Speedo and Tacho dashpot?

[Seabeowner]

Can someone explain how the dashpot on the tach and speedo meter works?
Have a tach apart. The needle is driven up by a fight between the magnetic coupling at the bottom and the coiled spring on the shaft. That bit seems simple. Above the spring is the dashpot. What goes on in there? The dashpot itself seems fixed to the shaft as does the needle above it. So it all moves together.
Plus tips for refilling with silicone oil.

[McCabe-Thiele (Ted)]

Only thing I know is there is a damper arrangement so the needle does not flutter all over the place - I've sent my gauges to be overhauled by Kent400 as I'm no clocksmith! My old Triumph 21 had some sort of gear like speedo it flicked like the seconds on a watch up & down.

[Sesman]

Chronometric: Wonderful instruments.

[McCabe-Thiele (Ted)]

Chronometric: Wonderful instruments.

Yes they were and I belived it when it said I was doing  91 mph (before the 60 mph limit was introdunced on rural roads) happy days!

[AshimotoK0]

A little tube is fixed to the fixed bridge at the top....  this dips into the dash-pot (attached to the needle spindle) with high viscosity silicone oil in it. When the needle shaft/dash-pot rotates, there is  viscous drag between the oil in the dash-pot and the tube fixed to the top bridge. This provided the needle damping.

[Seabeowner]

Thanks for that Ash. I found a thread from 2018 where you and Peter talked it over. Ordered 30000 cSt, but I see Lecram uses 100000. Not sure how I'm going to get it it. Don't think they will let me have the syringe after my booster shot.

[kent400]

You don't mention whether the gauge is ND on Seiki. The filling of the damper can be different between the two types and the on some of your machines on your profile if could be either of the two types. You silicone you need is 100000.   

[Seabeowner]

They are Nippon Seiki 500s. Presume 100000 is really thick, how do you get it in?

[kent400]

Assuming the gauge design is the same as shown is the image you will find after removing the face there is a small 5mm brass nut. If you unscrew that you will be able unscrew the damper bush. You need to take note of how much thread is protruding from the brass nut to ensure it goes back with the same amount of thread
visible.

 You can then 'dribble' some silicone fluid down the needle pin and put some on the damper bush. Check that improve the damping effect. There is another type of Seiki mechanisms where it not possible to unscrew the bush.  Although the mechanisms appear the same so you have to either remove the front of the mechanism by folding back the tabs that are formed from the frame or drill a tiny hole in the damper body and put some silicone in it. Reseal the hole with some mastic. Obviously great care is required, a centre drill bit and a hand drill.     

[Seabeowner]

Absolutely perfect. Thanks very much Peter for taking the time. Looks just like mine. Will have a go at the nut tomorrow, best not started late in the day. Oil available on ebay.