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Hi I am going to fit a after market heater
in my 1953 TD,
Where is the best place to pick up from
top hose ? and run back to from the heater
Any infomation would be get help.

The hottest water can be taken from a tapped hole in the elbow just below the thermostat housing.
Jim Merz

This just slipped below the posts....


And just down the page under 'Just checking' is a bit more....


Last year I fitted a heater to my TF. I read all the stuff about not taking the coolant off of the back of the head and thought: “Foooy, the whole system is hot and there can be little or no difference in temp at that location.” I went ahead and installed it by drilling the back plate on the head and installing an elbow. Guess what the heat from the heater was minimal. So back to the drawing board and I re-plumbed it, taking the coolant from the thermostat housing in a similar way that the Arnolt heater was designed. WOW heat this time such that top down in the fall is almost in comfort.
Godspeed in Safety Fast
John Crawley

John, interesting topic since I am at the same position right now: to prepare the installation of a heater in my 51 TD.
Gordon, I am sorry but I do not understand your comment:"And just down the page under 'Just checking' is a bit more...." Can you help me/us out a bit?
Greetings, Huib

Huib Bruijstens


By 'just down the page', I meant a post called 'Just Checking' has more heater information....

Is your thermostat housing for a TF 1250 or the one for the TF 1500. If it was for the 1250, can you please post a photo of the change you made? Thanks.
David werblow


Has anyone fit a TF wateroutlet (the one with the heater tap) to a TD? And if so, how did you do this?

I have a TF water outlet with 2 threaded holes and would like to try to fit this to my TD to install the heater.

Also, does anyone make a quick conect that could be attached to the heater for a quick removal?


RG Taylor


Do you have an e-mail address, I can send you pictures.

James Neel

If it will not work on your TD, have you any interest in selling it?
David werblow

Here is a picture showing how my heater is connected to the front of the engine. This was done 30+ years ago and the heater puts out more heat than one could use.


James Neel

Here is the rear connection.

James Neel

Thats interesting...I have the same hook up...same connections and get very little heat...thats why I have gone to taking the heat from the thermostat elbow....

One could get by with no drilling, tapping, cutting & brazing tubes.
For what its worth, I swung the thermostat housing around so the bypass now aims towards the driver's side, and will feed the heater (or 2 heaters, haven't decided yet) with this.
That makes the bypass branch available for the heater return into the pump feed tube from the radiator.
That's the simple explanation. In reality, you'd probably think my fabrication of tubing & hoses is a plumber's nightmare, but that'll be concealed under the dash. But then again, I'm a steamfitter, and not a plumber.
jrn Northrup

David, is something that you could use? Marvin

Marvin Stuart

I installed the Arnolt type heater with the inlet and outlet adapter located between the bypass and lower radiator hose, a common period installation. More than enough heat. Marvin
Marvin Stuart

Thanks, Marv. I don’t think I can use it, it looks like it’s the ”water-outlet” for a TD. Because of its pressurized radiator, the TF’s also serves as the thermostat housing. TF 1250s , like mine, have one tapped hole for the temperature sending unit. TF 1500s have two, one for the sending unity, the other for the heater connection.
David werblow

Marvin: Check your email.

L Karpman

i did my heater just like james neel's and have more heat than i need...which i guess is about 1/2 of what my wife wants. LOL! seriously, plenty of heat. regards, tom
tom peterson

Gordon, thanks for the other interesting thread. Pictures are very clear and I will follow that way.
I checked the archives and there is (of course) a lot about heaters in there. But I never before saw the suggestion as jrn Northrup made in this thread; to simply use the bypass.
Looks surprisingly nice and simple but how will it influence the cooling system in cold and hot situations? Greetings, Huib
Huib Bruijstens

The bypass pipe is where most commercial heaters took their water (and returned it)...they took the pipe and put two outlets in it and between the outlets they put a washer type disc with a small hole in it...
The bypass still 'bypassed', but most of the water went through the heater hoses...
This used the hottest water ... same temp as the thermostat elbow!


Here are a couple photos of heater plumbing in progress.
Members here enlightened me on the particulars of the XPAG block cooling channels, or I'd have mistakenly plumbed off the back of the head.
Utilizing the bypass tube or bypass elbow is logical, BUT the original thermostat is designed to block this off when it reaches temperature. Counterproductive for the heater system.
I'm running 7/8" cotton covered hose back into the tub. I think it has a vintage look to it. One could simply run a short stub of hose and clamp 3/4" copper pipe in, but I'd like to keep it insulated on the way in. On the way back to the waterpummp, I have stainless tubing.

jrn Northrup

Photo attempting to show the heater return tucked in nicely between the intake manifold and the head. It drops into the pump intake where the bypass branch is. I did fabricate a new intake pipe back when I had the heater originally supplied from the back of the head. I would have gone to a larger tube to match the oversized 7/8" hose, and now since I may try to stick in a pair of Arnolt heaters.
I also plan to design a small bypass between the supply and return under the dash with the heater bulb within. That way, I'll be able to shut the heater circuit off in hot weather and still circulate a small amount of water to facilitate both the bypass and thermometer functions.
jrn Northrup

I'll try uploading that 2nd photo again.

That neat little drip oiler with sightglass on the blower was courtesy of Gene Gillam! I found they're available from McMaster Carr.

jrn Northrup


Taking the heater flow from the back of the head won't work, but returing it there will!

The below set-up allows for lots of heat.

For over 50 years, I've had a set-up in which I take the hot water from the lower rad pipe, feed it across the engine in a copper pipe that passes under the exhaust manifold where it picks up more heat, then into the heater. The return goes into the back of the head.

I was aware when I did this in 1957, that water is also flowing into the head from the front of the engine. The flows meet somewhere in the middle of the head, and then down into the block.

This set-up also has the added benefit of using no hoses running through the engine compartment. In fact, unless one looks cloely, one doesn't even see the heater plumbing. Clean and neat.

I've appended a copy of the picture I put into the TSO that shows both ends of the engine but not the middle, so the plumbing is clearly illustrated.

The black "device" in the pic on the right, is a temp. sensor to operate the electric fan. I long ago removed the traditional "hand-chopper" blades!

And I run with 'Water Wetter'in the summer.

Gord Clark
Rockburn, Qué.

Gordon A Clark

The back of the head is at pressure and the pump inlet pipe is under a suction. Coolant would flow FROM the back of the head before it gets hot and returns TO the front of the engine, as yours is plumbed.
jrn Northrup

Is the following about the bypass job/function correct?
a: if the engine is still cold, so is the thermostat and so, the thermostat will block the flow from the engine water towards the radiator. By allowing a flow of water through the bypass back into the engine again, the thermostat will be able to sense the actual temperature of the water. So, bypass enables the thermostat to sense the actual temperature and to re-act accordingly.
b: if the thermostat is still closed, the pump by its design keeps pushing the water into the engine and if there is no way the water can go, the pressure will rise. The bypass prohibits the waterpressure to get too high.
greetings, Huib
Huib Bruijstens

Your first part is correct...thermostat closed, the water goes through the bypass back to the pump and back to the engine...and around and around...then the thermostat opens and the majority of the water goes through the rad and cools....then back to the pump and through the engine...then, depending on the water temp, the thermostat will close and again through the bypass, etc, etc.
I always wondered if taking the water from the bypass reduced the water temp when the thermostat opened?

The original thermostat is supposed to close off the bypass as the thermostat opens. In other words, just asthe engine gets hot enough to make the heater effective, it cuts off the flow of coolant. Rather counterproductive, but then again, the cars were not designed for a heater.

This is a good reason to drill & tap a branch below the thermostat, as many point out, and restrict the bypass.

I like the idea of using a newer style thermostat that can't block the bypass, and then utilize the brass bypass elbow for the heater supply and the bypass branch on the pump inlet tube for the return. The heater will act as the bypass circuit.

To confuse the issue further, so I can block circulation to the heater in hot weather and still have some bypass when the thermostat is closed, I intend to add a small bypass shortcut between the two hoses under the cowling . The water temp gauge bulb will be located in this little circuit so I don't have to run it up front to the thermostat housing or the radiator.

A small hole in the thermostat will bleed any air that happens to be trapped in the engine for some reason, and also add a bit of circulation when the stat is closed.
jrn Northrup

...the bypass remains unblocked when the thermostat opens, just wouldn't have the same amount of water passing through (am looking inside mine right now)....

I put a hose valve just at the firewall that prevents water entering the heater.

I don't like the idea of not using the bypass, or doing anything to restrict it.....

If I understood it well from this BBS, than a pipe diameter of 1/2" is sufficient for allowing enough hot water to the heater. If this is true, than the unblocked bypass facilitates a too big stream of hot water to enter the block again. Didn't we have a cooling capacity problem on hot days???
So, one should be able to reconstruct to original intended situation where a hot engine all the hot water gets cooled down, eather in the radiator or in the carheater. What do you think? Greetings, huib
Huib Bruijstens

The bypass allows water to circulate when the thermostat is closed...when the thermostat opens, most of the water then goes to the rad and cools...
I think we are making it more complicated then it was designed to be.
As the heater was aftermarket, the designers came up with a simple method of grabbing water...a new bypass pipe with the inlet and outlet separated by a restricted washer...(small hole in a blank)...not the best place to take water, but works...!
When the TF came along, they added the outlet at the thermostat elbow cause it was the hottest water....

i never charted out all the thermal/fluid dynamics of the MG TD cooling system..but i do know i have a set up like, if not indentical to, gord clarks and it puts out plenty of heat. it appears this is one of those situations where there is more than one way to skin the cat. regards, tom
tom peterson

Thanks, Dave; my TD came out a month too early.

Never really understood the dimmer switch on auto lights anyway... I don't think I've ever had a car I didn't always run with the rheostat fully on...
Geoff Baker

It wasn't until a few months ago these knowledgeable guys woke me up to the realities of the MG block design/water passages and the bypass function.
Out of our 3 different thermostat housings, none has the guts inside, so it wasn't intuitively obvious at first how they were originally designed. I always figured that leaving the stat out would be the cooolest operating scenario, BUT, after browsing posts here, I took a second look. I rummaged through my spare parts and took a close look at 2 vintage thermostats intended to replace the original (provided you buy the replacement housing). The original MG T thermostats incorporated a sleeve that rises up and cuts off the bypass. Without that sleeve, the bypass is a large, wide open circuit. It is conceivable that as much water would take the shortest path through the bypass as would have to squeeze through the thermostat and make the slow, long journey through the radiator. Running without one would offer a lower path of resistance, but still a percentage of hot coolant is running right back into the engine.
I'd been deceived into thinking our MGs have been running pretty cool, about 175F, but this temp has been taken off of the back of the head, which is coolant just starting its journey through the engine and is not a true representation of the real engine temperature.
Fortunately, we also use a motometer in the radiator cap, and luckily, it has always showed medium temps and we've never had an overheating issue. BUT, the thermowell was too short to reach down into the water! I added a copper tube to reach down into the coolant, and on one radiator, added plastic tube onto the overflow tube in the radiator to raise the level.

Bottom line is, I will check the water temp as it exits the engine; pull hot water from the front of the engine also, for the heater; provide a very minimal bypass from now on.
jrn Northrup

I would still be careful blocking off the bypass...whether cold or hot the engine has to have 'circulating' water...without it moving you will have hot spots and it will take longer for hot water to even get up to the thermostat so it can do its thing and open....
The bypass is an 11/16" opening at right angles to the water flow...once the 2" thermostat (granted there is a large brass unit in the middle) opens, the water is going to go straight up rather then make the right angle turn...although some will still take the bypass.

(I don't have my tools in front of me, but I do have the thermostat...the original...and looking down the opening, there is no sleeve of any kind blocking the bypass outlet) Will remove the bypass elbow this morning and take a photo....

Always one to admit when i'm wrong...yep... the brass sleeve of that brass thingie closes over the bypass hole...which is actually a 'slot'about half the dimension of the outlet.
Now that is interesting for the heaters that take the water from and back into the bypass...not sure how the water circulates through the heater when the thermostat opens?
gb lawson

If anyone needs a heater there is one on e-bay, a Smiths which is what I have and it really puts out the heat. Item number 160383642634.

No connection to the seller.

James Neel

Now I learned something! (again). Several years ago, in the beginning of my TD restorationproject, I bought a new thermostat housing since the old one was completely rosted. It is a aluminum one and the supplier was Angloparts. In their catalogue it came with a brass plate, called hole reduction plate that goes between the housing and the brass elbow. So, now I understand this is because the new thermostat housing does not have that wunderfull sliding-block-the-bypass-feature and that is why this small hole (I measured 4 1/2 mm) permanently restricts the flow through the bypass. The picture shows how it is presented in the catalogue. Greetings, Huib

Huib Bruijstens

Perhaps the opening in their casting is a full circle, this is the actual bypass opening...with and without the brass slider.


reactivating "Heaters" for the cold weather
jrn Northrup

All these posts on a simple issue......guys you are making this too hard. In my TF we have a modified connector pipe (see pix) with hose running under the carbs, thru firewall to heater (Bosch). The return comes out and into an inlet on the back of the head. Valve is for turning off heat in summer.
This supplies the Bosch heater unit with 3 speed fan, with plenty of warmth in the cockpit.
colin stafford

Heater connections 1

colin stafford

The XPAG cooling system is more complicated than it appears, and the fact it was not designed for a heater circuit makes it more difficult to sort out if one is added.
The circulation is actually the reverse of your desciption. The connection on the radiator return pipe is the suction side, returning water to the pump & engine.
Where you are tapping water on the back of the head is the supply of water to the heater. It also is fresh coolant straight from the pump routed to the back of the head (via the passage along side of the block), before it picks up heat.
Other questions concerning heater performance are whether there's a thermostat and does it have a sleeve blocking the bypass. ...and whether the bypass has a restrictor installed if there's no sleeve.
ORIGINALLY- the bypass shunts all circulation right back into the pump when the thermostat is closed and is blocked off directing virtually full flow to the radiator when the stat is open.
New thermostats do not have the sleeve so the bypass does not shut off, circulating hot coolant right back into the engine when you don't want it to. Hence, the restrictor plates to limit flow. The bypass also has presents another consequence in that when you want lots of circulation through the heater, the majority is flowing through the bypass.

jrn Northrup

John M. Knight,

Please contact me off board; jneel43(at)hotmail(dot)com

James Neel

by my count we have at least three guys tapping off the rear of the head and being satisfied with their heater performance. jrn, thanks for the temp readout at the back of the head. 175 degrees explains why my heater works so well off the rear head tap. many thanks. regards, tom
tom peterson

I've reactivated some old threads on the subject.
Gene Gillam generated the following great explanation of XPAG coolant flow:
I think Gene also provided photos depicting infrared temp readings on a running engine and illustrated the marked differences, but for the life of me, I can't find it. Anyone have a lead on this?
I was casually replumbing a heater the way it was previously installed and the temp gauge bulb, to the back of the head until Gene and others wised me up.
Tapping water from the hottest source is one thing, getting it flowing is another.
jrn Northrup

This thread was discussed between 24/11/2009 and 22/01/2010

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.