Chicagoland MG Club:Photos
MGB Heater Rebuild and Upgrade
Oak Park, IL - March 9, 2003

Corrosion indicating location of heater core leak, which was dribbling on the passenger's feet.

Photos and web page courtesy of Barney Gaylord
Click on small images for larger images. - Larger pics average 38KB.

The subject MGB belongs to Wade Keene, and we had a few more club members show up to assist on a cold winter morning. Thanks to a well insulated garage and a torpedo heater we were warm enough. Start by draining the coolant and save it for reuse if it's not too old. The heater valve had been closed and the return hose clamped off to stop the flow to the heater where the core was leaking. Take a good look at the original heater motor. The replacement motor you see later will be a little longer (and a little more powerful).

Remove screws securing heater box to shelf and firewall. One screw in particular below the blower is a bear for access, requiring a very short right angle drive. In a pinch maybe a long nose ViceGrip will get it loose, and if you're lucky you might unscrew it the rest of the way with your fingers.

Disconnect the defrost hoses under the dash, which may require remove of a few other parts, like under dash cover panels. Having the center console in the way doesn't help, and if there was a radio there it could be even tighter. Eventually you have to dig in deep enough to unplug the stub pipes from the back of the heater box, which happens to be inside of a steel body pocket. Hope the kids didn't hear the "funny" language, and patch up the skinned knuckles later. Then you get to pull the heater box out the top, maybe.

Nope, not yet. Back inside to disconnect two control cables. The one under the box isn't accessible until the box is lifted. If you didn't disconnect the back end of the cable first you get a nasty kink in the front end that has to be straightened out before reassembly.

Once the box is out the job gets easier. Pull off several clips, and lift off the side of the box with the blower attached. Or you can remove three screws and remove the blower first (or last). Notice that the squirrel cage fan comes out through the hole in the side of the box, but this may not be the situation later. Lift out the heater core, and remove the foam rubber packing piece. Compare the original heater core (bottom) and the new high capacity core (top). Notice the original core has five water tubes in the first row, and the next row of five tubes is staggered in between. The new high capacity core has eight water tubes in each row, giving up to 60% increase in flow capacity. Also compare the new high capacity fan. Notice the new squirrel cage has a few more blades. It is also larger diameter, so it will have a higher outside surface speed when turning at the original speed. And we hope that the new uprated motor will turn it a little faster as well. From this point of view the fan turns in a counter-clockwise direction, so the air escapes from the outer edge of the blade traveling in the forward direction at a higher velocity than the surface speed of the fan. If you run this fan backwards it would move a LOT less air, so be careful about the way the wires are attached when finished. The later model MGB fan motor will run backward if the wires are switched.

Notice again that the new fan is larger than the original one, and it won't fit through the hole in the side of the box. No, you don't have to enlarge the hole. Just place the fan inside the box before installing the cover, then install the motor from the opposite side, and tighten the set screw on the motor shaft once it's all together. This raises another interesting point. For future servicing, the heater motor cannot be removed without first removing the entire heater assembly from the car. The replacement motor came with long wires and a separate mounting plate which you have to screw to the motor. Presumably this is a universal motor, and the separate mounting plate allows it to be used in various applications. That should keep the cost down (yeah, sure). When the motor is in place, and the fan is pressed full down against the shoulder on the motor shaft, and the cover is installed, we found that the fan was rubbing slightly on the cover. I was wondering if there should be another gasket between the motor mount plate and the box, but there was no gasket and no mention of any gasket with the parts. So a little creative sheet metal bending was in order to stop the rubbing.

Time for reassembly. Wrap the new heater core with new foam packing and place it in the box. Squeeze the side panel into place and apply the perimeter clips. Then run a bead of silicone adhesive around the air inlet port, set the inlet screen in place, and stick on the foam rubber packing gasket.

Notice the thick rubber pad which will attach to the back of the heater at the bottom. This is to seal the outlet side of the box against the body bulkhead in the foot well. It also provides the mounting anchor point for the stub pipes to connect the defroster hoses. This is a piece of high density foam rubber, quite stiff, and before we were finished we would be wishing that it was a whole lot softer. Run the damper control cable through the hole in the rubber pad, attach the inner cable to the damper arm, anchor the outer jacket to the box, and then glue the rubber pad in place with some more silicone adhesive.

Just a few more gaskets to install. Glue three thin foam rubber strips to the underside of the mounting flanges on the box. One more piece not shown in these pictures is a thicker foam rubber strip to be glued to the back of the box just above the flanges to seal the fourth side of the box against the vertical bulkhead. As you set the heater box in place, run the damper control cable through the lower bulkhead hole into the footwell where it will later be connected to the control on the dash. Then comes the unexpected tough chore, stuffing the heater assembly down into the hole in the shelf. The problem here is that thick rubber pad on the back of the box at the bottom. This pad was so stiff as to dearly resist compression to the point of making it very difficult to press the box back far enough to drop into the well. But four hands, five minutes and a few choice words later the deed was done. Somehow we were lucky and didn't kink the control cable, as it would still operate freely after the fight. Then back to crawling around under the dash to insert the stub pipes into that now familiar block of rubber, reconnect air elbows, defroster vent tubes and control cables. Check that the dash controls actually work before final installation of under dash panels and anything else you may have dislocated earlier in the job.

There was another short delay when it seemed appropriate to extract the oil pressure gauge signal pipe from behind the heater box. That may not have been necessary, but the pipe looks better running around the large end of the box rather than behind it. There was also a little tab straightening at the top before the last screw could be installed. That tab had fallen victim to the four handed muscle used to push the box into the well against its will. Almost finished. Install new heater hoses, and pour the coolant back into the radiator. Cut the motor wires to length, strip the ends, solder on the two bullet connectors and plug them into the wiring harness.

So did it work? Of course it works. We never quit until we have a success story. But we are still waiting for some more cold weather so we can get a report on how much heat the newly revised heater will put out. With the upgraded water core, the larger fan and larger motor, we have high expectations. Credit most of the work to Wade Keene, Terry Gaskin and Cowboy Dave Bralich.

P.S. - Sept 03, 2003
Part numbers, brought to you by popular demand. Wade Keene reports:
"I did get the parts from the MGOC. The core is part number ARO1994HE, called Heater matrix uprated on the invoice, and cost me 32GBP; the fan and motor kit was called Heater motor+fan uprated, part number 37H7913HE, and cost me 51GBP. If anyone has any questions about the heater stuff they can just contact me".
Wade Keene,

P.P.S. - Oct 09, 2003
Barrie Robinson reports:
I have a way of doing it thanks in part to Mike Winney. I looked at the photo you marked but my cable kinked up when I got further and butted up against the bulkhead (see photo) No way would it go in without a BIG kink and thus useless. The picture you marked shows there is a kink in the cable !!!

Here's how I eventually did it:
1) Checked that all nuts were cleaned out (paint etc) and correct bolts available
2) Stuck round soft foam seal on with contact glue.
3) Stuck heater-to-body seal on with Dum-Dum.
4) Ground down top of top bracket (see photo)
5) Cut out 1/4 strip where cable was fouling (see photo)
6) Fitted cable to damper/flap
7) Threaded cable through and made sure it was in the "cut"
8) Lowered heater box in VOILA !

trimmed bracketcut for cable
Trimmed bracket. Cut for cable.
(These photos courtesy of Barrie Robinson)

It all works perfectly now - Knob turns easily !!!!
Barrie Robinson

Then Barney Gaylord noticed these notes in the Haynes MGB shop manual:
For the early models:
"Lift out the heater assembly".
"Take care to re-route the air control cable correctly".

For the later models (GHN5 and HGD5 models from vehicle No 410002):
(That would be the start of the 1977 model year in June 1976)
".... withdraw the heater unit from the car".
"Feed the temperature control cable through the bulkhead slot before fitting the heater unit".

This sounds like the factory must have wised up after a while (only 14 years) and added a slot in the 1977 and later models, similar to the cut shown in Barrie's picture. A request to the mgs e-mail list for pictures of the heater well area of a 1977 or later MGB brought back the following.

P.P.P.S. - Oct 11, 2003
Chuck Renner sends these pictures of his 1979 MGB:
Notice starting with the 1977 model year the bulkhead holes are now both round, and there is a wide slot exactly in the location where the slot was cut in the photo above. This shows how and where to cut the slot in the earlier cars to make installation of the heater much easier.
Also notice the deep indent depression in the later bulkhead, leaving a rim around the outside to be in contact with the thick rubber pad. This will likely make it much easier to compress the thick rubber pad while inserting the heater box into the well.
1977 and later heater well1977 and later heater well close up
1977 and later MGB heater well showing the factory slot in the bulkhead for passage of the control cable.
(These photos courtesy of Chuck Renner)

distriburion flap removed Addendum 6/13/2007:

At 06:47 PM 6/13/2007 +1000, David Hoskins wrote:
"I have attached a photo of the distribution cable to the heater, viewed from the driver's footwell - difficult to see and reach but at least there is no risk of kinking the cable and can be done up using two 1/4 drive sockets in a screwdriver handle driver. On LHD cars, it would be even easier as you do not have the pedals to hit your head on! One of those small strap-on l.e.d. torches is also useful in this confined space."

©2003 Chicagoland MG Club, All rights reserved.