Morris Minor Owners Club

Does anyone know if it is possible to rebuild/uprate a C39 dynamo to C40 output?

From what I can tell, from having them side by side, the casings look near enough the same (from when I rebuilt me C39 and my C40) so it should be potentially possible to transplant C40 internals into a C39 casing?

I doubt there is any significant difference! The output is all down to how hard you push the field windings.. but with the risk then of frying the battery. What load do you need to balance ? If your motoring is all 'about town'/short runs at lowish speed (say never over 60 mph) - then you could fit a smaller pulley and rev the dynamo faster. This would be fine if you don't use high revs and would boost charging in town conditions.

As the pulleys are B section and the fan blades are on the dynamo pulley which has a different PCD, I am somewhat restricted in my choice of pulleys but the swapping of internals may be possible

To no electrical advantage I suspect. As far as I can see - the difference is in the mounting arrangement for the drive pulley.... so if you want to keep the deeper B pulley - you need to stick with the C39...

A C39 dynamo produces 17 amps, a C40 produces 22 amps

Perhaps simply due to spinning faster!!

Only 200 rpm faster

What would you do with the extra amps....?

I would be able to tow the trailer at night without the ammeter showing a discharge

A trailer uses hardly any extra current - for it's few extra bulbs, which can only add up to an amp - surely ?? Easy answer is to fit the trailer with LED bulbs ! But I suggest if you tweak up the Reg box slightly - it should be able to cope.

A small trailer with basic lights IE a light board will draw 6.5 amps with the side ligts on brake lights on (at junction) and 1 indicator. If the trailer has marker lights etc then its more

Indeed- but brake lights and indicator are only on for seconds !! And unlikely the dynamo would be charging then anyway - with engine idling.....

That is why I need the extra amps when it is charging. It also runs at a small discharge on main beam with driving lights on.

Don't bother with 'driving lights' ! Just get decent halogen headlights!! Simples.....

I already have Wipac halogens, the driving lights are Lucas Silver Sabres wired through the dip switch which give a good spread of light on twisty country roads. See signature photo

I didn't realise you went rallying!

The roads are very twisty round where I live and one slip and you are in a 12 foot ditch!! Lincs is the same, you definately want good wide lighting.

Alternator....

On a Series MM? No way