XS400 Junkie
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Southern NJ
It's been 350 miles since Trusty Rusty was put back on the road, and the tach dial developed a noticeable shimmy while riding. A short while later, one of the screws holding the dial face in place completely backed out followed by the other screw a few minutes later. So, I took the tach off, capped the drive cable and have been riding without the rev counter while I researched what my options were.

Luckily, it appears that most of the instruments on Japanese bikes from this era were built by Nippon Seiki, and there is a lot of info on the web about how to open them up for service that applies to the XS400, even if the instrument is off of a Suzuki, Kawasaki, Honda or a different Yamaha bike. So after looking over some web sites and viewing a few videos, I took a chance and cracked my tach open by prying the crimped bezel open with a small screw driver and paint can opener. Here's everything apart:

Tach Refurb3.JPG

I didn't get a picture of getting the bezel off, but I will include that when I open up my speedo. Anyway, I cleaned everything up, concentrating mainly on cleaning up the old lubricant in the drive shaft and the needle shaft areas using some mineral spirits. I followed that with application of some sewing machine oil in those areas to refresh the lubrication. When the dial face was put back on, the two screws that backed out were installed with a drop of clear finger nail polish that my wife kindly donated to the project. Finally, the bezel was re-crimped in small increments using a water pump pliers.

Tach Refurb8.JPG

And we are done.
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The tach is back on the bike and working well - probably responding faster than before, even. As I mentioned, I am going to do this same thing to my speedo, mainly to get some new lubrication in there, and I will get some shots of opening up the bezel and how the odometer gearing looks and is lubricated.

Vintage bikes have two kinds of parts, those you have already touched, and everything else you thought was okay, but given enough time will bite you in the ass....
The weather here has been and is forecast to be wet for a while, so it is time to do the speedo. Here is a bit more detail on how I get these open. Here are the things needed ready for the job.
Speedo Rehab1.JPG

First off take off the trip meter reset knob. It has left-hand threads, so it unscrews by turning it clock-wise. Also now is a good time to protect any surfaces from scratches - I use tape on my tools and the gage itself.

Speedo Rehab2.JPG

To get the bezel ring off, I start with a small flat blade screwdriver and work it under the bezel and then around the gage several times prying up a little at a time.
Speedo Rehab3.JPG

I pry until I can get a paint can opener into the opening and start working my way around again while opening the bezel more each time around.
Speedo Rehab4.JPG

This is pretty tedious to do, but the ring will slowly open up and I finish straightening the crimp with the end of my needle nose pliers in order to get the ring off. Finally, after all of that work, I use the other end of the paint can opener which conveniently has an opener for one of my favorite adult beverages. Next up is to get inside to clean and re-lubricate the bearings and gears.
So I left off with the gage cracked open.
Speedo Rehab5.JPG

The needle has to come off next and I used two small flat-blade screw drivers to lever the hub off (be careful here as it took some effort and when the needle pops off, it will really go flying. It took me 2 hours to find the tach needle in my shed after that first go around.)

Speedo Rehab6.JPG

After that, the faceplate can come off and then the internals can be removed.

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Speedo internals.jpg

The way these work is the drive shaft turns a magnetic flywheel inside the gage. That flywheel is surrounded by a drag cup, but they are not directly connected. Instead, the magnet in the flywheel acts on the drag cup and the faster the drive shaft is turning, the more the drag cup will try to turn on the needle shaft until the wound spring matches the force exerted on the drag cup. The magnet on the flywheel also acts on a reed switch each time around. That switch opening and closing is the speed signal that is sent to the auto cancelling circuit for the turn signals. Finally, the odometer is driven mechanically from the drive shaft by the shafts and gearing you see.

I cleaned everything with mineral spirits, lubricated all of the shafts and bearings with oil and put a small bit of white lithium grease on the gearing before reassembly. That proceeded exactly like the tach, above.