The Mk2 Jag heater in original form is the subject of much (partially justified in my opinion) derision. Many British cars from the same era suffered similar afflictions. True it didn’t produce much de-misting or rapid heating effect, but it did get the job done — eventually. There’s no doubt that the performance is poor by modern standards, so anything that can be done to improve things is worthwhile.

Anyway, I’ve had a heater upgrade kit sitting in my workshop for several years now, which I purchased from the Daimler and Jaguar Spare Parts Club. Clayton Classics produce this kit in the UK; I assume they still provide it, although it’s not mentioned specifically on their website. Lin Rose used the same heater upgrade, check out his site for additional detail including an article from the January 2010 edition of Jaguar World on this upgrade. The Kriss Motors Mk2 Restoration details a more sophisticated approach.
For me, heater performance isn’t a top priority, so I’m happy with the Clayton Classics upgrade.

Heater upgrade

The upgrades to the heater are outlined on Clayton Classics website:

  • A new copper heater core
  • A more powerful fan motor coupled with a larger diameter fan wheel
  • The inner air flap is sealed permanently shut
heater airflow, the inner flap is shown directing air to the Hot or Cold position

In the Jaguar World article they state the reason for sealing the inner flap shut is that, over time, the seals on the flap deteriorate and the warm air is diluted by cold leaking past the seals. I also wonder if the system of cables and rods operating the heater valve and vent cause problems as well. I’m certainly no expert, but I’ll see if it’s possible to get this all working properly.

Of course, another option is to have the original heater core reconditioned … hmm sounds too simple.

Overall condition

Here are some images of the heater before I removed it from the engine bay.

Whatever the final approach, the first step is to assess the current condition of the heater unit and pull it apart — always the quickest part of anything to do with classic cars!

Heater case – bulkhead side and squirrel cage fan
Connections to inner and outer flaps (hot/cold and screen/car)
Front view (inverted)
Side view – flap return springs


I started the disassembly process by removing all the linkages to the heater valve and control flaps.

Water valve — The water valve is attached to the heater body with two machine screws, the valve was stuck in place (hardly surprising after nearly 60 years!) but it came out as I rotated it back and forth. Later I realised that the inlet pipe to the heater core had come out with the valve.

Water valve
Water valve complete with pipe (uppermost) which should still be attached to the heater core

Fan motor assembly — First I removed the nut holding the squirrel cage fan onto the fan motor, I needed to lock the cage with a screwdriver to get the nut moving. The motor is held in place by three set screws accessible from the inside of the heater body. Three nuts attach the motor mounting points to these screws along with two rubber isolating washers on each screw. There is a separate bracket to which a wire wound resistor, for the low fan speed setting, is mounted.

Fan motor mounting bracket, quite a neat and robust arrangement
Fan motor removed with fan

Heater core removal — Eleven self-tapping screws hold the lid onto the heater case, these unscrewed easily. I then carefully prised the lid from the case to expose the heater core. I wasn’t quite sure what to expect when I first peeked inside, but the interior was quite clean, with the remains of only a few dead insects.

Case lid screws
Case lid screws
Heater core

The heater core slots into position and was easy to remove, you can see in the images below where the inlet pipe pulled out of the old core.

Core removed, not too dirty inside
Pipe missing from core, insect remains
Underside of core
Case interior

I then gave the now stripped down case a good blow out with compressed air to make things a bit cleaner for the required modifications to the case.

Case interior after a good dose of compressed air
Bottom view after cleaning with air
New heater core
New heater core

Heater case modifications

The kit came with a printed set of instructions, and here is a colour PDF version which I downloaded from Lin Rose’s excellent site. First, I tackled the case modifications allowing the larger fan and more powerful motor to be fitted.

Enlarge the air intake — The intake is enlarged to 120mm diameter to accommodate the larger fan. I masked the air intake with tape and marked out the cut line needed for the increase in diameter. I used a jigsaw to cut along this line.

Masked off air inlet
New cut line marked for larger fan

Up-rated fan motor — The new motor comes with a plastic mounting bracket and a steel mounting plate, three 12.7mm holes are drilled through the heater body for the new mounting plate. I spent a bit of time lining things up so that the holes weren’t too close to the very edge of the case; this meant that the motor shaft wasn’t precisely centred in its hole, but I made sure that the fan fitted ok without fouling the side of the case before drilling the holes. I ran into one issue which was that the motor mount supplied in my kit didn’t have the edge cut-off, and therefore wouldn’t fit as both the mounting plate and the motor mount need to sit right up beside the edge of the heater box.

New motor mounting bracket and plate, note the bracket doesn’t have the edge removed

I used the mounting plate as a template to draw a cut line on the base of the motor mount. I assume my kit had the wrong part in it. I asked Clayton Classics about this; but apart from confirming that cutting the edge off the motor mount was the correct course of action didn’t clarify if this was the wrong part, and I didn’t follow up.

Holes marked for motor plate and mount
Edge to be removed from mounting bracket. One hole, like the plate, is very close to the edge of the bracket
New motor, modified bracket and fan

10mm access hole for fan grub screw — This hole is 20mm below the fan mounting face on the case, I used a stepped drill, a new toy which made a nice clean cut.

Motor mount — The motor mount and mounting plate mount onto the heater body with three number 6 x 3/8″ self-tapping screws provided in the kit. The motor is bolted directly to the mount. When it came to fitting the fan, I found it simpler to position it in the enclosure first and then loosely fit the motor shaft to the fan before tightening down the mounting screws.

Fan motor mounted on case
Fan fitted in enlarged air inlet
10mm hole for fan grub screw

Resistor — Clayton Classics provide a new 2.2 ohm resistor for the low-speed setting which can be mounted anywhere on the case by the motor. I left this step to last.

Resistor for low speed setting

I guess you could say that a resistor is a pretty crude way of providing the low fan speed option, but it has the benefit of being simple and effective.

Bench test — Once completed, I gave the unit a quick bench test. At full speed, the motor and fan seem to provide plenty of air.

Bench test – fan whizzing happily around

Preparation for painting — I stripped the paint and rust from the case in my blasting cabinet, well the majority of it anyway. I haven’t quite got the cabinet working as I would like yet, so this wasn’t entirely successful. To keep things moving, I took the case out to The Surgery for final preparation and painting.

Heater Con’t – January 2021

After a bit of a hiatus working on other aspects of the Jag, I recommenced work on the heater.

As mentioned above, The Surgery completed the final preparation of the case for painting. I was pleased with the results and the quality of the matt black epoxy finish.

Case prepped for painting
Case prepped for painting
Lid prepped for painting
Painted case – would look better if i had wiped the dust off before taking picture!

Once I had the painted heater box back home, I started the final assembly of the heater components, beginning with the motor and mounting hardware. I used three 4mm nuts, bolts and washers rather than the #6 sheet metal screws supplied in the kit. I had to drill the holes out a bit, and it took some fiddling around to fit, but I felt the motor was quite heavy for the sheet metal screws, although they are simpler to install. I applied some Loctite to the threads as the downside of this modification is the inability to tighten things up if they loosen off.

Motor mounting bracket and plate with 4mm bolts
Motor fitted
Fan fitted so that it sits “just” inside the enlarged hole in the case

Clayton’s instructions state that self-adhesive foam is applied to the outer flap and around the heater core, and the inner flap is sealed shut. I decided to stick with the original design, with both flaps operational. My reasoning is that the intended use of the Jag is in (hopefully) warmer weather and therefore I want the maximum flow of fresh air into the cabin.

I purchased some self-adhesive 3mm Neoprene foam which I hope will provide a good seal over time. Getting the foam into position on both sides of the inner flap was pretty tricky, I practised with some paper templates to get the hang of things, before applying the self-adhesive foam. Once that was done placing the foam on the inner side of the outer flap was very straightforward.


Self adhesive foam fitted to both sides of inner flap, and inner side of the outer flap
Outer flap closed

The next step was to fit the heater matrix; as per the instructions from Clayton’s, I applied the foam supplied in the kit to the outside of the matrix. As there wasn’t enough to go completely around the matrix, I used some of the black Neoprene foam I purchased for the top. The foam cushions the matrix against the heater casing and lid. I tried to fit the matrix with 1.5mm foam wrapped around it, but the fit was too tight, so I used the foam supplied in the kit which had more “give”.

Foam wrapped around bottom and sides of heater matrix
Heater matrix fitted in case. Rolls of neoprene foam.
Neoprene foam on the top of the heater matrix, I needed to add a bit due to poor measuring

I then fitted the lid onto the heater case with 11 x 6G sheet metal screws.

Heater lid in place – starting to look like the real deal!

The next step was to fit a new 0 ring and water valve onto its bracket on the heater lid.

New 0 ring
New water valve fitted
New water valve

I haven’t as yet ordered the correct flap return springs, but in the meantime I found some that will do the job at my local Bunnings Warehouse.

Flap return springs from Bunnings