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MG MGB Technical - Smaller Alternator pulley

I just temporarily replaced my failed alternator (rebuilt coming from Bob at Brit Tek (800) 255-5883)with a used one that I got from a friend. The pulley is much smasller than the one on the old alternator.

My car is a '73 'BGT. I know that the 77-80 pulleys are a different part number and I understand that they may be smaller.

This replacement alternator also has a disk in front of the alternator fan that completely covers it as you look from the front.

My concern is that this might not even be an MG fan. Are there dire consequences to over revving an alternator (because of the small pulley)? What does the 77-80 pulley look like? What's it's diameter?

There are no numbers anywhere on this alternator that I can find.

By the way, my voltmeter was invaluable! I converted my square oil pressure gauge to a voltmeter (looks just like Smith's) and replaced my temp gauge with the Smith's dual oil pressure/water temp gauge. If anyone wants to see the gauge, e-mail me.

Thanks in advance!

Steve
Steve Lipofsky

A smaller pulley will make the alt spin faster for a given belt speed i.e. rpm. Whether this causes a problem or not depends on how much smaller it is and how fast the engine is revving, the MGB is not a high-revving unit, of course. But the strictly correct answer to your question is: yes, over-revving the alt may physically damage it, and you may get charging problems (i.e. brush bounce) before actual damage occurs.
Paul Hunt

I wonder if I have a later model alternator. If so, should it have a smaller pulley? Have you heard of the disk on the front of the alt fan as I describe above?
Steve Lipofsky

Steve; I have one like you describe, same type fan and small pulley. I was told it is an MGB 18ACR 43 amp alternator. If you compare it to your old alternator the external part of stator is wider in the 43 amp than in the 34 amp unit. I have a 43 amp on my 68 GT but the pulley was transferred from the old alternator.
The new alternator came without a pulley. There was a pulley change around 77. Haynes states the 18ACR output is 43 amps at 6000, but they do not give the speed for 34 amps in the 16ACR units. I have four alternators and three different pulley sizes. Are they MG alternators, I don't really know? They all fit my cars. FWIW, Clifton
Clifton Gordon

Steve,
The pully might be from a MGC.They used a smaller pully and fan than the MGB did.If it is a C set up there are hard to find in the U.S..
Tony
Tony

Latter MGB's (don't know the break year) used a larger crank and water pump pulley- I'm assuming the alternator pulley would also be increase in size, making a large diameter pulley a later rather than earlier model
Greg Fast

The 18ACR alternator uses a smaller pully regardless of the year of the car. It needs the higher RPM provided by the smaller pully to keep the current tevel, thus the voltage up at idle. Cheers - Dave
David DuBois

This thread was discussed between 25/10/2003 and 26/10/2003

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