not have this circlip, nor did the transmission I got from eBay.
I'm wondering if this being absent is not the underlying cause for the 2nd gear failure. With this part missing, the entire shaft is shifted slightly toward the right side. This would lessen the area in which the dogs of 5th gear have to mesh with the slots of 2nd gear. When fully meshed the measurment is only 0.220 in. When the gear begins to slip, this bends the shifting fork. Now with each successive shift the area of contact between the dogs and slots becomes less, there is more slipping and rounding off of the dogs, and subsequently the gear fails to hold at all. Some have talked about increasing this engagement area by moving the thrust washer to the side opposite the circlip on 2nd gear, but I'm not sure this is a good idea to have this circlip acting as a thrust surface. Doing the dovetail undercut on the dogs and slot helps to keep the gears engaged.
I'm going to place a circlip or washer onto the right end of the secondary shaft and see if this causes any clearance problems with any of the other fork shifted gears. I'll post an update when I get this done.
Last week I decided to do the in bike teardown, and discovered the expected rounding of the leading edge of the 5th gear dogs and to a lesser degree, the rounging edges of the 2nd gear slots, and the shifting fork was bent and gaulded. Another thing I DID NOT find (and something I haven't read about in other posts) is the circlip on the right end of the secondary shaft! The parts diagram clearly shows a circlip on shaft end next to the middle drive gear that rides between the gear and the case bearing. My bike did not have this circlip, nor did the transmission I got from eBay.
I'm wondering if this being absent is not the underlying cause for the 2nd gear failure. With this part missing, the entire shaft is shifted slightly toward the right side. This would lessen the area in which the dogs of 5th gear have to mesh with the slots of 2nd gear. When fully meshed the measurment is only 0.220 in. When the gear begins to slip, this bends the shifting fork. Now with each successive shift the area of contact between the dogs and slots becomes less, there is more slipping and rounding off of the dogs, and subsequently the gear fails to hold at all. Some have talked about increasing this engagement area by moving the thrust washer to the side opposite the circlip on 2nd gear, but I'm not sure this is a good idea to have this circlip acting as a thrust surface. Doing the dovetail undercut on the dogs and slot helps to keep the gears engaged.
I'm going to place a circlip or washer onto the right end of the secondary shaft and see if this causes any clearance problems with any of the other fork shifted gears. I'll post an update when I get this done.
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