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Thread: Wrong Dynamo Pulley
Posted 19 March 2017 at 20:53:53 UK time
Lindsay Sampford, Cambridgeshire, United Kingdom

I had just fitted the spare engine I had rebuilt but on starting it, I had a horrible clattering noise from the front of the engine. Initially, I thought I'd left a spanner in the works or that there was a problem with the timing chain, then I noticed the bent blades on the dynamo pulley fan. The cause of the bent fan blades was they're coming into contact with the dynamo adjustment bracket (see picture). But this dynamo was from my original engine, why had I not had this problem before?


Posted 19 March 2017 at 21:27:02 UK time
Willem van der Veer, Y5888, Netherlands

Could it be that the bracket should be on the other side of the flange?

Posted 19 March 2017 at 21:28:15 UK time
Lindsay Sampford, Cambridgeshire, United Kingdom

Here is the reason, the previous owner had done this neat modification to prevent pulley fan adjustment bracket contact (see picture)!


Posted 19 March 2017 at 21:36:12 UK time
Lindsay Sampford, Cambridgeshire, United Kingdom

Managed to find the correct fan and pulley in my box of bits and this is the way it looks now (see picture).

Willem, no, it doesn't work that way. It would be impossible to slacken or tighten the adjusting bracket nut. The previous owner had fitted the wrong pulley and bodged the adjusting bracket to compensate.

Posted 19 March 2017 at 23:22:54 UK time
Lindsay Sampford, Cambridgeshire, United Kingdom



Posted 20 March 2017 at 09:29:26 UK time
Colyn Firth, South Yorkshire, United Kingdom

Glad to hear that the problem was high on noise but low on damage Lindsay.
I am usually not so lucky :-)

Have you had chance to run the car yet? And if so, how did it go?


Posted 20 March 2017 at 09:56:37 UK time
Colin Manley, Surrey, United Kingdom

I am confused. How come this did not evidence itself when originally in use on the original engine?


Posted 20 March 2017 at 23:37:04 UK time
Lindsay Sampford, Cambridgeshire, United Kingdom

The original engine had the 'modified' adjustment strap. I hadn't noticed the modification, as my only interest in the old engine was removing the parts I would need for the 'new' engine. The new engine came with a normal, un-modified adjustment strap, so the problem manifested itself as soon as the engine turned with the fanbelt fitted.
What puzzles me is what that dynamo pulley was meant to be fitted to. The pulley is less than 3" in diameter, causing it to turn faster than it should with the MGA crankshaft pulley, whilst the fan diameter is 5", which would make it foul in any situation using the same adjusting strap arrangement as the MGA.

Posted 21 March 2017 at 03:28:37 UK time
barneymg, USA at large, barneymg@mgaguru.com

There were originally to different generators. First one had slightly smaller end plate and used the smaller fan. Second one using the larger fan has a larger end plate with the link bolt hole farther out so the fan doesn't hit the bolt. You apparently have the earlier generator with the he closer bolt where the larger fan will hit it. They are all original pats, just matched up at a different point in time.

Posted 21 March 2017 at 06:36:43 UK time
Lindsay Sampford, Cambridgeshire, United Kingdom

Thank you for that Barney. I thought you would have the answer but I wasn't able to find it on your website. I have never come across the problem before, and I have been playing with A and B series engines for over 40 years, but I will be on the lookout for it from now on!

Posted 21 March 2017 at 10:24:35 UK time
Lindsay Sampford, Cambridgeshire, United Kingdom

I was trying to think how my MGA, in its earlier life, might have acquired an early dynamo with a late pulley and came up this theory: As the pulley diameter is only 3", I would imagine it came from an A series car, maybe a late Morris Minor or Mini. The dynamo, complete with its pulley, would have fitted the MGA without a problem, other than very little scope for belt adjustment thanks to the small pulley diameter. If then, after a period of time, the dynamo was replaced with an earlier unit and the pulley was transferred from the later unit (re-con units normally come without a pulley), the fan would foul the adjustment bracket, calling for some drastic metal removal from the bracket to make it work. Just a theory but quite likely!

Posted 21 March 2017 at 10:44:10 UK time
Steve Gyles, Hampshire, United Kingdom


Perhaps the previous owner thought that fitting the smaller pulley would spin the dynamo faster and give him more leccie.


Posted 21 March 2017 at 13:03:58 UK time
Lindsay Sampford, Cambridgeshire, United Kingdom

You could be right Steve. I always wondered why I never saw the IGN light on, as you would expect, at a tickover of 750rpm. It comes on now! The downside of running the dynamo faster than designed, is the strain it puts on the windings as they try to climb off the armature due the increase in centrifugal force!

Posted 21 March 2017 at 15:56:56 UK time
barneymg, USA at large, barneymg@mgaguru.com

It is on the website,just have to poke around a bit to find it:

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