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MG TD TF 1500 - Bypass Surgery

Just wanted you all to know that patient number TD 1851 has had successful bypass surgery. A diverter valve and a "T" were inserted into the water circulating bypass hose. The patient was out of the recovery room today and went for a nice long drive. With the engine hot and the valve in the summer position the fan blew nothing but cool air around the cab. Later, with the valve in the winter position the fan blew copious amounts of hot air.

I posted my proposed schematic earlier and this was one of the replys';
E.B. Wesson, Georgia, USA
 I like the idea of a "bypass for the bypass", but I'm not sure how much space you will have for the extra "T", and hoses, plus clamps. The heater hoses are very heavy walled...You may end up really close to the fan...
I was trying to keep the extra plumbing to a minimum.
I would be interested in seeing the valve, when you have a photo.


Ed, you were correct in that there is very little room to work in, especially with the supercharger taking up so much space. I tried to use the smallest practical components and arrange them closely. Hard to get a decent picture of the system.
More details, parts and pictures available on request.

PS: I will re-post the schematic in the next post.

Mort TD 1851


Thermostat Closed
All water circulates thru engine and heater only. Maximum hot water to heater.

Thermostat Open
Most of the water circulates from engine thru radiator. Some water is diverted down to the heater. Keeps a good flow of hot water to the heater.


Thermostat Closed
All water circulates thru engine. No water to heater.

Thermostat open
Most of the water circulates from engine thru radiator. Some water is diverted down to bypass and back to the engine. Same as standard TD with out a heater.

Works great. I love it.

Mort TD 1851

"Summer Most of the water circulates from engine thru radiator. Some water is diverted down to bypass and back to the engine. Same as standard TD with out a heater."

Not quite. The standard TD thermostat had the sleeve to cut down the bypass flow after warmup. Yours will be a direct short circuit of hot coolant dumping right back into a hot engine, unless you put a significant restriction in your bypass branch.

The heater and bypass circuits may be small, but they have the advantage in circulation where their returns are directly into the pressure drop at the pump intake; the coolant into the radiator is pumping uphill through the thermostat against atmospheric pressure, no suction.

I'd choke that down to maybe an 1/8th " hole.

I've got to grab a different computer to post a pic of the "French horn" I installed.
Jim Northrup

I make my comment , not to rain on your parade, but I know where your temps are.
Yesterday, it got up maybe to 80F here in Michigan. We were cruising at 70mph in our Magnacharged TD. The temp got up to 210F. I'm running a strong antifreeze concentration, so boiling may be in the 225 range? I will check that soon on the kitchen stove.
darn, went to post my pic but sent the post.
gonna continue.....
Jim Northrup

As I was saying, I'm afraid, with a wide open bypass, we'd have been boiling the coolant away.

I reached under the dash and shut off our heater, but if it got very much hotter, I was going to run the heater to cool the engine.

I'm going to post a picture of my "French Horn" which is sandwiched just above a stainless steel (rubber roofing electrical insulation glued on)cover under the dash, but don't expect anyone to understand the contraption. The valve is within reach under the dash. It incorporates a small bypass circuit past the temp gauge thermowell (thermowell restricts the bypass)and there's also a bimetal switch for an overheating idiot light.

Jim Northrup

While we're on the subject of cooling, I've added another quart or two of coolant to the radiator capacity for the cost of less than a dime.

I took off the radiator cap and stuck a 1/4" vinyl tube over the overflow within the radiator, and then cut it off just beneath the cap. This was really an attempt to raise the water level up to the motometer thermometer. I still added a piece of copper tubing to the motometer to get it down farther (picture). As the coolant heats up, excess water runs out the overflow and exits the bottom of the radiator. When it cools down, the level goes down a good inch or so down.

I have yet to do it, but intend to run a tube from the overflow below the radiator into a catch tank somewhere, like current vehicles, so it catches any exiting antifreeze and sucks it back into the system when it cools down.

Jim Northrup


That's a very pretty set-up, and I imagine that being supercharged, your engine would produce a little more heat than unsupercharged.

You may have seen my set-up which is similar except that I deliberatly passed the main hot water pipe UNDER the exhaust manifold, so as to pick up more heat. Your by-pass of the bypass, is a clever idea.

Like yours, mine is working well, and has done do since 1988 when I installed it. I have posted the pictures on the forum and it was written up in the TSO.

However, I might make one suggestion: Yours like mine has a fitting quite close to the exhaust manifold, and I see a rubber/neoprene/etc connector also quite close; and in my case, to minimize the heat transfer, I wrapped mine with asbestos lamp wick.

Gord Clark
Rockburn, Qu.
Gordon A Clark

I stand corrected about the "standard TD". They did have a sleeve to cut down the flow when the thermostat opened. However, mine like so many others uses a replacement thermostat that does not have a sleeve or any other restriction. Although I have not pushed the car to its limits(yet), both before and after my conversion I never had an overheating situation. I can only theorize that there is sufficient flow thru the radiator and back to the engine and minimal hot water thru the bypass. I also moved my thermocouple from the radiator to the elbow just below the thermostat so I am reading a temperature much more indicative of the engine water temperature than the radiator temperature.

I can understand why they made a restriction as part of the thermostat. It reduces the flow ONLY when the thermostat is open. What you and others have done, is to reduce the flow even when the thermostat is closed. Is this good practice?

Like other modifications to these cars, there seems to be no one right way. Many claim to be satisfied with their different configurations. I guess there is no "one right way", but only the way that satisfies our individual need to tinker.

As always I appreciate your comments and inventiveness. Keep it coming.
Mort TD 1851

Did I understand you right in that you have a way to manually bypass your restriction?
Mort TD 1851

The copper line coming from the engine to the heater does pass under the exhaust manifold. The line that is on top is the return from the heater.
Mort TD 1851

I didn't want the temp gauge capillary tube running through the engine compartment, so the plumbing incorporates a small circuit past the water temp bulb full time, ie my bypass. The heater valve is located above the right knee to open/close from within the cockpit.

"What you and others have done, is to reduce the flow even when the thermostat is closed. Is this good practice?"

While playing around with the thermostat, I just removed the brass air valve from the thermostat, leaving a tiny hole, as well as a bit of clearance around stat. With the engine just idling and warming up, the hose and radiator were getting hot well before the stat opened. I since replaced the thermostat and sealed it in the housing so now the complete flow goes through the heater circuit. A small hole in bypass or thermostat will provide sufficient circulation. I believe if it were completely blocked off, there'd still be decent circulation in the block through the openings where the pump sits in front of #1 cylinder. I just looked at an Arnolt bypass fitting with no hole between the two heater hose branches; there'd be no bypass at all with a closed valve (I've got 2, and keep forgetting to see if either has a hole).

We've never had an overheating problem, but the '50 now has the MGB 3.92 rear and is set up for the highway. While cruising 70 mph (2-3 lbs boost), 210F at 80F air temp is fine, but what happens in the summer when the temp soars over 100F, especially climbing a grade?

Within the next 2 weeks, it will be out on Michigan International Speedway... should be interesting.

If push comes to shove, I'll get the radiator recored to 4 rows for insurance. I'm not inclined to go pressurized cooling system, but plan an oil cooler when the original XPAG (now 1466cc) goes back in. I also plan on installing a coolant catch-can, soon.

Please report back your temp readings per driving conditions!

Live & learn.
Jim Northrup

Did you consider the much talked about dangers of asbestoes?
I was wondering if anyone knew of an alternative product?
Mort TD 1851

Is there an aftermarket thermostat that I can install in my TD? where can I find one? Moss wants $120 for a copy so I'm looking for an alternative. My Heater is plumbed from the bottom rad hose to the back of the engine (0r vice-versa) and my temp runs 40-60 degrees c so my heater is worthless unless I'm climbing a hill! Where can I find a restricted "T" to put in the bypass hose or where else can I get some pressurized hot water?
I see a lot of great designs on these posts, but I don't want to reinvent the wheel if I don't have to!
cj schmit

Why does someone who lives in the sunshine state need a heater? Buck up man! These cars were made without heaters to be driven, with a stiff upper lip, in English weather!
M Sutton

A conventional thermostat isn't difficult to install. The only real challenge would be if your housing still has the crossbar. It'll need to be removed.

The opening in the bottom is a nominal 50mm, I measured it at about 1.96." I grabbed a 2" ID snap ring that lists as 2.21" OD and cinched it in place with snap ring pliers. I sealed it off with an O-ring & silicone seal to insure maximum flow through the heater before the stat opens; before adding sealant, the radiator hose & radiator got a lot of hot water way before it warmed up.

The only photo of the thermostat I could find is below. The thermostat is located just above the bypass.

Jim Northrup

cj, see Bud
Bud Krueger (TD10855)

Jim Northrup,
Yesterday, ambient about 70F 1 hour in local traffic with some stop and go rush hour. Could not get water temp above 80C.
Today, Ambient 85F 1 hour or so at 70-75 mph got the water up to 95C but only when I was pushing it(2-3 lbs boost). Cruising 65-70 drops down to 85C. Off the Hwy. drops down to 70-80C.
I am running pure water since I have been draining it down so often. I will add some antifreeze now.
You or anyone else have any opinion on converting to a Texas Kooler?
Mort TD 1851

...slightly off topic, but pertaining to a post... I ran for about a week on straight water and it took about a month of flushing to get the rusty look out of the water/coolant.... was amazed it rusted up so fast... went quickly back to coolant....
gblawson(gordon- TD27667)

I've got a "stiff upper lip" it's the little lady who gets so cold!
I verified last night that there is no thermostatic function in my car. Started it up and looked in the rad and there was a swirl of coolant when I reved it up. Next stop to buy a thermostat.
cj schmit

Texas Kooler for Healeys, might not be any help

Personally, I went with just a single blade- at highway speeds, the fan isn't much of an influence, in my opinion. The brand new water pump (from India) wobbles so bad, I wanted to cut the weight, and am seriously considering an electric fan to extend the life of the pump, but don't like the image of an electric fan there.

Ours '50 TD has just hit the road recently (maybe 200 miles now)... radiator & engine have been setting dry for about 10 years. Now that freeze warnings are behind us (Michigan), I've drained antifreeze and rust, flushed thoroughly, and will run water and keep flushing for a while. We can drive the car and boil out the radiator at the same time!
Jim Northrup

Amazing how many different plumbing ideas for such a simple heater....
After seeing Jim's, yours looks pretty simple...
I like the way you worked it out.
I have a theory about the method that I am using, with the "diverter valve" in the bypass....(similar to the original mfg's heater valve).
With a "replacement" thermostat, the restriction in the diverter causes the thermostat to open earlier, because less coolant is moving through the water jacket...Once the thermostat is open, the diverter has no real effect on the cooling capabilities, since the main circulation is though the large hoses and pipes....Some hot water is still going into the bypass, and thus the heater works in either thermostat position....
Each time the thermostat closes, more hot water is diverted to the heater, and this keeps the heater from getting too cool to be useful.
I know this is less than scientific, but I'm no engineer....
So far, my '52 is running in the 75C-85C range on hotter days, this , in the Summer position of the main valve.
Photo added for clarification.

E.B. Wesson

Here's an image that I found, of the original Arnolt setup, from the instruction manual:

E.B. Wesson

This thread was discussed between 23/05/2011 and 02/06/2011

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