My gauges were working well and look fine except for the area under the glass. The area under the glass had considerable gunk and not only looked bad but made it diffiult to read the gauges. To clean the inside of the glass, the chrome trim ring which girds a Nippon Denso gauge must be removed and the glass bezel top half must be separated from the bottom half that contains the gauge mechanism. Between the two halves is a rubber seal that provides water proofing integrity. I found the condition of my seals excellent. They were both soft and pliable on both the tach and speedometer. If the condition of these rubber seals is poor, and they are not pliable, you will be forced to fabricate or locate new ones. I believe they could be cut out of any piece of thin mill rubber using the gauge as a template.
This is what my tach looked like prior to clean up:
This is the underside of the ring prior to starting work. Note that it is dimpled on this side where on the far side, is smooth. I recommend starting work on the dimpled side.
I will not try to detail the steps needed to remove your gauges but in general, you will need to:
1. remove small screw that holds on the back gauge bucket
2. pull out light bulb holders (two per gauge)
3. remove two nuts and associated hardware (lock washer, flat washer, rubber grommet and metal sleeves)
4. remove cable
Once you have the gauges out, lay the first to be cleaned on a flat work surface face down. Put a rag under the face to avoid scratching the paint. You will note that the ring that retains the two halves has a dimpled half and a smooth half as viewed. It is best to place the screwdriver between the gauge body and dimpled area. Pry about 1/4" up where the edge of the ring is bent up. Once you have the edge started up, use a longer screwdriver to pry it all the way around. If you don't go to the larger screwdriver you will probably bark your knuckles on the sharp edges of the gauge itself. If you do stay with the smaller screwdriver, watch those knuckles.
After prying up the edge of the ring, you will be able to slip a screwdriver between the gauge halves and the ring. Working your way around, you should now be able to pop the top out of the ring and remove it from the assembly.
At this point, if the seal is in good shape, I recommend leaving it alone and in place and clean the inside of the glass with the seal in place. You may wish to blow the inside surface out with compressed air to dry it well or let it dry under a light bulb for awhile to ensure any moisture is out. Put the two halves back together; you may have to use a screwdriver to again slip the base section back into the face section which contains the ring. When the two halves are in place, use a pair of pliers to turn the bent section of the ring back over the edge of the two gauge halves. Pull up slightly using a rolling motion to pull the ring as far towards the inside of the gauge as possible.
I found the best way to get the ring tight is to pull and crimp it in about 5 places spaced equally around the edge before crimping it down all the way around. Once you have it in about 5 places, go ahead and crimp it all the way around, pulling upwards slightly with the pliers and crimping. Here is one of the five initial crimps I made:
Here is a shot that shows all the crimps complete:
At this point, to make a professional job of it, I used a small flat ended punch and carefully went around the edge and tapped the edge down to smooth it out.
If you are careful with the pliers, you won't mess up the side of the ring that shows. In this picture, you can see some very small knicks in the ring which probably could have been avoided if I had put a rubber band or something around the ring to protect it as I used the pliers on it. A bit of steel wool took most of these knicks out.
Here is picture of the completed speedometer and then a picture of the completed speedometer/tach as well. Looks a lot better with the glass crystal clear again!
You will now replace your gauges using the reverse order you removed them. I also recommend you take the opportunity to lube your cables. Good luck and if you have a question, don't hesitate to drop me a note!