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MG Midget and Sprite General - Frogeye Radiator Cap Confused?

Help just bought 2 new radiator caps one is 1 inch in length, the other 3/4 inch, both 7 lbs which one is the correct one? For a 1960 frogeye with an original radiator?
Chris Banton

Hi Chris and welcome.

The correct one is the one which doesn't lose water.
According to my parts list the correct part number is ARH1039 but as that is no longer available, to use new part number is ARH1542.
Is it for Laura's car or have you got a new Frogeye?
Gary & Gaps

It's for Lauras car been suffering poping water! Out of overflow pipe After driving saw mothy had the same problems but had two different height radiator caps!
Chris Banton

Was this right after you filled it up?
Frog and early midget cooling systems are "total loss" systems.
This means you fill it up
Run the car to operating temp
Let the exess water out through the overflow
Done.

Now if you remove the radiator cap you will see water level is lower. This is your normal level.
Only when it gets hotter than normal more water will be expeled.

Later systems have an expantion tank to deal with this
Onno Könemann

Thanks have done this but the radiator cap I realised has been 4 lbs for 15 years!

So bought 2 caps one qh and one show quality...but they are two different heights, saw mothy had the same problem which one is correct for sprite mk1......
Chris Banton

Where are you measuring the length from?

I have a 7ib cap on my '66 sprite that's years old. I measured it at about 1 1/4".

Mines a Unipart. Doesn't have a number on it, although it's so old it might be invisible.

Lawrence Slater

I have measured from below the cap...I read there are different length caps for vertical flow and crossflow rads
Chris Banton

Hi Chris,

I can measure mine tomorrow, it has the virtical flow rad.
I know that when I swapped back from the crossflow to the virtical flow rad I used the cap from my now redundant expansion tank and that was the wrong one. So if someone measures their expansion tank cap, that will be to wrong one.
Gary & Gaps

Why not just measure the depth of the neck of the rad?
Guy

because we don't know by how much the spring compresses when the cap is fitted and screwed down....
David Smith

The same situation exists with the MGA/MGB of the similar period.

The answer to that however was simple and equally I think the answer is maybe the same for the early Frogeye.

The 2 lengths for the MGA/MGB radiator.

If you got the shorter one (fitted to early MGBs) then it didn't even make the bottom of the rad for the MGA so was obviously the wrong one. So the longer one was the one to choose.

So I think you need to check the distance and hopefully if the distance is such that the shorter one does not actually fit to the bottom then use the longer cap. If both would reach then I would use the short one.

The pressures of each cap are a little arbitory in so much as either pressure will work. The benefit of the higher pressure is that water will only boil at a higher temperature However if you needf to have water boiling at well above its norm of 212degrees then you need to address blockages in your cooling system.

As pointed out by Onno expect to lose some liquid as the system does not use a recovery header tank as more modern vehicles. I would personally install a modern system simply for piece of mind, I did has many others have done on my MGA it is a simple process.
Bob Turbo Midget England

My Rad cap is from my Vertical radiatior. I'll make a fresh measurement from inside the cap.

Rob,
Is it possible to install an expansion tank alongside a vertical Rad?

Could one for example, remove the spring and seal, and use that as a cap on the vertical rad, and then use the overlow pipe to plumb to the expansion tank, and use a 7lb pressure cap on the expansion tank instead?
Lawrence Slater

The measurment inside the neck of my vertical flow rad is exactly 1 inch from the sealing face to the seat.

It's not that clear, but the picture shows the measurement form inside the cap, to the metal flange below the rubber seal, to be 1 inch also.

When you include the rubber seal (mines worn thin) it's about another 1/8 of an inch.

Lawrence Slater

Lawrence
Yes you can!
But make sure you do not use a higher rating cap on the expantion tank as the rad might not take very well to that
Onno Könemann

Yep Lawrence that is ""exactly"" how to pipe in an expansion tank.

And in your case the shorter radiator cap would be of no use in the present system as it would not seal the bottom neck of the rad and so would not pressurize the system

Obviouly then a simple measure could identify the cap required.
Bob Turbo Midget England

another option is to instead of converting yours to an expansion tank (pressurized along with the radiator), is to fit an overflow tank (how all modern cars are)

the only difference is that the overflow tank is not pressurized (simpler, cheaper)

you run a hose from the overflow pipe on the radiator neck to the bottom of a jar somewhere

that's it

Take a close look at the radiator cap on our radiator and you'll see a small "pop valve" in the middle, that allows air to go back into the radiator as the coolant cools off and shrinks while the engine is shut off. That vacuum will draw coolant back in, from the overflow tank, keeping your radiator topped off in case there is ever a small leak somewhere or loss to evaporation over time, and the jar, if translucent, is easy to keep topped up.


Actually, they both work fine, an overflow or an expansion tank. But as a retrofit to an existing radiator, an overflow is just simpler is all.

Norm
Norm Kerr

Thanks for those confirmations Onno/Rob.

So just to clarify, as unintentionally, I seem to have altered the content of this thread. (Sorry Chris).

Are there two different versions of the length of neck on the spridget vertical flow Radiators?

Is this why there are 2 different length 7lb rad caps?


Norm,
That sounds a great idea. I had no idea that was the case. If so I'm surprised it wasn't done to begin with. When my coolant overflows, it just dumps under the chassis. This has already resulted in my having to repair rust damage to the ARB mounting point on the nearside becaus my overlfow pipe was a bit short. (Simnce extended).
Lawrence Slater

This supplier has the same 1 inch(ish) long 7lb rad cap. Listed as part number GRC101.

http://www.classicparts4cars.co.uk/mg-midget-radiator-cap---grc101-131-p.asp.

It's a QH £3.49 Plus postage.

I've decided to get one and keep as a spare in waiting, irrespective of changing my setup to an expansion or overflow capture system.
Lawrence Slater

Thanks for the info but two different suppliers have supplied two different caps one long one short...the short one said midget 1958-1974...the long said midget 1961-1974...will download pic!
Chris Banton

A radiator cap should be the same from the frog until the early mkIII midget.
I think they switched to the crossflow rad in 1970 certainly not 74.
Onno Könemann

Chris,
there was a thread on this subject not so long ago so have a look in the Archives, sorry I can't remember the outcome

the X-flow rad (with expansion tank) has a 15lb cap on the tank and came in around '67 I think

N Atkins

FINAL Outcome

The longer cap QH FC41A has sorted the problem no more loss of coolant after driving car!

Thank you for all of your advice..

Chris Banton

Lawrence,
if you're getting a new cap you might as well put it on the car

. . . and keep the existing one just in case the new one has rubbish rubber seal and cauese trouble

are you sure you want to risk replacing your existing one the seal is only slightly cracked :)

being serious now - if your coolant oveflows could it be that the pressure cap has gone weak with age

ETA: well done Chris keep a note of that number !
N Atkins

Glad that's sorted then.

Hi Nigel,
No I think it's as Onno says. When you fill up, it finds it's level and discharges the rest. Of course these systems do still require a top from time to time, so I get that discharge after I fill up, and then it's good for a while. Evaporation I would assume acounts for the gradual decline in the coolant level, and in the course of driving some must get expelled anyway in the normal course of events.

As this is so cheap, I will get one for Ron.

Pitty you can't just buy the rubber gasket, I thought you used to be able to.
Lawrence Slater

Lawrence,
I find once the car finds its coolant level it stays there

forget Ron (he need never repays the cigs) put it on now

or as an experiment put it on for say a year and see how many times you need to top up with the new cap

you can still keep your existing cap and then after the new cap's year put the old cap back on for a year see which one really is your reliable friend
N Atkins

Nigel.
But I thought you had the later type of xflow rad, with the expansion tank? That catches the expelled coolant, and discharges it back into the rad when the engine cools down. That's why you rarely have to top up yours.

Lawrence Slater

Just a word of warning with downflow radiators. Don't assume that just because you can see water in the top when you open the cap, that the rad is actually full. Depending on the angle of your rad, the angle that it's parked and the condition of the filler part, the radiator can still be quite empty with only a small resovoir of water sitting under the filler cap hole.
Gary & Gaps

Nige
the x-flow rad came in april 68

And if an vertical flow rad needs to be topped up depends on how hot it gets.
Mine does not need any topping up if temp stays below 90ish
If it goes higher topping up is preferable (not instantly needed)

Onno Könemann

Lawrence,
possibly if you change from your present pessure cap to a new one then you might need top up less quanity and less often - you'd have to try that experiment to find out :)

Gary,
a girlfriend of a friend had a French 80/90s car that didn't have a temp gauge but did have a see-through plastic expansion tank that was full of clean coolant yet around the engine bay was evidence of previous rust coloured water, to cut the story short the rubber supply hose that came out the bottom was totally blocked so none of that clean coolant made it to the rad but gave the impression the rad was full

Onno,
I don't normally remember dates by '67 was firmly in my mind but I don't know why so I can't justify it :)
N Atkins

Yup, probably will nigel. I'm not aware that I have to top up any more than I ever used to, but you might be right at that.

Actually, when I do get the replacement cap, I'm going to try Norms' idea of the overflow reserviour. I like very simple uncomplicated solutions, they suit my simple uncomplicated mind.

Lawrence Slater

An upate to this.

I was at my local parts shop today, and they had an old QH Brochure. It listed all the Rad Caps including the Long neck 7lb and the short neck 7lb.

Part number QH FC41A is the long neck version
Part number QH FC41 is the short neck version.

There is a picture in the Brochure that shows the correct way to identify them. (I'm going back there on monday and will take a pic to post here.).

The measurement is taken from the OUTSIDE of the cap to the outside of the vacuum release valve, (as shown in my first picture below Posted 14 October 2011 at 08:46:09).

The correct measurement is 35mm for QH FC41A long neck.

The measurement is 25mm for QH FC41 short neck.

I believe the short neck version fits the expansion tank on the later 1275's fitted with x-flow rad.








Lawrence Slater

Well done Lawarence the addition of the 'A' to the part number explains it

perhaps you need to put up a thread called something like

Rad cap 7lbs part numbers

to make the info easy to find when searching archives

one thing though pressure cap for (expansion tank) X-flow 1275 rads is 15lbs
N Atkins

Good idea Nigel. when I get a pic of the caps measurement diagram in the brochure, I'll post it.

But there is still some confusion then.

15lbs would raise the running temp of the engine by some amount. --- (x) degrees per additional lb of pressure.

So the later 1275s with a x- flow rad will run hotter.

Is that right?

I thought only the 1500 had the 15lb cap and that does run hotter.
Lawrence Slater

Only the x-flow rad had the 15 lbs cap for bothe the 1500 and 1275.
It does not mean it runs hotter it means it can get hotter before boiling.
Onno Könemann

Onno,
I have to confess, my understanding of the expansion properties (physics) of water when heated, is not as good as it could be.

Why was the original 1275 only fitted with a 7lb cap?

Was it because the original rad, was too weak to stand the increased pressure of a 15lb cap?

There must have been plenty of other engines of the time running at higher pressure, so why were the early spridgets running at a max of 7lb coolant pressure?

Why not make a rad capable of taking a greater pressure?

Why increase it to 15lb for later engines?
Lawrence Slater

the reason why is

- companies gradually increased the pressure to deal with overheating issues in export markets when the system was not in full health (especially with the lower grades of antifreeze available back then, and folks using tap water instead of distilled water, that the performance of a system could degrade in just a couple of years - higher pressure helped reduce the number of warranty claims, a little bit).

- but, higher pressure required tighter quality control to prevent leaks that would occur, so they were reluctant to go there and only increased it as needed. Higher pressure costs money to manufacture (both in the cap, and in the cooling system itself, as the quality control of all of the components must be held tighter, to prevent leaks)

- eventually, the whole industry transitioned from "atmospheric" cooling pressure, up to 5 ~ 7psi, and later to 14~15psi, to some today that run up at 20+ psi.

It is all a matter of economics. If the system is kept up to snuff, then the extra isn't needed, except in the more extreme markets (like Phoenix).

At first, BMC adopted the larger, more expensive cross flow radiator to deal with overheating issues in the colonies, but it wasn't quite enough, so they eventually also adopted the 15psi working pressure too.

Maybe someone who worked in BMC, or in the dealerships, at the time could shed more light, but that is my take on the history, based on my understanding of the general industry through the '60s & '70s.

Norm
Norm Kerr

It is a bit of what Norm said and a bit of something else.

It is a mix of these factors

The vertical flow tank has a big reservoir that might not hold the 15lbs

But the switch to x-flow was also done to facilitate the airpump etc for the smog midgets.
The big rad tank of the vert flow would clash with these systems.

The x-flow system has a slightly smaller volume than the vert flow and so needs the added pressure to as Norm said give it some safety margin
Onno Könemann

Excellent thanks guys.

I suppose the complexities of the cooling system, poundages and the like, was never that much of an interest/issue, until these cars aged and parts had to be sourced other than from main dealers.

With part numbers getting changed and linked incorrectly to the original part numbers, it's no wonder that we can end up with the wrong part for the right part number.

So it seems now to be worth better understanding how it all works, in order to avoid fitting a bad or incorrect part.

As with the front wheel bearings saga.
Lawrence Slater

This thread was discussed between 13/10/2011 and 16/10/2011

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