Modifying a PA speedometer.

[Robert Foreman]

Has anyone with a vertical drive PA speedometer modified it from a 0-60mph to a 0-100mph?
  Any information how to carry out this modification would be very much appreciated.

[bob46320]

The PA mechanism uses a "WATT" type of governor - as used on steam engines etc.  The deflection of the pointer is governed by the weights and the coil spring of the whirling governor.  To change the speed range, you would need to either reduce the weights or make the spring stiffer.

   

["Slack Alice" Simon]

I changed the spring in my PA, Robert.

When I first had the car, it only read about half speed - perhaps it was a Nippy's?

My spring was home made and so the needle wobbles about a bit.

But if I can do you, you will certainly be able to.

Cheers

Simon

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"Nippy's"  ???   probably not.   But - whatever.

[Alan]

The other alternative would be to fit a corrector gearbox from Speedograph. The mileometer isn’t gong to be correct but you should be able to work out the gearing needed to put the needle in the right place.

[Robert Foreman]

Thanks for the answers gentlemen. 
  Bob, how do you remove the bob weights to lighten them or change the spring?
The reason I ask is that the pivots are crimped so they can't be easily/safety removed.

[bob46320]

Never had to remove the pins but it looks like the the 2 pins above the weights would tap out IF you gently ground the tails of the pins down a bit with a Dremel type grinder. Don't remove too much as you will need to peen them over once you have replaced the spring.

[Pearls not a Singer]

Getting the correct calibration is likely to be a problem and trial and error could be a long process.

It can be done professionally . I have no connection but am aware of https://www.magnetic-speedometer-repair....uction.php of this company who despite the name can deal with governer type speedos or you could try Steve Hodgson who has offerred this service in the past

[Charles P]

Getting the correct calibration is likely to be a problem and trial and error could be a long process.

Wouldn't the solution be to borrow the right one and measure weights & spring rates?
I'm sure someone has one on a shelf that they'd trust to you Robert. (sadly, not me though)

Charles

[Austin in the Shed]

I have asked several instrument people,including Steve H and non could accurately recalibrate a 60 to 100 mph.100 mph PA's  are like finding rocking horse droppings as they were fitted to few vehicles.