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MG MGB Technical - Fan belt diameter....

Just put a new alternator in my 71 BGT (18GG engine) and noticed that the drive pulley on the new one is a tad smaller than the old original one. The alternator was already on the limit of adjustment with a 937mm dia. fan belt, which is now too big.

What size should the belt be? I seem to remember that the last time I replaced it the one listed for it at Halfords was either too big or too small (can't remember which now!). Moss, SC parts etc don't give the size in the catalogues. What size should it be?

Dave.
Dave Smith

Try a 900mm, the Unipart number I have is GCB 10900. For the later cars with the big crank pulley the part number is GCB 10950.
Rick

The first belt I purchased for my 70 MGB was two small and would not fit. When I returned to the MG parts supplier he explained that I had an oversise pully fitted to my alternator. I think it goes like this there are two belt sizes, one to fit an engine with a generator which is the larger of the two and one to suit an engine with a alternator which is smaller. Of course my engine has an alternator with a generator pulley fitted.
Hope this makes sense and helps
David Levy

dave
how did you get a 937mm belt size
there was only meant to be two sizes 900mm and 950mm
the last three numbers of the part number are the belt size in millemetres
you will have to change the pulleys over as the alternator was used on all mgs/minis/thriumphs
ste
Ste Brown

The new pully was only a fraction smaller but just enough to leave a bit too much slack. The new alternator is the Moss repro part (not quite repro enough it seems!).

I know that there are two water pump pullies, one 'dished' deeper than the other but were there different diameters too?

I've just picked up a 900mm belt from Halfords (they list both the 900 and the 950 for the later cars), they've got all sorts of sizes there if you need to experiment! I'll give the 900 a go and see if it fits as it should.
Dave.
Dave Smith

OK, the 900 fits as advertised, but I now remember what the problem was last time! With the alternator loose and fully down, the 900 belt is a sod to get on (maybe the last one I tried was a little smaller).

However, once on, the bottom bolt sits right in the middle of the adjuster slide, so it's spot on really.

The ideal solution would be to use the 913 belt I saw. That would be easy to slip on and still leave adjustment range either way. Think I'll carry one as a spare!

Anyway, thanks for the input chaps.

Dave.
Dave Smith

As far as the belt being a little too small to get on goes, I had the same problem with a Scimitar GTE many years ago. I hadn't had the car long and the belt broke on the motorway. First AA patrol had a belt that was just too small, so called up for further assistance. Eventually we had about five AA vans lined up along the motorway, all with belts too large or too small. I then had the idea that if we removed the bolt from the alternater slider bracket altogether, that would allow the alternator to move a little bit closer to the engine. It did - and the first belt went right on!

Since then I've always carried a spare belt, and checked it would go on! Sod's Law dictates that in the nigh-on 30 years since, I havn't had a breakage.

My 73 roadster spare is a Ferodo VS945, listed for Marina 1.8 with alternator, amongst many other makes.
Paul Hunt 2

Hi
It should be a 925, I have recently built a 64 roadster, also have a 72 GT, the GT belt broke I asked a dealer to suppy a belt they sent me a 900, this was to small, when I told them they said this was listed as the right belt, as is usual with a dealer,: your car must be wrong: I fitted a 925 from Halfords who also have 900 listed for an MGB, when I came to fitted a belt to my roadster again had the same problem, 925 again fitted, so are both my cars wrong.

Colin
Colin Lanning

colin
in a nut shell yes both your cars are wrong
either your pulleys are not standard or you are on a full adjustment
ive rebuilt a 63 and a 70 and they are both 900
all the books say 900 and so do allthe motor factors so they all cant be wrong
ste
Ste Brown

This thread was discussed between 27/05/2007 and 02/06/2007

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