Shift Drum Stopper Replacement When Doing a Clutch Rebuild

I recently learned about a CT90 failure mode that was new to me of a part called the Shift Drum Stopper that is part of your shift mechanism in your CT90.  I'm also finding that this isn't a failure mode that is all that uncommon and from now on I'm am considering replacing the shift drum stopper whenever I rebuild a clutch on one of my CT90's.  While this approach may be overkill, the part is cheap and its an easy replacement when your in doing your clutch, so I think it is well worthwhile.

I recently rebuilt my clutch on my 1975 CT90 and after I put everything back together I took the bike out for a spin in the neighborhood and the bike wouldn't shift correctly, so I pulled the cover off and started poking around.  Nothing jumped out at me as being out of place, so I started surfing the web and came across several postings related to poor shifting and something called the "Shift Drum Stopper".  I went back and looked at my bike and low and behold the roller on the end of my shift drum stopper had broken off.

The picture below shows the new shift drum stopper installed next to the broken part that I removed.

My initial approach to fix my bike was to see if I could salvage a part from a few motors I had picked up off of craigslist for parts (3 motors for $40.00, which I thought was a great deal), so I went and pulled the cover off of one of the motors to salvage a shift drum stopper and found that it was also broken as was the other motor I checked (the third motor was not that complete). So I had struck out trying to salvage one and was going to have to find one online.  It also started to sink in that maybe this problem isn't all that rare.



The purpose of the shift drum stopper is to correctly position the gear shift drum each time you shift.  It accomplishes this by being spring loaded which forces a roller at the end of the arm on the shift drum stopper to engage a plate with a number of lobes and detents (called the drum stopper plate).  Each time you shift you cause the drum with the plate to rotate and the roller rides over the lobe and then settles into the next detent which correctly positions the gear shift drum.  At neutral and forth gear, the drum stopper plate has a feature that hooks the roller on the shift drum stopper and prevents any further travel of the gear shift drum.

The shift drum stopper is located on the clutch side of the engine towards the upper rear of the housing and is hidden behind the primary gear.  In the picture below you can see the bolt that holds the shift drum stopper on at about the ten o'clock position on the primary gear.

After you have removed your clutch to rebuild it, all you have to do to get access to the shift drum stopper is to remove the external retaining ring on the shaft at the center of the primary gear and then just slide the primary gear off the shaft.

With the primary gear removed it only takes a minute to unbolt and replace the shift drum stopper. You need to remember to make sure to hook the spring on the lower part of the arm of the shift drum stopper so it will be preloaded against the drum stopper plate.

When you are in doing this along with a rebuild of your clutch this is a very simple part replacement and could save you a lot of grief if your shift drum stopper failed when you were a long ways from home.

The latest Honda part number for the shift drum stopper is 24430-943-010 and is good for years 1966 thru 1979. I was able to pick one up off of eBay for $16.00 delivered to my front door, so this is really a low cost addition when you're doing your clutch rebuild and I think is one of the best preventive maintenance items you can do.

Update as of 8/16/17
After I made my initial post on Facebook someone shared that they also check to make sure the screw that holds on the drum stopper plate is tight and also suggested that the screw be removed, cleaned and then reinstalled with Loctite to make sure it doesn't come loose, which I think is a great suggestion and something I plan on doing in the future.

Helpful Links (Shop Manuals, Wire Diagram, Model Information, etc.)

Link to page with listing of CT90 parts available on Amazon