1972 Suzuki GT750 Reproduction Gauge Shells

When I picked up my 1972 Suzuki GT750J rebuild project, there was an obvious issue with the gauges. The plastic gauge shells for my speedometer and tachometer were both cracked up into the lens, and when I started asking around I found this was a common issue and specific to the 1972 model. Some folks have tried to glue up the shell, but I do not consider this to be a long term solution – and of course if the crack has extended up into the lens then the shell is scrap anyway as per this photo to the right.

The rest of the instrument is not usually difficult to repair – but the plastic housing and lens is unique to that year, and from the research that folks like Allan have done, also unique to Suzuki for that model in that year. So a couple of years ago I started to look at replacement options – my first step was to have a set of original gauge shells modelled in 3D by a small local company( 3 Print Dimensions) here in Calgary. What they did was first laser scan an old housing, generate a CAD/CAM drawing with simulated texture applied to the wire frame drawing, and then print a copy using a 3D printer. On the left is a printed 3D model I had made from the CAD/CAM drawing. It is in an opaque plastic as the intent of the model was to double check fit, shape and finish.

The 3D process has really improved a lot in the last short while, and affordable table top 3D printers are almost within reach of the average home hobbyist. But if you want to make a product that has a highly polished surface like a lens for example, it currently isn’t a suitable manufacturing option, and as well is quite expensive on a per unit basis.

Originally I had thought of using a one piece injection moulded design done in clear plastic, and then gauge restorers would have had to paint the outside of the shell to suit. The tachometer and speedometer housings are basically identical, with the obvious exception of the hole for the reset knob in the speedometer, which would have had to be drilled. I had considered duplicating the water temperature housings but they generally are not in bad shape, they clean up well and they seldom crack the way the tachometer and speedometer housings do so for the moment I have not pursued the idea.

After a number of design iterations and working with a plastics injection moulding and engineering company in Edmonton (Flexcim Services) I finally decided to make the shells in two parts as was originally done by Suzuki. I should mention here that the engineering design and construction was handled by Gordon Baker and Gary Penney who were both more than helpful, offered great suggestions and support, and never got tired of answering my many questions (or if they did, they at least hid it very well !!).

Here is an example of the early test batch done to validate the fit and shape – these were shot in clear for convenience. In the upper left you can see a test piece mounted on the metal base and inner sleeve. In the foreground are the blanks shot for the housing and in the upper right the lens. In the lower right is an original part I’ve used as a base comparison. Note that the housing is filled from the middle, across the lens face and then equally down sides. The idea is to ensure a uniform strength shell which should be stronger than the originals.

The process then is to shoot the black housing, CNC machine the face of the housing to receive the clear plastic lens, machine the hole in the side of the speedometer shells at the correct angle and recess the inside of the shell to ensure proper clearance of the reset knob tube, then glue the lens into the housing.

These show the assembled lens and housings, with the housings shot in the black plastic. Original housing on the left and the reproduction is on the right in both photos.

The last step was to get the texturing of the polished injection mould done, and here to the left we see the copied part with the texture added – original on the left, and copy on the right. The two small nubs you see on the top of the one on the right are locator pins for the CNC fixture so ignore them. Also – the one on the right doesn’t have 40+ years of hard life and sun damage. 😀

So – I there you have it ! I now have copies of the original shells for my own restoration and also have a few for others also restoring these iconic machines. This next photo shows the shells fresh off the production line at Flexcim. If you look at the photo closely, you can see both types (tachometer and speedometer). The material used is polycarbonate for the black case and an acrylic for the lens, which is then solvent welded to the case. Each solvent welded seal has been water tested to check that the joint between the clear lens and the black case is complete.

Here is a photo of the trial fit of a new speedometer shell, re-using the reset knob spigot from the old shell.

Replacing the shells is not difficult, but typically that isn’t the only repair required. Usually the needle needs to be repainted, the dampening fluid needs to be replenished (to dampen needle bounce), the instrument mechanism needs to be cleaned and calibrated, and the trip and odometer reels often could stand to be rejuvenated with new numbers. Possibly the face will also need repair as well. So:

•  you can order a set of reproduction GT750 J plastic gauge shells at this web site
• while most people can do the basic shell replacement themselves, for the best results and a set of clocks/gauges that look like new, I strongly recommend that you use the services of a clock/gauge restorer – at the time of writing, both Gary Mellors in the UK and Allan Tucker in Barbados are known by myself and others to do good work.
• I cannot accept any responsibility for your results, or the results of others.

Now – there is this T500 Cobra that I need to get back to …………………………