Joining two V groove pulleys together

[badabec]

Hello, this is for my 1972 Citroen DS.
Citroen used two V groove steel pulleys for the drive belts to the water pump, alternator and hydraulic pump (belt and braces, two belts to water pump and alternator, two to the hydraulic pump). On the semi-automatic versions they used a three V groove pulley on the hydraulic pump, the additional groove was used to drive a centrifugal regulator.
I use the third groove to drive the air conditioning compressor which I installed. All the drive belts are driven by the camshaft, meaning the hydraulic pump runs at half engine speed. So too does the compressor drive.
I would like to increase the diameter of outer groove from 100mm to 125mm to increase the speed of the compressor.

So far I have toyed with the idea of machining off the outer V of a 100mm three groove pulley leaving just a stub and then machining the inside of a 125mm pulley to be a heat shrink fit.
Or take a very light cut from the face of a 125mm pulley to align the 100mm pulley centrally then pay for someone to TIG the two together.
Or bolt them together.

Has anyone done this before?
Has anyone any advice or suggestions?

 

[mike os]

Can you reduce the other pulley instead ?

 

[badabec]

Can you reduce the other pulley instead ?

Unfortunately no, if I reduce the pulley size the hydraulic pump won't spin fast enough. The DS lives and dies on hydraulic pressure.
The picture shows the arrangement. The lower large pulley has a smaller pulley riveted to the rear. The smaller pulley drives the hydraulic pump, the larger pulley drives the water pump and alternator.
The small green pulley to the left of the picture of the engine is the one I wish to attach to a 125mm single groove pulley.
Citroen DS, complicated? Nah!

 

[mike os]

Decreasing one is the same as increasing the other

 

[Agroshield]

Unfortunately no, if I reduce the pulley size the hydraulic pump won't spin fast enough.

You are proposing to increase the size of the driving pulley.

He is suggesting you reduce the size of the driven pulley on the compressor.

It is a sensible idea as it is a single pulley that does not interact with anything else.

One or the other or both will increase the speed.

 

[badabec]

I looked at changing the pulley on the Sanden compressor but it incorporates an electric clutch. I don't want to mess about with that, hence increasing the size of one groove of the drive pulley.

 

[mike os]

In that case machine the mating faces and bolt together the keyway will maintain alignment