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MG MGB Technical - water loss mgb - more

Further to discussion earlier in the month. Noticed this morning that the "new" radiator cap fitted six months ago does not appear to require pushing down; that is it does not appear to seal on bottom of filler neck. I have thrown old one away. Does anyone know correct length of cap from underside of top seal to bottom of lower seal?

George
G Burrow

George - Remind us again what year your car is. Some of us are getting old enough to suffer from short term memory degredation :) Cheers - Dave
David DuBois

It is a 1971 with 10lb cap. From memory on the midget I had to press down to put the cap on. This one just turns easily with no downward pressure. Might be solution or only highlight real problem

thank you

George
G Burrow

I have had several caps that were just blatantly made from the wrong material, Every time you tapped the ears up to compress the spring they just sagged back again to where like yours it was only gravity holding the thing in. I got one from a local motor factor instead of the "mg specialists" and its been fine.
Stan Best

George

If you don't have to press down to seal the cap then the spring-loaded "pressue plate" is not engaged and you have a "zero pressure" pressure cap. I do remember that the earlier MGs (? B?) used a "longer" radiator cap than commonly used in US cars. Does anyone remember when they changed to the US "standard"?

You can measure the distance between the two sealing surfaces on your cap, and the corresponding length in the radiator fill neck. It will be obvioius if you have too "short" a cap.

HTH

Larry
58A
72BGT
Larry Hallanger

George. Your car uses the same radiator cap as the mid-70's Chevys. Only difference is the pressure required to cause the cap to break seal, and they are fairly close to each other.

I am not sure what you mean by having to push down on the cap. A somewhat subjective thing I would think. On my cars, I do not notice having to provide any downwards force to install it. It does, however, require that I put some downwards pressure on the cap to remove it. When turned onto the radiator (or expansion tank), the pressure cap can be felt to move upwards slightly as it goes into the locked on position. Take a look at the design of the filler neck to see what I am speaking of--it is a ramped surface that you are turning against, making the cap easy to turn on, then locking it in place.

Les
Les Bengtson

>Does anyone remember when they changed to the US "standard"?

Prior to '69 -- my car has the shorter neck. George, if you have the deeper neck, then someone has installed an earlier radiator in your car.

HTH!
Rob Edwards

Do you know what dimensions are?
George
G Burrow

Seal to seal distance is 13/16" on the 10# cap of my 74 B.

Clifton
Clifton Gordon

Clifton
was that from bottom of uper seal to top of lower seal or bottom of upper to bottom of lower?

do you know depth of radiator neck on your car?

Rob wondered if i had older radiator and newer cap?

thank you
george
G Burrow

I measured bottom to bottom of the seals as they fit in the radiator neck. I'll measure the neck depth later this morning and post.

Clifton
Clifton Gordon

My radiator neck mesures about 3/4" from top sealing surface to lower sealing surface.

Clifton
Clifton Gordon

thank you

I am off to compare measurements

george
G Burrow

Clifton

my neck is a smidge over 3/4" so same as yours.

My cap, off vehicle, and without any compression is only 23/32". That is it doesnot touch the bottom.

if I fully compress top rubber (doubt if it would fully compress in use) then face to face is 25/32 so some contact but not much.

I will try and find another cap to compare with

Thanks for your help
George
G Burrow

FWIW my filler neck is also just a smidgen over 3/4" between top and bottom seal surfaces. The cap measures exactly 13/16" between the top metal seal and the bottom rubber seal. The difference between these two is how much the rubber and spring are compressed with the cap fitted, i.e. just over 1/16", and this is just about the thickness of the rubber.
Paul Hunt 2

New MGB owner. (Me, not the car; it's a '72 roadster.) I have a 7 lb. cap on my radiator and there seems to still be air bubbling through the engine and radiator. The engine runs quite hot, half way between the Normal and High on the guage.) Also it is backfiring. I'm not much of a motor head...please help!!
Ken Bonn

Ken, if you are getting air bubbling thru the rad you may have a blown head gasket. Have the cooling system pressure tested. If the cooling system passes the pressure test you may just need a tune up.

The radiator neck on an "A" is also the long reach neck. I have a stock Stant cap I bought from NAPA. There book has all the dimensions for the caps listed.
John H

John... Boy I hope you're wrong about the head gasket!! Would that cause the overheating problem or is it just that the cap is too high a setting (7#) and is not letting the air bubble out into the overflow enough?
Since you're here...Could the backfire be caused by gunk in the SU float chamber and can I remove the cover on them without taking the carbs off the engine?
Will something fall out otherwise?
Ken Bonn

Welcome to the board, Ken. I think we'll have to deal with your problems one at a time, things can get confusing otherwise.

Do you see bubbles coming out of the coolant when you're looking down into the filler neck? Maybe you should take it to a radiator shop and have them look at it. They probably wouldn't fix the head gasket, but would probably tell you if they think that's what the problem is.

You must have a backfiring problem, and that could be lots of things. Maybe start another thread for that.

It would be a good idea to look in your float bowls for gunk no matter what, it's really easy. Be careful though, whatever you're doing. As a novice, which we all are to a degree, things can surprise you.

Do you have a good manual to refer to?
Tom

Ken,

FWIW, Clausager states that MGBs had a 10lb radiator cap from inception till a change to 13lb in conjunction with a radiator design change for the 1976 model. However, my Leyland Australia manual lists the 7lb cap for the 18G/GA engines with no change to that indicated up to the end of 1972 production. I know there is a body of opinion here that Bs should be running a 7lb cap but mine has a 10lb and is running at just under the 'N' here in the last month of winter - no oil cooler fitted at present, motor possibly running a little rich.

I mention this only because a 3lb cap increase will increase boiling point and so may be a factor in the running of your car. Of course, if your head gasket is in fact suspect that 3lb difference may bring on a failure quicker (but not rather than) a 7lb cap might. On the other hand it just might reign in your temperature a little.

Hope its not the gasket, but not the end of the world if so.

Regards
Roger
Roger T

I've had my roadster 17 years and all that time, when the engine is hot and running, there is a small escape of air from the overflow pipe. When you switch off, the escape gradually reduces, then stops, then becomes an *intake* of air as things cool down.

Never stopped to think about it for years until I suspected I head a head gasket problem on my V8 which was causing air bubbles to get into the radiator. Unlike the roadster (which has the filler/pressure cap on the radiator) the V8 has the remote pressurised expansion tank, and the air bubbles were pushing coolant from the rad into the expansion tank, and eventually out of the overflow, emptying the rad. Because the roadster has the filler cap at the highest part of the system there is only air there, so no loss of coolant.

As I say I only started to get concerned about the roadster after many years, but could not really understand a) why it was happening and b) why if it was a head gasket problem was I not getting any other symptoms. That was several years ago, it's still doing it. I've stopped worrying about it.

If you have visible air bubbles in the rad with the engine running get a combustion leak detector, that will be more indicative than a pressure test. Mine (V8) wasn't combustion gases, I can only surmise it was the water pump as I replaced that when I did a top-end rebuild and the problem went away.

High temp gauge reading could be any number of things - temp sender, voltage stabiliser, mixture, timing, gunk in the rad or engine, cooling fan type and fitting, etc.
Paul Hunt 2

Ken, if it is backfiring thru the carbs it could just be in need of a proper tune up or you may have an air leak in the intake manifold or the carbs. An air leak in the exhaust can cause backfiring from the exhaust. As mentioned above do a good tune up first, ignition timing can effect engine temp, backfiring, etc.

Changing the presure rating of the radiator cap does not change engine temp, it only changes the boiling point of the system.
John H

John,

I agree on the pressure cap, I mentioned the increased boiling point. However, if the temp sender unit has a fixed response to temperatures, then raising the boiling point *may* affect its representation of a given temperature on the dial ie it may reduce the dial reading of Ken's actual temperature.

If not, wouldn't we have to presume an intelligence for the sender unit ie that it was able to read the cap pressure and adjust its response?
Regards
Roger

Roger T

Electric senders vary resistance to temperature and so are independant of pressure. Capiliary gauges use pressure, but unless the capsule in the head deforms in response to varying pressure it also will only indicate changes in temperature. I submit that both respond to temperature only. Raising the boiling point won't in itself have any effect on the temp gauge, but raising the actual temperature will - i.e. the gauge will read higher. You should be able to get all the way up to the start of the H section and still not get boiling or coolant loss, the 'normal range runs from just above C to just below H, it isn't limited to being right over the middle of the N. In practice under most circumstances it should run pretty close to the N, but in extremes of temperature it could be either side by a significant amount and still not be a problem for the engine - for the occupants maybe .

In normal use the system pressure will be less than the cap pressure, the cap pressure is only intended as a safety valve. While investigating problems on my V8 I was getting pressures that would exceed any cap I put on the car (a 20lb cap caused a spectacular bursting of the bottom hose, the standard cap is 15lb) but once resolved the normal running pressures ranged from 2 or 3 lb in free air to a maximum of 10lb just as the electric fans cut in, which showed as mid-way between N and H on the temp gauge. Electric cooling fan cars also had a 15lb cap.
Paul Hunt 2

Great info everyone. Thank you. The air could be because I forgot to open the heater valve when I refilled the rebuilt radiator. (OOPS) That could reduce the amount of coolant in the system, no?
Could it be that the temp guage is out of calibration, and what would any of you use to check the temp of the coolant?
The timing is right on. New wires and plugs. I will do a valve clearance tomorrow. Does everyone use .015"? I've seen .013 and .014 argued.
I have a Haynes manual and I bought a .pdf of the factory maintenance manual. Internet a wealth of sources.
I've drained the gas and am putting in fresh with dry gas and a lead substitute to see if maybe there was water or rust or just old gas still in the tank.
Thanks once again...Ken



K Bonn

Some feedback; I fitted a recovery bottle as suggested and after 1100 miles water level static when cold. Thanks for advice
george
G Burrow

This thread was discussed between 25/07/2007 and 07/08/2007

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