Wednesday, December 21, 2011

Removal of a Smiths chronometric speedo or tacho bezel rim.......

This could be the last post from me for the year and it is an item long overdue from me to add to the several posts I've done on the Smiths chronometric speedometer and tachometer.
The bezel of the 80mm diameter (3") chronometric has a rim with an 80mm x 0.9mm thread. The thread is relatively fine for the diameter and removal of the bezel from the case by unscrewing is not always easy.
You need a good strap wrench...the best I came across was a type and you'll see it illustrated, that was made in the USA and has s mall rubber type suction pads.
I dont think they make them like this now as years back I tried to get more and they had gone over to plastic which as you read down the blog, you'll see is not suitable.
Putting too much pressure onto the rim will distort it and that in turn causes the bezel to grip harder.
So what is one of the major reasons a bezel is stuck?
The 'O' ring seal used by Smiths was made of natural rubber and these perish over time and appear to 'melt' effectively glueing the rim to the glass and oozing onto the glass carrier or slipring/mask.
The rim wont move under normal pressure if it is like this.
The trick is to use careful heat, played around the rim edge until the remains of the 'O' ring really do melt and then quickly fit the strap wrench and in 99.9% of cases the rim quickly and easily unscrews and when cleaned up of the resulting rubber mess can be re-used.
As well there is no damage to the dial and internal components.
I suggest you make up, from some convenient angle piece a bracket to mount the instrument to and hold this in a vice.
Left click on the images to enlarge them for better viewing....

The smoke from the rubber melting indicates the bezel is close to ready for unscrewing...


Anonymous said...

It is much easier to unscrew the bezel breaking the glass.

Unknown said...

Why break it when you don't need to? I've used Dennis's method and it works very well.

As an aside, I've learnt a lot from this site and feel much more confident attempting instrument repairs.