Topic: Tach/Speedo lenses?

[750K4]

I have a K0 with a tach that has a lens in terrible condition. Is it possible to replace just the lens? Does anyone rebuild instruments? I know there are several sources for new faces, but I have searched and cannot find a rebuilder or source of replacement parts. Help!

[Bob Wessner]

I have a K0 also. Being plastic, both lens and case, I don't think it can be done, unless a prof. restorer perhaps. What kind of condition/damage are you talking about, cracked, abraded, etc?

[Jonesy]

There's a guy on eBay offering "professionally restored" speedo's and tach's... at a Buy-It-Now of almost $500.00, it's cheaper to buy new ones from a dealer....

They can be dismantled, just not not sure if the lens is flat or bulged out slightly. If it's flat, a thin sheet of polycarbonate plastic or glass could be cut and installed.

[750K4]

Bob
The tach lens was damaged by the previous owner - it has two holes that have been filled with clear silicone. The case, face, and band is in good shape.

Jonesy
I, too, saw the guy with the restored gauges. But all I have seen are newer styles with glass lenses. My K0 has fragile plastic lenses.

I'll just have to watch eBay - maybe I can buy a poor one to dismantle and try replacing the lens.

[Zane]

I'll just have to watch eBay - maybe I can buy a poor one to dismantle and try replacing the lens.

That's what I did to replace a lense I scratched up badly.  Even though my lenses are convex glass, I was able to find a gauge at the wreckers which had the same lens.  Although it cost me $10.00 (Can) I was not only able to use the lense, but salvaged the steel ring, some wiring and sockets, the needle, and even a couple of working bulbs as well.

I wonder if it's worth considering (or even if it's possible) for you to replace the plastic lens in your gauge with a glass one. 

My gauges are smaller, so that concave glass magnifying effect is really quite useful and effective (especially for this old fart).  But I think your gauges are quite a bit larger, so you can probably get away with those plastic lenses a little easier than I could.  It's just that the glass lenses are so much more scratch resistant.

Good luck with it all .....

wz

[putnaja1]

Can you guys post a pic of a gauge?  That way, I can see how complex the lens is..  It might be possible to vacuum form a new lens, or I have a couple other ideas..

Jason

[Zane]

Sounds interesting Jason.

I'll leave it up to the other chaps to post a picture, since my little gauges are not the same as what's being discussed.

However, if you think it might be helpful to see a 400F clock exploded, please let me know and I'll take a few shots and post them for you (us).

Thanks,

wz

PS  I have a question.  Does the casting process you're talking about involve milling of the casting in order to finish the lense?

[Bob Wessner]

The K0 plastic lens is not flat, but the convex curve is pretty shallow. See attachment, hope this helps;

[martini]

I searched in vain for a year and a half for a new lens for my K0 before swallowing hard and buying a new guage off e-bay. Good Luck.

[putnaja1]

PS I have a question. Does the casting process you're talking about involve milling of the casting in order to finish the lense?

Nope, vacuum forming (I'm no expert, but I think it can be used) would involve making a plug out of some material- this could either be using an existing lens to make a plug outta plaster of paris or "sculpy" clay that would then be baked to harden, etc.  Alternatively, a plug can be carved/shaped outta wood, and reinforced with fiberglass or even bondo.

This plug is then coated with a release agent and positioned on a vacuum forming jig.  The plastic sheet material is warmed in an oven until it gets soft (it's fastened inside of a wooden frame that holds it from it's outside edges)  Once the plastic is soft enough to sag, it's removed from the oven, placed over the plug on the vacuum jig.  The vacuum is then turned on, which pulls the plastic firmly over the plug until the plastic cools and remains in that shape.

It sounds pretty hardcore, but alot of model airplane modelers make vacuum forming jigs, using a shopvac for vacuum, and their kitchen oven to do the warming- in other words, it's doable by us, and wouldn't need a large factory and lots of money.

The K0 plastic lens is not flat, but the convex curve is pretty shallow. See attachment, hope this helps;

 

That shape should certainly be doable- it's not too complex.  My only question now is, how thick is the plastic of the lens?  Anyone got a broken lens that they can see about how thick the material is?  I should note that I've never done vacuum forming myself before, just read up on it, and I've seen the results, which look pretty promising.  The only thing I don't know is this:  How thick of plastic can be successfully vacuum formed..  I know that if the initial vacuum doesn't get the shape 100% nailed, while the plastic is cooling it can be "persuaded" along with a heat gun..

Jason

[martini]

I would say that the lens is about 2-3 mm thick.

[Zane]

Please excuse my ignorance re: things 750, but is a "K0" only one specific year?  Or can a K0 be any one of a number of years?

Please tell me what year(s) we're talking about.

I'm asking because within the next day or two I'm making a trip to one of Canada's biggest salvage places.  I'm on a quite friendly basis with the owner, and if I know what model and year I'm looking for I don't mind spending a bit of time seeing if I can find a lens (or even an entire gauge) or two.

Also, Bob, it kinda looks like from the pictures you posted, that a fellow might be able to change the lens in one of those gauges without cutting (or other wise removing) the metal band.  Is that the case, or must the gauge be opened up by removing the metal ring/band?

As for vacuum forming - it sounds interesting but I'm really curious what kind of optical quality you would end up with in a 3 and a half inch diameter, vacuum formed plastic lens.  If you did it really well, my guess is you might get close to the quality found in a relatively inexpensive pair of safety glasses.  It might be good enough.

I do know that decent eyeglass lenses are made from milling a plastic blank, and then polishing with a pretty sophisticated polishing machine.  There's apretty friendly guy downstairs from me who makes plastic lenses.  If I can find a plastic gauge lens at thesalvage yard, I'll take it in to him and see if he can duplicate it economically.

[Bob Wessner]

Also, Bob, it kinda looks like from the pictures you posted, that a fellow might be able to change the lens in one of those gauges without cutting (or other wise removing) the metal band.  Is that the case, or must the gauge be opened up by removing the metal ring/band?

I've never had occasion to try and open mine. The metal band on the K0 gauges seems to be nothing but a bracket that holds the whole gauge unit on the bike. The lens appears to be cemented or otherwise affixed to the black plastic case so I'm not sure how you would go about separating the two.

As for the K0 designation, though something of a misnomer, I believe it is associated with 750's with various 1969 build dates on the VIN plate, possibly some early 1970's as well, but I'm not sure.

[750K4]

The K0 designation generally refers to the 1969 and 1970 models. I believe that all the gauges after the K0 were made with glass lenses, so if you are looking at a gauge with a glass lens, it's too new.

I'm intrigued by the idea that an eyeglass lens manufacturer may be able to duplicate a lens.  I have an eyeglass lens manufacturer in my town that I drive by on the way to work every day. If I had a sample lens, they might be persuaded to manufacture some. I know a pair of eyeglass lenses from them costs around $100 a pair. I'd gladly spend $100 for a new pair of lenses if it's possible to fit them to the cases. Anyone have a junk K0 speedo or tach that they'd be willing to disassemble to get the lens out and send to me?

[martini]

I have a cracked lens and housing that I replaced a few months back. I had to buy a whole new tach unit to do it though. I have had the old one apart. I'll have to have another look at it tonight when I get home but I recall the lens actually being fused to the plastic body. Not sure if it would be possible to seperate them or not. As I said I'll have a look at it tonight and let you know tomorrow.

[750K4]

Great!
I haven't had one apart - how do you open it up?

[Zane]

Hey, that's great men.  I'll definitely have a good chance of finding a lens or two. I'm almost certain to find one or two in unusable shape for a bike, but good enough to show a potential manufacturer.  I'll also keep my eyes open for usable ones.  What is the consensus among you for a fair price for a used lens in good to excellent condition?  (My friend is almost certain to either give or charge very little for a lens like I describe above - just to show around to possible makers - but he's in business as far as useful parts go, and would charge a fair price for a good lens.)  

I might be full of beans too, though.  I might not be able to find anything remotely resembling any condition K0 gauge lens.....        we'll see....

That's neat eh?  The K0's (is that K - oh, or K - zero?) are the first 750's.  So my guess is that the gauges were fitted with plastic face covers for the same reason the first few thousand had sand cast engine cases - Honda really wasn't sure how the 750 was going to do early in 1969, sales wise, and until they saw that it was going to take off, they cut financial corners in a few spots.

I didn't think of it before, but after your responses, I looked up the K0 in a book I have.  I'll see if I can attach the paragraph that talks about value.

The last thing I would find very helpful, I think, is a few measurements.  The lens diameter - butt to the inside edge of the gauge face, and a second diameter across the gauge front - from the peak of the front, leading edge (likely 4 to 6 mm greater than the first, inside diameter).  The last measurement that could be helpful would be the pitch of the lens convexivity.  (From Bob's shots, it looks like that might be even more than half an inch.)  So if somehow I can get that info, it would be helpful

[Bob Wessner]

Zane,

(From Bob's shots, it looks like that might be even more than half an inch.)

If you are talking about the amount of curvature, it is very shallow. The shot I posted is misleading in that regard. If you like, I can take one at 90 degrees to the lens surface to give you a better idea.

[Zane]

Bob, I don't think another shot's neccessary.  (I appreciate the offer though.)  Just give me your best guess of the pitch by looking 90 degrees on.  mm's or inches - I'm an old enough Canadian to be bi-distance-al (or whatever you call it.)

[Swede]

I just thought I would throw this link to Carpy's site out there.  He has his process for dismantling gauges all laid out.

http://www.cb750cafe.com/parts.php?id=5

[Bob Wessner]

Zane,

The lens itself appears to be 2-3mm thick and the center, dome if you will, is only about another millimeter, just enough to not be flat.

[Zane]

Great.  Thanks B.W.  That sounds like it would be right - based on the small degree of curvature of my (smaller) 400F gauge lenses.  (I wonder why the curvature looks so much more extreme in the photo?  Weird eh?

I plan to head to the slavage place tomorrow, weather permitting.  I printed off the shot of your two gauges for a K0 750 from your website Bob, and I'll take that along to help me look.

Enjoyable site, by the way.....

[martini]

I had a look at my old guage last night. I think you might have a bit of a problem seperating the lens from the plastic body. It appears that the two are somehow fused together. Can't really tell if its glued or melted or what but I could see no way of getting the two pieces apart aside from possibly trying to cut it out and that did not look like it would be an easy task.

BTW, the link on Carpy's site referred to above will not help you with dissassembly of a K0 (zero) tach. The set up on the K0 is completely different than subsequent models. Mine actually came apart fairly easliy. I removed the entire unit from the chrome ring that holds it in place and then I very gently pulled the plastic housing off. I was suprised how easily it came apart. It appears that the only thing that really holds it together is friction and the chrome band which holds it on the bike. I did apply a bit of silicon when I re-assembled it for my own piece of mind.

Let me know if you want measurement I can get them for you tomorrow.

Martini

[Zane]

Excellent info for me Martini - thanks a bunch.

I was going to ask about the construction - I was inclined to guess (from Bob W's pix) that it was different than most of Honda's subsequent gauge housings.  I sure hope I can find a screwed up but complete one tomorrow - just so's I can see how it works.

What you describe would also explain "searching in vain" for a replacement lense -

[martini]

No problem Zane.

Let us know how the search goes, good luck.

Martini