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MG MGB Technical - Fan belt tension MGB

Hello,

Please can someone tell me what the tension of the mgb fan belt should be?

The haynes manual states two different tensions (or deflections) but not sure which one applies to my engine:

My engine is an 18GG (from a 1971 mgb roadster)

Many thanks
Alex Davies

Prepare to be confused.

The Leyland Workshop Manual in the Cooling section states for both dynamo and alternator systems it should be possible to move the belt laterally in the middle of its longest run by no less than one inch! Is that in one direction which seems a helluva lot? Or a total in both directions i.e. 1/2" each way which seems more reasonable? The Electrical section just says to "remove any undue slackness". Haynes in its cooling section says there should be a sideways deflection in the middle of the longest run of 1/2" on 'early' models, and 1/4" under an applied load of 7.5 to 8.2 lbf. Both 'lateral' and 'sideways' imply to me to be pushing or pulling it towards the front or rear of the car, and not towards or away from the other side of the loop which is always the way I have done it. Also is it in one direction or both? It seems likely that when using a spring-balace to apply the stated force for later models it would be in one direction only. The distance for that is half that for 'early' models, so do the early ones have twice the play? Or is theirs in terms of total deflection in both directions? And for that matter what is the definition of 'early' and 'late'? (Dynamo vs Alternator?). FWIW my 1967 Mini Workshop Manual also quotes an inch in the longest run, but an 80s Metro Manual quotes a much smaller deflection of 3/16" and in the shortest run i.e. between alternator and water pump pulleys. It also shows it being pressed down towards the crank pulley i.e. not sideways, and only in one direction from rest. Interestingly it also quotes a torque wrench value of 11 to 11.5 lbf when applied to the alternator pulley nut to produce slip, which is perhaps the most useful approach.

The Lucas Fault Diagnosis Manual has a drawing, which is repeated with the same deflection distance for dynamos and alternators, and shows 1/2" to 3/4" (13-19mm) in one direction, and towards the opposite side of the loop.

For 40 years I've checked my belts by seeing how far the middle of the longest run can be deflected from its 'at rest' position to or from the other side of the loop with light-ish finger pressure, and set it to about 1/4"-1/2". I've never had belt squealing in 40 years, and only had to replace a water pump once each on two cars quite soon after coming to me after many miles with other owners.
PaulH Solihull

I've always done what Paul does - on the B and any car I've owned, though I'd probably say 'firm' for the finger pressure. It's probably one of those 'the more you do it the more confident you get' tasks.
Roger T

This thread was discussed between 13/11/2011 and 14/11/2011

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