MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

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 MGB Technical - Bad parts

I had BOTH the thermostat and radiator cap fail on my '79 B luckily I didn't burn up the engine! I bought them a couple of years ago from Moss I don't know where they get their parts but I'm not happy at all to say the least! I just want to pass this along so it doesn't happen to some one else.
patrick bailey

Patrick. The MGB uses the same sized thermostat and radiator pressure cap as the small block Chevy engines of the 1970s era (and perhaps later models). Parts available at any auto parts supply store, although you may have to go with a slightly different pressure rating on the radiator cap than the original MGB rating.

Les
Les Bengtson

I know I already replaced them I bought the parts from Moss while buying other parts and I can't believe they both failed at the same time! The thermostat wouldn't open and the rad cap wouldn't seal which makes for a very bad situation!!
patrick bailey

I feel your pain, Patrick, but if the water heats up, the cap is supposed to let the pressure out. Glad you didn't drive too far.
Tom

Go to Napa and buy a Superstat and a plain old Stant cap. You won't have that problem again.
Steve Simmons

"but if the water heats up, the cap is supposed to let the pressure out"

The water (or coolant) will *always* heat up when the engine is running, and that should *always* result in an increase in pressure. The radiator cap allows that to happen, only lifting if the pressure in the cooling system exceeds that of the cap to release *excess* pressure, closing again when the pressure drops below that of the cap. Except in extreme situations, or a fault, the system pressure should always be less than the cap pressure.
Paul Hunt

I think Moss now sells a thermostat that fails open not closed. I only needed to replace thermostats because they wouldn't close fully. I've never had one stick closed. But, I've heard lots of stories of stuck closed thermostats especially on GM cars.

I have a Stant cap(7 psi) and Stant 192 deg standard thermostat that have given no problem for over 12 years.

I've had few problems with Moss parts over the years. But, you experience may vary.
Robert McCoy

This is the first bad part I have had with Moss in 14 years but when it can ruin your car I think that makes it serious I just drove the car up and down the mountain and all is well!
patrick bailey

Paul, thanks for making this clear. But I got the feeling that Patrick thought his radiator cap was faulty because it let some pressure out. Obviously the temp and pressure was excessive if the stat stuck closed.
Tom

Tom. You seem to misapprehend how the radiator pressure cap functions. At least from reading your postings.

The radiator pressure cap is designed to seal the cooling system up to the rated pressure of the cap. If the rated pressure is seven pounds per square inch (7 PSI), the cap will provide a secure seal up until the 7 PSI is reached within the system, then, the seal will be forced open and the excess pressure will be vented outside of the cooling system.

The cooling system, when in good working condition, will normally be operating at less than the pressure cap's rated capacity (7 psi for our example). This is because the standard boiling point of pure water (212 deg F/100 deg C) is modified by the "anti-freeze" added to the water (raises boiling point proportional to the exact mixture used) and the boiling point of the coolant mixture is further modified by the pressure build up with each 1 PSI increase in system pressure raising the boiling point of the coolant by 3 degrees F. Thus, our 7 psi pressure cap raises the boiling point of the mixture by 21 deg F.

Pressure caps fail in two ways. The first is where they fail to hold the designed in pressure (what seems to have happened with Patrick's example). In this case, the system is not pressurizing as it was designed to do and the coolant mixture may boil off even though the rest of the system is operating to correct design parameters.

The second, less common, method of pressure cap failure is when the pressure cap does not allow pressure to escape when the design pressure is reached, over stressing the cooling system, pressure wise, beyond its design limits. This can cause hoses to burst or connections to leak which, had they been operating under the design pressure specifications, would have operated in the correct manner. This same condition can be brought about when one uses a pressure cap of higher than factory designated pressure rating.

Patrick has a legitimate complaint and is quite fortunate that there was no engine damage involved. I know of several other cases where only one of these problems has occurred and engines have been damaged as a result.

I am assisting an owner who recently had his engine overheat as he was driving at highway speeds. The cylinder head gasket was damaged, the cylinder head showed .007" of warpage when removed and, when the machine shop removed the valves to mill the cylinder head, they found cracks inside the valve throats.

A bad pressure cap--one which does not hold rated pressure or one which does not release at its rated pressure--is a significant problem and can lead to engine damage.

Les
Les Bengtson

I took the rad cap in and had it checked and it wouldn't hold pressure so I did have a faulty cap
I don't know which failed first the cap or the tstat
but as far as I can tell the engine is ok I took compression and they were all within 10 lbs of each other the plugs look good and there is no smoke or anything I just hope it wasn't overstressed and I will develop a problem later.
patrick bailey

My bad, Patrick. I misunderstood your cap problem. Again, good to hear all's well.
Tom

This thread was discussed between 15/09/2008 and 19/09/2008

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now