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MG MGB Technical - Fan belt length

The MGB Register News Letter has a comment that the fan belt supplied by Moss for the MGBGT is too short.
The part number of the belt is GCB900 and the information is that the numbers refer to the length of the belt. Apparently the belt has to be 925 in length.
Can anyone advise me on the correct length of the belt?
Mike Christie

Mike there is more than one length belt used on a B,CB up to 1975, and RB from 1976 on, I only know of these 2 sizes, I don't know if any export ones are different, what are your car details.
Andy Tilney

CB up to 1975 900mm RB from 1976 0n 950mm, forgot to add that bit to my reply
Andy Tilney

From the Parts Catalogue:

GFB103 900mm for 18G and 18GA (3-bearing) engines, and 18GB 4-bearing engines
GFB176 900mm for 18GD to GK engines, 18V engines (except as below) and those for Germany and Switzerland from 1975 on
GFB205 950mm for 18V 797/798 and 18V 846/847
GFB255 950mm for all engines and all markets from 77 on

Modern equivalents are GCB10900 and GCB10950 the first two digits indicating the width (10mm) and the last three the length.

There can be variations if the alternator and/or pulley is non-standard.

And even if all correct you may struggle to get the belt over the alternator pulley last - even with the adjuster bolt fully removed - so try putting it over the fan blades first, then the crank and alt pulleys, and only finally the water pump pulley.

Not all fan belts are equal I've found, cheaper ones can vary a bit from specification. I've found Gates to be a good make and (nearer?) to quoted size. A little more expensive but not much and worth it I feel for better longevity.

The last generic part number belt I got was only just off too long and it stunk the small shed out with its smell.
Nigel Atkins

After getting caught short when a fan belt broke on a new (to me) car just after purchase I bought a spare as soon as I got each of my MGBs - and more importantly checked it fitted! The one in the back of the roadster is 945mm ... and has now been there 30 years.

But despite my engine being a Gold Seal equivalent to an 18V582, which according to the catalogue should take a 900mm, 945 does fit.

Looking at suppliers they are all over the place. Brown & Gammons says the change from 900 to 950 was with electric fans. Moss says it was with rubber bumpers (as does the Parts Catalogue), and MGOC says it was for 76 on.

I have to agree with Nigel, and I always use Gates belts, their gogged belts are the best quality I have found, the same goes for there cam belt kits, at less than 4-00 I treat them as a service item.
Andy Tilney

My 30 year-old is 'cogged' as well, Ferodo.

Yes it is surprising how long cogged belts have been around,and yet people still use ordinary V belts.
Andy Tilney

The change in length is due to the larger diameter crank pulley. I have fitted countless 900mm belts to different cars with the small pulley and never found one too short.
Agree with Paul, always fit to the pump pulley last.
Paul Walbran

This thread was discussed between 03/04/2018 and 05/04/2018

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