Taking apart a speedometer assembly is not as hard as it seems…

Before I tore into it, my speedo was actually on its way to a specialist to have it repaired. This guy who was going to do it turned out to be sick, and since I took the speedo out of the bike anyway…I thought I could just as well give it a go.

The symptoms were classical. My tach worked fine, the speedo was stuck at 50km an hour, and would move irregularly while riding. At the base of the needle you could see something like copper wire spaghetti through the hole in the face. I thought that needed some rectifying.

The major problem when taking apart the speedometer is the chrome bezel, which is crimped around the housing. How to solve this ? First remove the rubber seal which will be around the speedo body, wedged to the crimped part of the bezel. Put the assembly face down on a non slip surface. The back of a mouse mat is ideal. Take a flat screwdriver that isn't too large, put the tip behind the crimped edge of the bezel and gently pry, using the speedo housing itself as a pivot to be able to put some force. Like here. Go easy…it uncrimps far easier than I thought, at least on my speedo it did. There are no horror stories of impaling your hand with the screwdriver, or shooting the speedo through the room. Just gently uncrimp the bezel a little more than halfway around the circumreference. Don't uncrimp it all at once. Pry it up gently, if you're halfway around the speedo, go back around the rim prying some more. If you don't do it easy, you'll crack the bezel. This is not a disaster, but can be avoided if you're really careful. If you work your way round, eventually the bezel will start to move if you pry, because it has more room to move now it is partly uncrimped. Try keeping it as close to the rim of the housing with your other hand while uncrimping it. Another picture : halfway around the bezel. I finally succeeded in uncrimping a little more than half the circumreference with just one crack in the bezel. Then you need to convince the bezel to come off the speedo housing. I used two screwdrivers for this. Locate the middle of the uncrimped part. Go half an inch (a good centimeter) to the left or right. Wedge your screwdriver between the lip on the housing and the uncrimped edge of the bezel. Then pry upwards, like here. You'll probably need two screwdrivers to get the lip partly over the bezel. If you succeeded doing that, then work your way round until the lip is over the bezel on nearly the complete uncrimped part of it. Then move the housing about a bit until it's loose enough to shove it over the uncrimped part of the bezel like this.

Undo the screws at the back and take out the tachometer and speedometer.

This is the housing, with the bezel, filled with some kind of sealing compound, the glass and metal holder, and the housing itself with the instrument lamps. The red gen lens is removed because it wouldn't stay on.

This is a closer look at the oil and gen lenses. It's a clever system… The gen lens is shorter because the lamp is bigger.

This is the tachometer as it comes out of the housing. I don't have a picture of the complete speedo.

I took the needle off the speedo. The round black needle holder was stuck on the small axle, and thus I couldn't get the face of the speedo off. Do not scratch the face of the speedo while undoing the two small screws in the front…use wd40 (the paint apparently can stand it). It may be best to leave it attached until you have the needle holder off. I could see that the spiral spring, which is supposed to look like this (on the tacho) was indeed spaghetti.

So I took it to the watchmaker who lives nextdoor (it's good to have connections) and he produced a puller much like a rotor puller but twenty times smaller. Basically it's a c-shaped piece of metal with a threaded hole in the back of the c, in which fits a screw with a very very sharp and tiny point. That way we got the needle holder off.

I still thought that if the spring could be straightened or replaced the problem would be solved. Of course I didn't know how the spring came to be like that in the first place…

Bare with me !

As the face of the speedo was off now, we could remove the odometer, which I cleaned in warm water with dishwashing solvent, rinsed and blew dry with a hairdryer. Using a toothbrush might be a good idea. The odometer holds the piece of metal which holds the spring. (here on the tachometer, of course without the odometer) These are pictures of the gears that drive the odometer. The bronze gear on top is part of the odometer. The axle with the worm wheel in the middle can be taken off by removing the screw. Look here for the view from below. The lower worm wheel can be taken out by removing the rubber plug on the other end and tapping lightly with a suitable drift. I've got no pictures of that. Reassembly is the reverse procedure. Lubricate this part.

My friend the watchmaker took the spiral spring off the needle axle (with a lot of prying with a screwdriver …it's really stuck as well) and pushed the small pin out that clamps the other end to the rest of the speedo. Then he cleaned it in white spirit and spent twenty minutes with a magnifying glass and two pairs of pincers to straighten it out.

In the mean time I took the rest of the speedo apart. You can see here what it looks like. This is an exploded view of all the components (after they have been cleaned). The wheel was removed with a suitable small drift.

The disk in the large grey part is a permanent magnet and directly coupled to the drive cable. If it turns, it tries to pull along the wheel that covers it. The spiral spring on the axle holds the wheel back. The harder the magnet turns, the larger the force on the wheel, the higher the needle will read, since the force practiced by the spiral spring stays constant. What puzzled us was the large amount of metal filings inthere (In combination with the magnet..hell to clean ! A vacuum cleaner does wonders) and the alarming sound the unit made when screwed together. The only thing that would keep the wheel from rubbing the magnet would be the small spiral spring…and that wasn't possible. Obviously something was wrong, and I opened up the tachometer for a look.

There were no metal shavings inthere…and what was more…there was a part visible that wasn't present in the speedo ! The small needle in the middle of this picture is exactly what is supposed to hold the wheel away from the magnet. As you can see here, there are still the remains of grease on the magnet. This part is supposed to be greased ! If it isn't…this is what you get. As you can see, the needle which should be present on the axle on the left side is completely worn away, and this causes the wheel to bind with the magnet, flapping the needle about wildly, possibly breaking the needle, messing up the spring and causing the speedo to fail. If your speedo or tach starts acting strangely…stop immediately and disconnect the cable to avoid more catastrophic failure. The axle will have to be replaced anyway, maybe you can save the rest of the instrument.

I got a local watchmaker student to turn a new axle for me, to the specifications of the old one. Took about six weeks, and sure enough, when I got the part back, the upper (thin) part, was too tin to hold the spring assembly or the needle.

In the mean time, I found a derelict speedo of unknown origin, in a very sad shape with 78000 km on the clock.

Sure enough, when I opened it up… the tip of the axle was broken off. It hadn't been loose in the speedo while riding though, so the magnet was in way better shape then mine was.

I ended up replacing my magnet with the one from the derelict speedo. I kept my wheel and the rest of the parts. The derelict speedo had been repaired before, because part of the gears was no longer brass, but cut out of a whitish type of plastic. The key to disassembling the lower part of the speedo with the magnet disk and the gears, is carefully tapping out the odometer gear from behind (where the rubber plug is), then compressing the ring that is holding the magnet in by friction with the sides of its housing. The ring will fall out, and you'll be able to lift the entire magnet and its axle, complete with gears out of the pot metal housing.

I also experimented with the face of the old speedo, to see if I could repair the scratch that I made on my way better looking speedo face.

The answer is no.

I tried flat black paint, glossy black paint, satin black paint, thick, thin, felt tip pen, alcohol marker, every black looking type of ink I could get my hands on, and it all ended up looking worse than it did with the scratch.

I also found out that the speedo face will stand WD40, but NOT aceton, in case you would want to try that out for yourself. I then decided to leave the scratch alone to remind me of what I'd done to the speedo when it would be back on the bike.

With a socket from a socket wrench and the axle in a vise, I tapped the wheel back onto the lower part of the axle. This requires some force, and you are strongly advised not to put the wheel on the table and try to hammer the axle into it. The axle will bend, and you'll spend a lot of time straightening it. Don't ask me how I know.

Then I could assemble the lower part of the speedo, applying a coat of light machine oil all over the magnet, the tip of the axle and around the magnet.

The next problem was how to get the spring assembly to go back onto the upper part of the axle where it wouldn't clamp because the axle was too thin.

The answer was simple. Plain old acrylonitrile super glue. A dab on the disk in the middle of the spring and a very light coating on the rest of the axle insured that it wouldn't break loose, yet would be removable again if the need would arise. The light coat of superglue on the axle insured also that the needle wouldn't flip around loosely, since that part also is clamped to the axle.

I attached the other part of the spring to the lip with the small pin (see pictures above), just hoping the best of it now. Then I screwed on the face, put on the needle holder with the needle just lightly, not seating it all the way, so it could go over its stop easily, pointing at its actual zero.

That's when I discovered the markings on the raised edge of the speedo face. There are three thin white lines there, and with the speedo needle pointing to the middle one, and the spring to factory specifications, the speedo should be as good as right.

This may work if your spring didn't look like mine when I got it out of the speedo, i.e. like a blob of copper spaghetti. The strength of the spring apparently was affected by this, as I discovered when I put the speedo in its housing, put it in the bike and went for a ride. I could easily attain 50kmph in first gear. That is a bit too good to be true.

So I reasoned that if I shortened the spring length, its spring constant would become higher and I'd be able to get it right.

I shortened it a bit. After I mobilised my father to calibrate my speedo against his (in the car), and after about four test runs, it turned out that tampering with the spring length didn't change a thing, however much it should according to the theory. Or maybe my theory is just rusty, or not applicable to spiral springs.

Then I did one more careful test run, like this : take electrical drill... find something that will fit into the drill that can substitute the speedo cable and will not damage the speedo, i.e. turn the magnet.


Record speed. (48 kmph)

Put speedo in bike, go for a ride with dad and the car behind you.

Ride at exactly 48 kmph on your speedo.

Ask car driver how fast you really went according to his speedo (40 kmph)

Adjust needle so that you get a reading while drilling of about 38 kmph. (To compensate for the car speedo being a bit fast probably, and to get the point where the speedo points correct around halfway on the useful scale) Forget about the spring strength. Result of another session with the friendly watchmaker nextdoor was that we both concluded that you can't change that anyway, and that you best forget about those marks on the speedo being useful.

Afterwards, go testriding (with car)

My speedo is now dead-on around 50kmph and reads about 10 kmph fast at 90 (I thus read 100). Not too bad I'd say. And I get a favourable idea of the bike's top speed this way

If you cannot live with this, buying a new, correct spring would probably solve the problem. You need to talk to Palo Alto speedo if you want to do this. They may or may not be able sell you the right parts. I didn't try it.

Then clean everything very thoroughly, take it all apart for yet another round of oiling (you don't want to do all of this again soon), clean again (you do definitely do not want dust or greasy fingerprints anywhere once you have recrimped the bezel), put everything back together like it was, double check the position of the idiot lights, then put the glass on, wrap the bezel around it, turn the whole thing upside down, double check again that there is no dirt or dust, and that everything is in the right place, then start recrimping the bezel. At this point, it is very handy if someone can press down on the speedo housing for you, as this will insure a much tighter fit and seal in the end.

If you got as far as this, all that's left to do is remove all the duct tape from the bare idiot lights that you taped to the sides of the headlight bucket, remove the turn signal relay and the headlight assembly (not in that particular order !), put in the speedo housing, attach the tachometer cable, screw the housing in place, put all the idiot lights and the speedo illumination back in their rightful places, test this (the charging lamp is easy since this is bigger than the rest, green will go out if you step the bike into first, blue will be out in the first place, yellow is always on. Check blue for function by flashing the high beam. Turn on the headlight after hooking the assembly back up and check if the speedo illumination is on too.

Then attach the speedo cable, put turn signal relay back in place, and close up the headlight bucket.

Congratulations, you just succesfully rebuilt your speedo.

For now, I got a working, fairly accurate speedo for the round sum of about 20 US$ (for the work on the axle), a couple of evenings’ work and a sixpack of beer for the watchmaker.

Now, who did it cheaper ? ;-)


 Update : in June 2001 me, my girlfriend and the bike got hit in the rear by a speeding car on the freeway. Luckily, both of us ended up all right. (That’s right … from freeway speed to zero over the asphalt … and this is Europe, so we drive 120kmph. Can you say "protective clothing"? Indeed ! WEAR PROTECTIVE CLOTHING. WEAR PROTECTIVE CLOTHING. And I won’t say it twice.) The bike ended up in a concrete ditch with rather a lot of smashed parts. Among those was the rebuilt speedo … I guess we’ll never know if the repair would have held up. I’m keeping an eye out for a rebuildable speedo right now, because the one I have in the bike now is a replica. The mechanics are a lot better, but the faces and idiot lights look slightly different. I’ll scan a picture once I find time to make one. I’m playing with the idea of grafting the motometer faces on the modern innards …but that would mean opening up a perfectly good (and very expensive) instrument.


1971 R50/5 toaster