Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Defrosting

I have two MGs, a TD and a MGB GT. I bought the latter for a winter car in Norway. So far we have had a very mild winter and no snow in the Oslo area.
Of course I knew that MGs do not have good heaters. In spite of that, I would like very much to have better defrosting on the front window. And I have spent so much money on this car now (rust) so I would like to spend very much money.
Any advise?
What about the Heater Upgrade I read about in the MGOC catalogue?
Raymond Wardenaer

If you will post your Email address, I will send you an article that I wrote about how to recondition the heater system of the MGB. It is too long to post here.
Stephen Strange

Stephen, this is my email address:
And thank you in advance!
Raymond Wardenaer

Moss do a heated front screen for £249.96GBP
Part number AHH8228H
Dave O'Neill2


I'd like a copy of your article if possible.


johnprewer at hotmail dot com
John Prewer

There are about a dozen aspects of the MGB heater that can contribute to poor output. An uprated matrix and fan may improve things if those items are faulty anyway, but others report little change. It's never going to be as good as a modern car, but I can't say I'm impressed with the clearing abilities, either inside or out, of my ZS180 or a pal's Seat Leon, and I used my V8 as a daily driver in all weathers for a number of years.
Paul Hunt


The standard system is fine, if in top condition.

In my experience the heater efficiency can be greatly improved by:
- thoroughly cleaning the heater matrix,
- replacing all the rotted or missing felt and foam in the housing,
- oiling the heater motor.

This will entail the removal of the complete heater unit; a job best done when there are no (grand)children within (their) hearing range. Otherwise they will learn many new, colourfull and in your case Norwegian swearwords. ;-)
The removal and fitting of the heater unit is in my top 3 of worst jobs on the MGB, together with fitting the (Tourer) windscreen and the fitting of the rear gearbox crossmember on an MGB with overdrive.

I was amazed by the improvement I got from the standard set-up when I cleaned all the thick red muck out of the matrix by copious (back)flushing and replacing the neccesary felt and foam to get better control of the airflow. Oiling the heater motor caused it to turn much faster, with obvious results on the airflow.
Willem van der Veer

I agree. The standard heater is fine when working properly. On my GT the defrosting isn't as quick as a modern car, but once the engine is warmed up it is perfectly possible to keep the car cosy inside with clear windows. However, removing and replacing the heater is a terrible job. I've done it twice and still have the scars, both physical and mental, to prove it.
Mike Howlett

On my B I've temporarily blocked off the passenger side warm air outlet (lump of plastic foam) which improved things for the driver's side and demister (I don't have a passenger for most of my journeys).
Brian Shaw

I'm with Willem and include most of that work as part of my simple thorough cleaning of the cooling/heating system

(email me if you want a copy of that)

the only problem I experienced was I couldn't get one of the screws that hold the heater box to body back in so I left it out, otherwise I can't remember it being awkward and I'm a non-mechanic

it is important to replace any worn seal both inside and outside the heater box

on my BGT I used an uprated matrix and fan motor and impeller and can't say I noticed much improvement

as well as looking at the heating you need to also look at the sealing of the car to reduce condensation such as the fresh air flap fully closing, the door and window seals etc.
Nigel Atkins

Brian, the earlier cars have hinged flaps over the heater outlets, so you can close one or both.

When I had a midget as a daily driver, I would keep both flaps closed until the windscreen had cleared.
Dave O'Neill2

"I've temporarily blocked off the passenger side warm air outlet"

Must remember that for when the Navigator is having a strop ...
Paul Hunt

Main culprit for lack of heat is the control valve on the head - the passageway blocks with corrosion and reduces water flow - I have seen this completely blocked, so it is worth undoing the two bolts and ensuring the head passage and the valve passage is clear.
Chris at Octarine Services

a good point

also that the heater valve is wired to fully open up

usually it's either wired to fully close or fully open but not both

then there's that the valve is seized or needs lubricating
Nigel Atkins

It looks to me that the front of the heater which includes the fan and motor, can be unclipped and taken away leaving the matrix accessible and even removed for a good clean, am I correct?????
I have heard how horrible it is to take the heater right out, trying to make things easy.
GJ Barker

Nope, not correct I'm afraid.
the front panel extends down below the level of the bulkhead with two further clips hidden from sight.

The heater is relatively easy to get out if all the screws come out..
I say relatively because relative the getting it back in which is an absolute pig of a job. LOL

Best of...
M McAndrew

as I put earlier I've taken a heater box out of a BGT no problems and I'm no mechanic but I never got the last screw back in because of its position, I have more tools now so I *might* get it in if I was to try now

replacing any disintegrated seals is the main thing, the one around the matrix (not always put back on properly by some), the seal to intake and check the seal under heater is solid
Nigel Atkins

Thank you for all the good response. Another member of the Norwegian MG Club invited me to his home to try and help me out. He rinsed his heater without taking it out, and his car is now warming up well. I shall also check the fan, maybe get a new one.
We still have a "warm" winter, so I hope I shall have it fixed before the cold arrives.
Happy new year to all of you from Oslo, Norway.

The image shows my GT on the Island of RÝst, north of the Polar Circle, this summer.

Raymond Wardenaer

Raymond - nice car - you got Winter wheels ?
Roger W

is doubtful that you'll need a new fan motor or impeller just take the impeller out and wash, run a little light oil on the shaft of the motor and clean the electric wire connectors

brush clean the matrix fins, make sure that the foam seal is in good condition and refitted correctly

flush the matrix until it runs clear water then back flush until the water runs clear and a final forward flush

don't forget Chris's point about the heater valve perhaps being blocked with crud and my point about the cable setting on the valve not always set to fully open the valve

also consider how sound the seal on the fresh air vent door seal is

often missed is to remove the engine drain plug and to scrape out the crud from the hole with thick wire as the coolant drains and during the flushing - lot of crud can settle and move from there

finally it's drain flush/back-flush/flush on the rad to complete the system cleaning and if you can brush the rad fins both sides all the better

I always like to drain coolant and oil when it's as hot as possible so that more crud is carried out with the hot fluid whilst draining

good luck
Nigel Atkins

Roger - yes, I have to have that in Norway during winter. I chose "the best" I could get, Nokian Hakkapelitta.
Raymond Wardenaer

Given the size of the inlet in the factory valve the cable doesn't need to fully turn the lever in order to get maximum flow, the inlet becomes the overriding restriction. WEven though mine is adjusted to fully close, hence only turn moves the lever about 3/4 of its travel, I still have to turn the control to the 'off' side of warm before it reduces the heat output.

With a bog-standard heater system at an ambient of around 8-10C/46-50F I get just over 60C/140F from the screen vent on the roadster, a bit lower at the footwell vents, but that is enough to burn our feet. The V8 is a bit lower as with bits of foam haven blown out of the vents in the past I suspect the foam seal around the matrix has perished.

Some dismantle the valve and fit several washers between the spring clip that retains the main valve, and that valve. But the pin the main valve rides on is tapered so as to give a fine control of the coolant flow at small openings, otherwise the heat tends to be either on or off.

Others replace the valve altogether, which does give a greater throughput in theory, but again you lose the fine control.
Paul Hunt


This link may be useful:

Raymong I tought Norway, like Canada, was a cold country, here it is -25C this morning with more than 1,5 meter of snow. Anyway a B-GT is quite impressive to behave in winter road. Your car look great, but aren't you afraid of ruining wire wheels? I drove a B-GT for 2 winters and spokes started to be loose.


Jean Guy Catford

Raymond - Yes Nokian has a very good reputation here.
Jean - Raymond means "it is obligatory to have Winter tyres" and yes, totally agree with you; the MGB handles better than many modern cars on Winter snow and ice - but the tyres are an important choice.
Happy New Year
Roger W

Jean Guy - many people believe Norway is a cold country, and it can be. But in summer we are sometimes the hottest in Europe, so no Polar Bears walking around. And today, + 8 C in Oslo.
Raymond Wardenaer

This thread was discussed between 29/12/2013 and 02/01/2014

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.