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Cleaning a tacho glass

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Alan Donovan12/04/2021 11:54:02
44 forum posts
22 photos

Hi all.

This may be a 'long shot'

I have a Honda (circa 1975) motorcycle tachometer which is dirty on the inside of the glass (which I would assume to be plastic). I looks like the tacho has had dirty water inside at some point. The unit appears to be of 'snap together' construction as there does not appear to be an obvious way to remove the glass. I would like to preserve the original part as it is, if I can, as it is still working.

Has anyone any experience of successfully cleaning the inside of a tacho glass using a non-aggressive method?

Initial thoughts are carefully pouring a suitable solvent (rather than soapy water) into the unit, through the hole for the night time illumination bulb, avoiding the tacho mechanism, and gently swilling around to remove the dirt. Has anyone any experience of this? Did it work?

It's not a big hinderance to leave it dirty, it is still readable - but it would be nice to have a clean glass.


All the best & thank you.


Edited By Alan Donovan on 12/04/2021 11:56:29

Edited By Alan Donovan on 12/04/2021 11:59:14

Hopper12/04/2021 12:19:44
5427 forum posts
134 photos

If the instrument has been full of water, its insides are toast and its not worth messing with. Putting more soapy water in there will do the same damage. But if it is still working, the damage you see is more likely from condensation and maybe some silicone lubricant from the instrument or sometimes that little disc in the middle of the needle is full of silicone oil to act as a fluctuation damper and they spring a leak.

And don't put solvent in there. Those plastic lenses don't do well with solvent. I still remember trying to use methylated spirits to remove the "Do not exceed 50mph for first 500 miles" sticker from the lense of my brand new 1977 Harley that had a very similar instrument and being heartbroken with the resulting half melted mess.

There is a ton of that old Honda stuff still available so a new secondhand unit might be the best way to go.

Otherwise you might find out on a classic Honda forum if there is a trick to opening those instruments up.

I know the late 1970s Harleys used very similar unbadged Nippon Denso speedos and tachos with the same plastic lense. Harley aftermarket suppliers such as JP Cycles sell a lense kit that you cut the old lense off and glue the new one on in its place. It might fit yours but not sure of the exact size being the same and all. And I imagine the glued joint would be noticeable unless you were very lucky.

PS and are you sure the mess is on the inside? Often they craze on the outside with age and get a dull matt surface that can be restored with Solvol Autosol metal polish.

Edited By Hopper on 12/04/2021 12:21:09

Edited By Hopper on 12/04/2021 12:23:31

Alan Donovan12/04/2021 14:30:21
44 forum posts
22 photos

Hi Hopper.

I did think it was a long shot, and that the use of a solvent was a non-starter, but I thought I would ask the question anyway.

I hadn't considered condensation as a possible cause rather than a soaking, but the reality is that although it 'works' I have no means of checking if it is accurate. I will probably just leave it 'as is' for the immediate future.

Thanks for your input.


Grindstone Cowboy12/04/2021 16:30:01
584 forum posts
57 photos

Not sure about Honda, but some of the older Smiths instruments had a screw-on chrome bezel. Difficult to get off, you would need to make a tool to do it, I think there was one described in "The Vintage Motorcyclist's Workshop" by Radco.


Jeff Dayman12/04/2021 16:42:39
2131 forum posts
45 photos

Years ago I did take the speedo and tach apart on a 1972 Honda 350 K2 I had. The front and rear halves of each were held together by a polished aluminum band rolled or pressed to a C shape to permanently assemble the units. A rubber gasket was compressed between front and rear halves of the steel shell. I got the speedo and tach housing shells apart by cutting the aluminum band. After cleaning the internals, repainting the faces and cleaning the glass (real glass, in those units, luckily) I drilled tiny holes in the aluminum band, put a dab of silicone sealant at the join, and wired the band ends together with some small dia copper wire. The wire and the join was placed where it would not be visible, under the units toward the steering stem. Worked for many years afterward.

Bear in mind both meters worked fine before, they were not corroded inside at all. The reason for disassembly was the paint on the faces was completely sun faded to the point you could not read the faces at all. Lots of condensation marks on the inside of the glass too.

Hope the disassy info is useful, if your gauges are fastened with a band as mine were.

duncan webster12/04/2021 21:38:58
3179 forum posts
55 photos

I once tried to clean the insides of a speedo by pouring solvent in. Melted the little plastic gears, cost me a new speedo, and I wasn't flush in those days.

Hopper13/04/2021 04:24:53
5427 forum posts
134 photos

I just had a quick look at the very similar instrument on my '77 Harley. (The great American Freedom Machine came stock with Nippon Denso instruments, Keihin carb and Showa forks!)

The plastic casing is in two halves joined around the circumfrence. Where Jeff Dayman above found a polished metal joining ring, mine has a very discreet rubber ring of some sort, set into the join so its barely noticeable.

I'm sure it will be common knowledge on the classic Honda forums etc. how to best pry that rubber seal out and separate the two halves of the instrument casing without destroying it, if it is at all possible. And if yours has the same joint.

The outside diamter of the lens is 81mm at the top in case you think about the Harley lens replacement kit. Available here in the USA for $20 but there might be a UK supplier or eBay seller

Edited By Hopper on 13/04/2021 04:34:31

Hopper13/04/2021 04:39:56
5427 forum posts
134 photos

PS there is also a heap of videos on YouTube about repairing HOnda instruments. Like this one:


Looks like they are not too hard to open up and clean.

These ones are a slightly different casing from the Harley ones so if yours are like this, the lens repair kit will not fit.

Edited By Hopper on 13/04/2021 04:40:35

Edited By Hopper on 13/04/2021 04:42:12

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