New Suzuki 1972 GT750J Reproduction Gauge Shells
The reason the shells crack goes back to two design issues with the original shell made by Nippon Denso - the mould used appears to have been injection filled from one side, and where the plastic met after flowing around the mould a slight cold joint resulted causing a weak spot. As well, the metal bases of the gauges often were not round, and when the shell was heat sealed onto the metal base, stress was introduced. The result was cracking starting at the lower edge of the plastic shell which then propagates upwards into the ultrasonically welded clear lens. Suzuki switched to all metal gauge shells for all subsequent models.
Please note that repairing clocks/gauges is not something that I do for a living, and if you do your own repair I accept no responsibility for the result. Nor do I accept any responsibility for work done by whichever clock/gauge restorer you select. You are welcome to use the following information, but it is entirely at your own risk.
The tachometer is easier to do than the speedometer, so lets assume you have a speedometer that looks something like the one below (just click on any photo for a larger image).
The first thing to do is remove the old shell - this is generally done by cutting away the plastic weld at the lower edge of the shell. Depending how badly cracked your shell is, parts may readily fall away - with the exception of the area around the trip reset knob on the speedometer of course as shown in the photo below.
There is a metal latch that locks the trip meter rollers - indicated by the pointer in the photo below.
If your shell is more complete, it can be easily reached using a screw driver through the bezel light bulb socket - lock the spring, and unscrew the reset knob - note that it is a reverse thread.
Save the part of the housing that the reset knob's shaft runs through - this tube is just solvent welded to the inside of the old housing, and can be carefully removed with a hobby knife or with a Dremel if you consider yourself especially skillful, and then glued into the new shell. The hole is pre-drilled at the correct angle to make this easy, and the back of the shell is slightly recessed to ensure adequate clearance.
The solvent glue I've had the best luck with is this one:
Now remove the indicator needle - I use the teaspoon method when my wife is not around. Just place the backs of two spoons on the heads of the two screws securing the gauge/clock face and then gently and evenly lever off the needle. The most important part of this is to first have placed a cloth over the face of the clock, otherwise your needle will now have disappeared into the neither regions of your work area, never to be found again (don't ask me how I know). The tip of the needle can be repainted with a day-glo orange hobby paint.
Below is a partially exploded view of the clock/gauge internals specific to the 1972J clocks (later gauges are different). The drive gear box in the lower part of the photos can be disassembled, cleaned and re-greased quite easily. The other components just normally need a very careful cleaning with a light solvent like WD40, and then a drop of light machine oil put on the drive gears. Only use a strong solvent like brake wash on unpainted surfaces as it will quickly wash all the digits off your odometer and trip meter reels if you are not really careful. Note that the 'bell' (indicated by the arrow) and the upper insert fitting into it should not touch each other - and do not add oil to the inside of the bell. Doing so will prevent your gauge from working at all.
The clock face usually shows signs of damper oil staining - be very careful cleaning this, using a very mild detergent and water rather than any strong solvents to avoid potentially damaging the silk screened face of the dial. If you do damage the face, then Steve at ClocknDials may be able to help with replacement vinyl ones. Otherwise your choices are eBay, your club, or one of the gauge/clock restorers already mentioned.
Checking the calibration of the instrument is beyond the scope of this discussion as it is far too easy to ruin the instrument completely - if your instrument's zero is off, or if it is not reading correctly at speed, then that is what folks like Gary and Allan look after. Also - different jurisdictions have different rules when it comes to changing speedometers on vehicles, but if you are repairing a clock to replace a broken one you previously had installed, you are usually allowed to match up the odometer reading of the original instrument. It is also possible to refresh the numbers on the trip and odometer to look like new. Both items are things best discussed with a clock/gauge restorer.
Putting aside calibration questions - if you have noticed that your needle bounces a lot when at speed, then it is possible to add some dampening fluid into the small pot indicated in the photo below.
To get at it, you need to further disassemble the top of the mechanism - not difficult to do, but also very easy to mess up and render the instrument completely useless if you make a mistake, so you are on your own ! The special silicone fluid you need for the damper pot is readily available at any decently sized radio control hobby car shop, as it is commonly used in the suspensions and rear differentials of R/C dune buggies. A small bottle will last a lifetime, as you only use a drop or two per instrument. Share it with your friends ! It comes in a range of grades - I've made a guess as to weight and the one shown below is what I plan to use on my next repair. It is a 30000cSt weight.
After any other repairs or re-painting, and with the cleaned instrument reassembled, you are ready to fit the new shell. I do suggest you do a quick check that the clock/gauge does function properly before fitting the shell permanently - a #2 Robertson bit (square drive bit) in a portable electric drill running in reverse will give you a reasonable indication of whether you are in business or not - but without a proper calibrated bench test rig such as a clock/gauge restorer would have, only a road test will tell you the full story. If you have access to a way to measure the RPM, then 1000RPM on your drive will be 4000RPM on the tachometer, and 2240RPM will be 60MPH on the speedometer.
The new shells are perfectly round, so spend some time to check that the new shell does not bind on the metal base plate of the instrument housing - if it does, file or sand the outside diameter of the metal base plate where it binds to ensure a proper fit to the shell. The locator tab in the new shell will align your inside green sleeve, the shell and the base plate properly. With your new shell fitted, thread in your trip reset knob on the speedometer (don't forget that it is a reverse thread), seal the base of the shell with black silicon sealant, and you are ready to go !
And before someone asks - the photos above were of trial fits, so I do know the one on the right is still missing its needle and the face hasn't been cleaned, and the tachometer on the left needs a dab of paint on the needle but they were the only photos I had handy !
Good luck !