Author Topic: Refilling Dashpots on CB750K0 Gauges  (Read 265 times)

Offline AshimotoK0

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Refilling Dashpots on CB750K0 Gauges
« on: July 11, 2018, 11:47:38 AM »
Anyone successfully refilled the dashpots on CB750K0 gauges? If so, couple of questions:-

1) What viscosity silicone oil did you use  30,000cSt or 50,000cSt ? ( I think Marcel at CB750faces uses 50,000 and I guess he's the man on these but other people suggested it's too viscous)
2) Did you unglue the lid on the dashpot and re-glue after topping up or drill a hole in the side, inject oil and re-seal?
“Alright friends, you have seen the heavy groups, now you will see morning maniac music. Believe me, yeah. It’s a new dawn.” Grace Slick, Woodstock '69 .. In the year of the Sandcast.

Offline kent400

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Re: Refilling Dashpots on CB750K0 Gauges
« Reply #1 on: July 11, 2018, 01:10:53 PM »
I've always used 30000 cSt on ND gauges effectively and drilled a small hole is the damper to replenish   You need to exercise care of course, once a few drops of silicone has been added move the needle around and add some more until the damping effect returns.

The damping effect does tend to vary from gauge to gauge slightly but what you need to see is a needle that does not drop immediately to the stop when the drive is spun. 

Offline AshimotoK0

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Re: Refilling Dashpots on CB750K0 Gauges
« Reply #2 on: July 11, 2018, 01:16:02 PM »
I've always used 30000 cSt on ND gauges effectively and drilled a small hole is the damper to replenish   You need to exercise care of course, once a few drops of silicone has been added move the needle around and add some more until the damping effect returns.

The damping effect does tend to vary from gauge to gauge slightly but what you need to see is a needle that does not drop immediately to the stop when the drive is spun.

Thanks Peter... the K0 speedo I have appears not to have a cap on the dashpot ..its just a brass tube within another brass tube .. I will try to photo as it may help others. The tacho dashpot is an alloy cup with an alloy cover and the cover is glued to the cup. So it's a case of make sure the gauge is always stored upwards.
“Alright friends, you have seen the heavy groups, now you will see morning maniac music. Believe me, yeah. It’s a new dawn.” Grace Slick, Woodstock '69 .. In the year of the Sandcast.

Offline kent400

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Re: Refilling Dashpots on CB750K0 Gauges
« Reply #3 on: July 11, 2018, 04:30:00 PM »
I think there were two different designs of damper type on ND gauges and I would have thought that two gauges from the same bike would have been the same design. The two types I have here is one with the aluminium cap that has a sealant around it, the other type that has no cap and is a smaller diameter. The second type I found in the spares box, I can see I have re-filled the damper and that's a speedo mechanism. Looks like I used most of it for spares and I rather think I may re-sealed the hole made in the damper as an experiment to ensure the seal was effective. Clearly it was.

Offline AshimotoK0

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Re: Refilling Dashpots on CB750K0 Gauges
« Reply #4 on: July 11, 2018, 04:58:16 PM »
The speedo has the open type damper but is definitely original to the bike, as it's consistent with old MOT's I have. The tacho may well have been changed by a P.O. but it has the sandcast type gearbox drive and it has the alloy style damper with the cover (cover secured to the body with dabs of dark brown adhesive or whatever). The damping doesn't seem too bad on the tacho, so I may leave well alone on that one.
“Alright friends, you have seen the heavy groups, now you will see morning maniac music. Believe me, yeah. It’s a new dawn.” Grace Slick, Woodstock '69 .. In the year of the Sandcast.

Offline AshimotoK0

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Re: Refilling Dashpots on CB750K0 Gauges
« Reply #5 on: July 13, 2018, 07:52:57 AM »
Well I ordered the silicone oil and the Revell  orange paint  for the needle tip. I found that the tacho drive was almost completely seized up. I removed the top bridge with the hairspring intact and no worries there. However, I couldn't remove the gearbox housing from the steel base. There is another cast alloy plate alloy  with three mounting screws that also carries a brass bush. This plate has a boss that locates into the gearbox and  I found this boss was seized to the gearbox housing. You can't remove the plastic worm gear and  its shaft without pulling the gearbox housing off at least  partially. This plastic gear prevents you from applying any real  heat to free it off plus I would image any heat might partially  demagnetize the rotary magnet. Long and short is I tried every method to remove it and then CRACK, the  plate with the boss on it cracked and I still can't pull the gearbox off the base plate. The only way I reckon I can get it apart now would be to somehow press the brass gear off the shaft but I'm  not sure if the gear is a press fit or pinned onto the shaft. In desperation I bought a  damaged K1 tacho mechanism from DK and I am hoping that I can graft this into my K0 unit, if the gearbox on it is serviceable. I guess Lecram may be the guy to ask but  his website says he's on holiday. Any advice Peter?
« Last Edit: July 13, 2018, 07:57:05 AM by AshimotoK0 »
“Alright friends, you have seen the heavy groups, now you will see morning maniac music. Believe me, yeah. It’s a new dawn.” Grace Slick, Woodstock '69 .. In the year of the Sandcast.

Offline kent400

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Re: Refilling Dashpots on CB750K0 Gauges
« Reply #6 on: July 13, 2018, 08:52:14 AM »
Hi Ash, I've never had much of a problem getting the angle drive off once I've removed the three screws and just twisted them. The problem is that the aluminium  corrodes on the small portion of the angle drive that's inside the base and expands. It's been a while since I had a gauge of that type though my hands so I can't recall the exact details other than the fact that once the angle drive you are left with just the plastic gear protruding from the base.

I suppose all you can do is to soak it in some rust remover for a couple of days, something that's mildly acidic. Heating won't do much good as the it will probably cause it to expand.