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 MGA - Blanking sleve, thermostat

OK, I bought a new Sleeve from moss and now I'm about to install it into my 1956 MGA. Does anyone have thoughts on the install? I'm thinking its really fairly easy? This I believe will take the place of the
thermost I have in it now which doesn't seem to help in the overheating, 190-210 alot. Tom
Tom Peotter

For overheating check mixture is not too lean, timing is not too far advanced and try disconnecting the vacuum advance at the distributor.

dominic clancy

My MGA was completely rebuilt by a reputable MG Dealer in Mlps.,Mn. in 2004.The engine was analyized and tuned on a Dynamo,the Rad. was rebult with additional cores added, it has an oil cooler and a recovery tank, the carb. were redone with the originalstandard needles
the Distr. is original with points,chrome slats are opened for more air and no badge bar to restrict air flow, rebuilt temp. gage,I've opened the heater for more cooling through heater core, used water Wetter,water and water coolent mix, I'm switching to a 20w50 oil by Penn with Zinc,The pumpkit is 3.9 or better, I have tried 1/2 doz. thermostats from 160 on up. It still overheats so this is the last straw before I pull the Engine and rebuild again.
Tom Peotter


I would suspect your radiator with its additional cores. The MGA is very sensitive to its radiator configuration and the closest to original design (including pipe diameter) the better the cooling.

Steve Gyles

I was told by some MG mech.'s that more is better?
This was done again by another reputable British auto
mech.tune and or rebuildable dealer in Neenah Wisc. that convised me this was the way to go after talking to the Radiator repair shop.
Tom Peotter

Dominic has pointed you in the right direction(s). Having the engine tuned and analysed in 2004, and carbs rebuilt, does not preclude timing and mixture issues arising. Try driving fast on the road, then stopping at the roadside and check the spark plug colours (all cylinders). These will show if you have a lean mixture (white to very pale brown).
As a sidebar, I would like someone to explain to me why changing the thermostat (from one temp rating to another) would have any effect on the running temperature of the engine. In my mind, a car running with indicated engine temp at say 190F-212F, is not influenced by a fully open thermostat which opened at 160F or 170F.
Similarly, an oil cooler has very little, if any, effect on engine temp. It may improve oil pressure when hot. Airflow is an issue with the MGA, and the fan shroud really helps at lower speeds or when crawling in traffic.
You could also try flushing the block with a suitable cleaner to check your water jacket is not constricted with rust etc.
Removing the engine seems to be an unecessary "last resort". If you don't have high oil comsumption, compression is good on all cylinders, and you have power out on the open road, then the engine would seem to be OK.
P. Tilbury


I reached the same position as you and had tried everything.In desperation I fitted a 45 DCOE Webber and the problem went away.I now run with a thermostat and the car does not overheat at all,no matter what happens, even a traffic jam on a hot day.
Must be the mixture.
D Townshend

Ideally, you'd want to use the sleeve and the thermostat together. Original thermostats had sleeves on them. The purpose of the sleeve is to keep the circulating water from getting into the bypass and decreasing the cooling efficiency of the circ. system. Once I started using the sleeve I noticed an improvement (decrease) in operation temperature. If I know I will be doing a lot of driving in high temperatures I will remove the thermostat entirely.

The sleeve drops right in, it's self-explanatory.

BTW, I don't consider 190-210 to be overheating especially if you're talking highway driving during the summer. On a hot day (80 deg+) with highway driving my car regularly runs 190+. In 60 degree weather it will run 170-180. I don't use a fan cowl or felt pad. If you're running 210 no matter the outside temp though, something needs to be addressed.

Just curious, have you put the temp unit in a pot of boiling water and checked it against a digital kitchen thermometer? It's not unheard of for the gauge to read incorrectly.

Mark J Michalak

If you are using a normal modern (unsleved) thermostat, there can be a significant amount of coolant that simply flows through the bypass port and completely misses the radiator. Barney talks about it here (read this page and the 2 that follow it).

I have a blanking sleeve as well as a modern thermostat on my car as Mark mentioned. I also installed the missing felt pad above the radiator on the under side of the bonnet, flushed out the radiator, and installed a fan shroud. Since doing that, I no longer have any problems with over heating.
Andy Bounsall


I am with Mark and Andy on this one. I was running on the hot side to the point that I was losing a lot of coolant. I had tried all of the fixes mentioned in the thread with the exception of the thermostat. After reading Barney’s piece on the function of the thermostat sleeve I pulled the standard style thermostat installed the Moss sleeved thermostat (434-156) – problem solved. I am succesfully running a 7 lb. cap although some articles indicate that the original style sleeved thermostat will only function properly with a 4 lb. cap. My NAPA cap is also designed for the long neck on the MGA radiator, has the valve for connection to an overflow tank and the proper rubber gasket.

For good measure I installed a cheap overflow tank. I simply connected the overflow tank to the end of the metal overflow pipe and ran the hose back up in front of the radiator to the tank. The tank is held off to the side by a couple of quick tie straps. Not elegant but it works.

You might also note Barney’s recommendation to drill a hole in the blanking sleeve when used with a modern thermostat, .


jjb Backman

I failed to mention that I also have the felt between the hood and radiator only its very thick. I found that my shroud was getting very hot and was distorting
to the point of fan contact so I removed it, also the Dyno testing was just done last year and from that the Carbs were also adjusted along with the timing but it still gets way to hot. As the temp. clims I start to smell in the car hot radiator fumes almost as if it leaking out somewhere, possible the small drain line on the top neck?
Tom Peotter

And the old chestnut of the correct longer radiator cap?

dominic clancy


I harp back to my radiator rebuild comment. I have had long conversations with Bob West (a UK MGA specialist, authority and supplier) and have directly seen the results of overheating caused by modern cored rebuilds. My colleague had one such radiator that caused all the problems you describe. After 2 years he went to Bob West, traded in his radiator for a standard layout and all his overheating problems disappeared instantly.

I will happily acknowledge that there are some good quality modern profiles that do work, but they need to be carefully researched. Some members on this board have such radiators; equally, there are many who will say that the original layout was designed at the time for the car and still works the best.

Whilst it is generally a lot cooler here in the UK, I run an 1800 engine with an original MGA radiator construction and I rarely reach 180 degrees. I have no additional fans, oil coolers etc.

Steve Gyles

Interesting thread. I agree with what has been posted. I am running an original radiator with a blanking plate and a 180 degree thermostat. The radiator slats are spread to ensure air flow and the everything is mechanically sound. I generally run around 180 to 190 during the summer months here in VA and rarely go over 190 during the hottest days.
Bill Haglan

I just installed the Blanking Plate last night from Moss and am running it today. I also put in a 160 thermo. All looks great without over heating. Thanks all for your input. Tom
Tom Peotter

And now the reverse of the question:

How to get a car that runs below 70C to run a bit hotter? And that's on a really hot day. On a cold day I get ice on the carb and an engine temp of 50C. Standard core rad, oil thermostat in the circuit for the oil cooler. 180F thermostat in the head (which made no difference at all...).

I have almost completed the installation of a roller blind in front of the rad, I just need a little bit from Bob West that I shall collect on Saturday. That will be a bit more elegant than a slab of carboard box stuffed in front of the radiator. I hope that will bring some improvements, but O'd love to get to the core of the problem rather than the symptoms.
dominic clancy

What about the reversal of the hot problem where one is told to retard the timing to help cool the engine to advancing the timing to make it hotter? Tom
Tom Peotter

Maybe repair the temperature gauge that never reads above 70dC, evem when it may be about to boil?
Barney Gaylord

The temp gauge is working perfectly - its been checked and has no variances across the gauge when compared against readings from a digial termometer, and reads dead on 100C when in boiling water. It's definietly not the gauge.

Timing is advanced as possible - there's even a set of electronics to automatically retard a cylinder in real time if it starts pinking/detonating under load.

I have been gradually weakening the mixture, so far with no change in operating temp. Running lean on a Supercharged engine is guaranteed to cause detonation and subsequent self destruction of the engine - hence the fancy ignition timing electronics. See here
dominic clancy

Dominic, I'm very impressed with your system for the MGA. Wish you were here in the states or in Wisconsin.
Tom Peotter

Same as Bill Haglen ,interesting stuff, been all through it all myself. I would like to clear up one important point ie, the designed running temp of a B engine ,which equals thermal efficiency. This I understand is approx. 190%, still well below water boiling point.
To achieve this I have a %180 thermostat, plus a bypass sleeve, to set the floor temp ,and a radiater that sets the upper temp via good air flow, engine fan and electric. My inital concern was the large change in temp in hilly country 195 going up and 150 going down, an eventual cracked head ? ?
S Sherry

If I remember correctly, the role of a thermostat is to maintain a block / head temp at or above the temp rating of the thermostat. (Yes we all know this stuff, but it may be worth mentioning any way.) For example, a 180° thermostat would or should limit the temp excursion from 180° down hill to 195° uphill, a relatively minimal change in temperature.
Possibly your 180° thermostat is not working...
Russ Carnes

Ross maybe I did not make myself clear. The low tempt. downhill was before I fitted the 180 thermostat. Engine now runs 180 to 195.
S Sherry

I'm going to try a thermostat blanking sleeve in the hope that less coolant will then flow through the bypass route.
dominic clancy

Excellent! er... Cool!
Russ Carnes

This thread was discussed between 11/08/2008 and 20/08/2008

MG MGA index

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