How much does the alternator pulley size matter?

[aaronwood66]

 

I am replacing my 100A domestic alternator - the model is pretty much a straight swap. Easy enough there. However, the pulley that comes with the new alternator has a 55mm diameter, where as the old one was 80mm.

 

As it seems to be VERY difficult to swap them over (I have tried myself, although keep reading that an impact wrench is the only way) - I was wondering if I can just keep the smaller 55mm pulley? Will that affect anything?

 

 

Edited November 20, 2023 by aaronwood66

[ditchcrawler]

The smaller pulley will probably be to your advantage with better charging providing you can get the adjustment to get the belt tight

[Alan de Enfield]

As long as the belt size / shape is the same, keep the 'new' pulley.

 

If the new pullety had a different 'grove' profile it could shred the fan belt.

[dmr]

All boaty things are a compromise, and this includes the pulley size. How much it matters depends upon your engine and installation.

In general a smaller pulley is desirable as it gives more output at lower speeds and better alternator cooling.

But,

some engines, especially smaller ones, might not like a big alternator load at low speeds and in extreme cases damage can be done.

The smaller pulley has less surface area and will reduce wrap to some (variable) extent and this just might give belt slip or temp you to over-tighten the belt.

 

The green paint might suggest a Beta engine ??? and they might have chosen that pulley size for a reason (or maybe not)

 

If you do change the pulley then undoing and safely re-tightening the nut on the new alternator might be a challenge 😀

[Tony Brooks]

Do/did not Beta fit freewheel pulleys to some larger alternators? If the old one is a freewheel, then fitting a solid one might not be good for longevity.

[Jen-in-Wellies]

35 minutes ago, Tony Brooks said:

Do/did not Beta fit freewheel pulleys to some larger alternators? If the old one is a freewheel, then fitting a solid one might not be good for longevity.

Just because the old pulley is green doesn't mean it has to be Beta! 😀

What is the diameter of the engine pulley? What is the idle speed and maximum speed of the engine? Provided it doesn't exceed the maximum speed allowed for the alternator (ratio of the two pulley diameters), which is unlikely on most diesel engines, then it should be OK, all other things, like grooves are the same.

[Wanderer Vagabond]

2 hours ago, aaronwood66 said:

 

I am replacing my 100A domestic alternator - the model is pretty much a straight swap. Easy enough there. However, the pulley that comes with the new alternator has a 55mm diameter, where as the old one was 80mm.

 

As it seems to be VERY difficult to swap them over (I have tried myself, although keep reading that an impact wrench is the only way) - I was wondering if I can just keep the smaller 55mm pulley? Will that affect anything?

 

 

Just trying to think logically on this, when the alternator was manufactured I'd be surprised (stunned even) if they fitted it together using an impact wrench (I could be wrong) so is there any access to the other end of the shaft with any means of holding it whilst tightening the nut? On my own (totally different) alternator there is a hexagonal recess in the shaft into which an allen key fits to hold the shaft rigid whilst tightening the nut.

 

On the other hand, as has been suggested, just go with the pulley it came with

[dmr]

17 minutes ago, Wanderer Vagabond said:

Just trying to think logically on this, when the alternator was manufactured I'd be surprised (stunned even) if they fitted it together using an impact wrench (I could be wrong) so is there any access to the other end of the shaft with any means of holding it whilst tightening the nut? On my own (totally different) alternator there is a hexagonal recess in the shaft into which an allen key fits to hold the shaft rigid whilst tightening the nut.

 

On the other hand, as has been suggested, just go with the pulley it came with

 

Ive just done an alternator swap (Iskra) a couple of weeks ago and this required a pulley swap.

On the Iskra (Mahle) the hex recess is actually a spline, an Allen key did fit but the pulley was tight and so I wrecked the spline (on the old alternator, and have now purchase the correct spline tool). The nut was then easily undone with a Makita impact driver but Iskra specify that this must not be used for doing the nut up.

[Ex Brummie]

If your alternator drives the rev counter, it will make a difference, to the reading.

DAMHIK

Edited November 20, 2023 by Ex Brummie

[beerbeerbeerbeerbeer]

If you’re having difficulty,

any friendly local garage should be able swop the pulleys over for you,

for not a lot of money,

 

if that’s what you decide you want

Edited November 20, 2023 by beerbeerbeerbeerbeer

[MtB]

1 hour ago, Ex Brummie said:

If your alternator drives the rev counter, it will make a difference, to the reading.

DAMHIK

 

Good point. And it probably DOES drive the rev counter. 

 

 

[nicknorman]

9 hours ago, MtB said:

 

Good point. And it probably DOES drive the rev counter. 

 

 

I’d say it probably doesn’t. OP says it’s the domestic alternator which implies there is an engine alternator and it’s normally the latter that runs the tacho.

[MtB]

5 minutes ago, nicknorman said:

I’d say it probably doesn’t. OP says it’s the domestic alternator which implies there is an engine alternator and it’s normally the latter that runs the tacho.

 

Good point. And it probably DOESN'T drive the rev counter. 

 

[aaronwood66]

On 20/11/2023 at 19:36, dmr said:

The green paint might suggest a Beta engine ??? and they might have chosen that pulley size for a reason (or maybe not)

 

Well spotted from the paint - it is a Beta Marine engine!

 

Thank you, all,  for your advice - I shall keep the 'new' pulley on the new alternator.

 

23 hours ago, Ex Brummie said:

If your alternator drives the rev counter, it will make a difference, to the reading.

DAMHIK

 

It is the starter alternator on my engine that feeds the rev counter / tacho, so all good there

 

[aaronwood66]

Just to update this thread... I was hoping to keep the 'new' pulley as, like someone said, it would likely be better charging. However, the old pully had a built in spacer that means the new would would not line up the same with the belt, so I had to swap over. I found a great little place called The Starter Bay in Gloucester that did this for me in the matter of minutes. So all is well now  Thanks for everyone's input!

[Midnight]

A tale of caution
When Midnight was being fitted out I was advised by a so-called expert at Crick Boat Show that if I wanted to add a battery management system to my Beta 43 I would be better swapping the alternator for a Mastervolt system with an Alphapro regulator. As I had just bought a Mastervolt invertor and charger I could have a show discount and a free MICC - still cost £600. The Mastervolt pulley would need swapping for a polyvee but so-called expert would have one made for me - doh!

Two years later on a holiday cruise the alternator stops working. When I sent it back to so-called expert for repair I'm told guarantee had expired and it needed new diodes - £200.

 

Two years on on our 4th holiday cruise the alternator burns out and destroys the charger!

So-called expert is called and visits but after offering no solution dissappears from view. So I call Mastervolt technical. After a lot of questioning about cruising destinations, Mastervolt Technical asks me to measure the pulley then tells me it's too big in diameter which wouldn't provide enough cooling at canal speed. My 1st and 3rd cruise was mainly on rivers so the alternator revs were high enough. 2nd and 4th cruise maninly on canals resulting in the overheat and eventual destruction of the alternator and charger.

 

I call so-called expert's company but alas he is no longer employed and they won't accept any responsibility for the oversize pulley. 

 

New Mastervolt charger - £ouch!
Prestolite alternator cos' it had a small polyvee diameter pulley from my local auto-electric company - £96

 

Moral - pulley size matters!

 

 

 

 

Edited December 2, 2023 by Midnight