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MG MGB Technical - 1977-80 Radiator Cooling Fans

I have just finished installing new electric motors and fans on the 1979 mgb roadster I have been working on for the last thirteen months. The next to last thing that needed to be done before taking it in for emissions testing prior to registration.

Both of the fan motors were non-functional. I purchased the vehicle seven years ago with a seized engine. It had been sitting for several years before I purchased it. Hence, most systems have been removed, cleaned, inspected and either rebuilt or replaced. The motors I decided to replace. The fans are in better condition than most and could be used, but are seized on the shafts of the old motors.

Ordered two new motors and three new fan blades from Moss last week. The screw for the fan was listed as a separate item. The customer service rep told me that the screw is included with the fan blade. He was correct on two of the fan blades. The third one came without the screw. Blades seem to be similar/identical to the factory blades. The center hole in the fan boss is slightly oversized when compared to the diameter of the motor shaft. But, when the set screw is tightened they work well and seem well balanced. At least, I did not notice any obvious out of balance indications. Fans are marked "Made in the UK".

Fan motors are also marked "Made in the UK" on the box. A few problems with the motors. First, the illustration in the Moss catalog shows the plastic connector as being part of the motor assembly. It is not and you need to re-use your old connector after removing it from a non-working motor. The supplied motors have two of the "push through?" style ends which mate into the female connector in the wiring harness. The wires are black and green and it is easy to match up to the black and green/black wires in the harness. The pointed end of the connector receives the green wire, going to the green/black wire, and the square end of the connector receives the black wire going to the black wire in the harness. Both motors operated well, and very smoothly, when tested in the car.

There is, however, one problem which cropped up in testing the first motor. The diameter of the original motor case is 3.000" while the diameter of the new motor is 2.975", a small difference of only .025". But, the mounts for the motors are designed to work with the 3" motor cases and, when fully tightened, do not secure the motor in place. When I tested the first motor, it flew forwards, causing the fan blade to strike the bonnet lock platform. No damage done, but, there may well have been if I had been slower at disconnecting the jumper wire. That was when I measured the motor bodies and discovered the different diameters. When fully tightened, the motor mountings will allow the new motors to slide front and rearwards.

The solution to this problem was to use some .025" brass shim stock on the lower portion of the motor mounting. This was cut to the width of the mounting and just slightly shorter than the distance from the separation point of the upper and lower halves. It was bent, by hand, into a roughly semi-circular shape, the halves of the mounts opened until the bolts were just engaging the captive nuts on the upper half, the motor inserted, and the nuts tightened down. This allowed the mounts to hold the slightly smaller motor bodies firmly and, for right now, seems to be working well.

The thermocouple in the radiator was not used. When I had the radiator re-cored, I had the hole patched over. I have seen too many blow out of the radiator to trust them. Instead, I used the fan control system sold by Brit-Tek which has a rheostat control to adjust when the fans turn on and a probe which lies just behind the radiator as its controller. The only problem with that system is that the sheet metal screws suppled to mount the bracket holding the rheostat are larger than the holes in the bracket. Two of the holes in the mounting bracket need to me opened up to just slightly larger than the major diameter of the sheet metal screws.

The unit was mounted on the radiator support panel on the left hand side. Previously, I have mounted them on the left hand inner fender, but this makes for a cleaner looking installation. I also did not want to obscure the factory decal on the left inner fender which this car still has in place.

Hope this information will be of use to others who need to change out fans and motors.

Les Bengtson

Your experience is interesting. I have not had to deal with your fan motor problem as my car is Feb '74. However, in similar situations I have lined clamps with black loom tape so as not to brake the paint layer when tightened. That would have been an alternative to your brass shim.

Mind you we get alot more rust here so my priorities are not the same.

I have an aftermarket radiator fan. The switching solution I have used is to get a threaded brass thermo switch and solder a suitable brass boss into the top tank. (I soldered a drain in the bottom tank while I was at it.) I just hope I bought a switch with the correct operating temperature.
David Witham

This thread was discussed on 18/04/2007

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