The only difficult part of working on a /5 speedo/tach assembly is getting the crimped chrome bezel off the casing. This is a tedious and delicate operation if you intend to reuse the old bezel. If you have a replacement bezel from the friendly corner bezel store then the old bezel can be simply cut away.
First, write down the odometer reading as you will have to return the odometer number wheels to their original position later.
Second, have a flat surface such as a solid workbench to work on. Put a hard rubber mat or other sturdy, not too soft, non-slip surface on the bench. You need this to prevent the speedo from shooting away as you apply pressure to un-crimp the bezel. Also it will prevent the speedo from slipping and the screwdriver from impaling your hand. I came close a couple of times.
Place the speedo face down on the mat. With a medium to small sized screwdriver blade tip placed between the speedo case and the inside back edge of the bezel start to un-crimp the bezel. (see fig. 1) Apply plenty of pressure with the other hand forcing the speedo onto the non-slip mat so it stays put as you apply force to the screwdriver or the whole assembly will end up on the other side of the shop floor faster than you can say "oh shit". Take your time.
Force / /
|| | / /<-----Screwdriver
______vvv_____ /
Speedo ass'y | | /
| | /
| | /
|_____________|/
/______________\ <------Bezel Rubber mat
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Figure 1
You will find that the crimped area will develop a few little splits radially outward as you work your way around, un-crimping. This cannot be avoided and if you are slow and careful the splits will not progress to the part of the bezel that is visible when re assembled and put back into the headlight bucket. Un-crimp only half way or a touch more than half around the face of the speedo, just far enough to enable you gently pull the un-crimped part of the bezel barely away from the speedo face. Then slide the bezel off to the side slightly at which point it will be free of the face. Watch that the glass, freed from its restraint, doesn't fall out and break. If you've gotten this far you have my respect. It takes a lot of dexterity and patience to do this properly.
Once the bezel and glass are removed, undo the screws on the back of the speedo and the complete assembly will come out the front of the housing. One look at the innards will show you that this is a unit made to be easily worked on. Just like the rest of the bike. The only difficult part was the bezel. The pointer and dial face come off easily and obviously. The little gears and worm drive to the odometer are easily accessible and workable. All can be taken apart and disassembled. (You did write down the odometer reading didn't you?)
Clean everything, check for wear on the little gear that is driven by the worm at the odometer number wheels. This gear can be removed from its shaft if necessary by pressing it off gently.
When reassembling remember to put a glob of grease on the worm drive or it will wear out quickly. Put a little drop of oil on the shafts and bushes. Not too much or it will migrate onto the odometer wheels and onto the dial face. If that happens at least you will know how to un-crimp and repeat the job. Check that all the parts work by spinning the speedo and tach drives by hand. The needles should move and the odometer should increment.
I have not gone into detail on the speedo, or the tach, for that matter since if you are competent and confident enough to attempt the un-crimping procedure you will not need detailed instructions on how to service the innards. It really is that simple once you are inside. It is only the scheissenbezel that is a problem. If your little gears are worn out and cannot be bought anywhere then a good watchmaker can machine new gears using the old ones as a model. The needles can usually be carefully straightened, repainted and if necessary secured to their shafts with a dab of Loctite.
When all is reassembled, and the dial face and glass is clean as a whistle, (you really don't want to find dirt or lint on the face or glass just after re-crimping the bezel) place the glass carefully back in place with the rubber sealing ring. Slide the bezel back on. Try to reinstall the un-crimped part so that it is toward the tach, or bottom, or rear of the bike side of the assembly. Th at way any of the little splits or imperfections caused by the un-crimping procedure that may be visible, when re-crimped, will not be obvious when the unit is reinstalled in the headlight bucket. Once again carefully place the assembly face down on the rubber mat and using a little metal or wood drift gently re-crimp the bezel. Do not hammer at it. Once again take your time and check regularly to see that the glass or rubber sealing ring has not slipped out of place.
When the crimp is complete reinstall the unit back on the bike. It gave me a real feeling of accomplishment to successfully do this myself on my /5 several years ago. Please feel free to contact me if you need clarification on the above.