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MG MGB Technical - harmonic balancer is smaller

I've just bought a new harmonic balancer for my 1969 MGB GT (ex USA but now in Australia). The new balancer is smaller than the one it replaces, having a diameter of only 130mm. Should I be worried that this will lead to undercharging via the alternator?
N Reid

As far as i know there were two (Maybe more?) different diameters of crank pulley. You can easily tell which yours should have by looking at the front cover. The timing marks had to be moved to accommodate the larger pulley. All things being equal it will affect the speed of the alternator and the water pump, more significant at low engine speeds. A bit of pie x diameter and working out the ratio!!!
Allan Reeling

130mm is the correct dia for a 69B. Some of the later USA cars had a slightly larger harmonic balance and your car may have had one of those fitted. I cant see a problem. Denis
Denis4

I've just measured a later type pulley at 6", i.e., 150mm.

If the front "timing" cover has the relocated timing marks you may have a later engine. What's the number?
Allan Reeling

The early covers and pulleys/dampers had the timing marks on the bottom.
They then moved the marks to the top/side and changed the pulley ro suit.
Both of these are the same physical size and easy to mix.
The last type is the very late pulley which has a step on it and is larger than the previous 2, but which has the timing marks in the same place as the 2nd type.

There were also different sizes of water pump pulley and dynamo/alternator pulleys to suit. You can try and source the correct pulley. Change the other pulleys to suit. Or just ignore it and hope for the best, but you'll end up with a weird off size of fanbelt that you'll have to source and measure on your own. The 'usual suspect' suppliers only sell the early/late types usually. None of which will fit if you mix pulleys up.

Unless you really push your engine, I can't see it being much of a problem. Your water pump will turn slightly slower, as will your charging system. You might notice this on a marginal cooling system or if you used a dynamo. Doubt you'll spot the difference.
Roadwarrior

Roadwarrior,
"but the timing marks were in the same place", not so. To accommodate the larger diameter pulley the marks were positioned closer to the edge of the cover, but still at 10 o'clock...ish!
Allan Reeling

Ah ha! A 25 year old mystery solved - probably...
Ever since I got it, I always wondered why my 18GG engine didn't take the fan belt recommended for it (910mm dia, if remember correctly?). I had assumed it was something to do with the alternator or water pump, never occurred to me it might be the bottom pulley! It's on it's third rebuild, so it might well have been changed at some point.
Dave Smith GT

Fan belts are tricky, you should always trial fit a spare to make sure it does! Over 40 years ago I had a Scimitar that lost a belt on the M6, and ended up for four AA patrols and two spares vans on the hard shoulder trying to find one that fitted. In the end it struck me that the first belt they tried, which was fractionally too small, might go on if we removed the bolt from the adjuster slot altogether, and so it proved! I've always carried a spare since, and so far have never needed it of course.
Paul Hunt

Paul,
Pair of stockings was the get you home in the 60's!!
Allan Reeling

"Pair of stockings was the get you home in the 60's!!"
I always thought that was just a ploy to get a girl to take her nylons off in front of me - especially if she was wearing panty hose ;-)) Cheers - Dave
D.W. DuBois

Yes Allan I didn't mean identical front covers, I just meant the same relative location as per timing the engine. You can make them fit and they can work together easily. The others obviously not so.
Roadwarrior

"Pair of stockings was the get you home in the 60's!!"

I don't think I or any of the AA patrol men would have admitted to wearing those :o)
Paul Hunt

If you were a runner, as I was, tights were a good ploy running in winter, you just hoped you didn't see anyone you knew!!
Allan Reeling

Oh dear me I could never run even at school, always came second to last no matter how hard I tried. Why second to last? My pal was determined to come last to be sure he wasn't picked for sports day.
Paul Hunt

This thread was discussed between 04/05/2015 and 13/05/2015

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