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MG MGB Technical - more poor parts

Had a trip out yesterday to a classic car do about 15miles down the road.
The car is due a service today so I started the usual checks when I got back.
Despite a check before I went out, the header tank was empty!
No sign of any leaks so I gave the pressure cap (newish item)a close look, i.e., measured the metal seal to rubber seal dimension and compared it to the header tank. They were very similar, so I checked all my selection of caps (5), all bearing the same part number but all slightly different! I installed the longest one but am still far from convinced.
When I run the engine up to temp to drain the oil, i'll put a container under the overflow pipe and see if the warm up merely pushes water past the cap rather than building up 15psi.
Allan Reeling

Unless the coolant level is too high, or you get it really hot i.e. more than boiling point, it shouldn't push anything out anyway.

After a normal run, where the cap has been fitted from cold, you should get a release of pressure if you remove the cap - carefully just in case. At this time of year and unless you thrash it it's not likely to get up beyond a few psi anyway ... unless you have a head gasket leak!

To go higher than that you need to block the rad off and let the temp gauge rise towards H, or until air or coolant starts coming out of the overflow. You can check for that by putting the overflow pipe in a container with a little water in the bottom. If the cap is releasing you will get bubbles first, then coloured (presumably) coolant.
paulh4

The point I'm making Paul is the variety of working lengths in supposedly the same product, with the same part numbers.
Allan Reeling

As long as the seal sits on the inside rim of the neck and the spring exerts the right amount of force to pressurise the system to its rated (cap) pressure then overall length is irrelevant.
Chris at Octarine Services

Allan
I was looking at pressure caps yesterday (GRC110 / GRC1100, G 1050, 15lb, for 3/4" neck IIRC), seemed to be 2mm difference on the one I was looking at 27-29mm overall height.

I was looking as I have a problem with another newly made part (that was a replacement for the previously supplied part that was faulty). The faulty part isn't fitted to a B but it is from a manufacturer who supplies B parts so if I continue to get the runaround and "yours is the first I've had back" I might put up details.



Nigel Atkins

I did take the point, Allan, I was referring to your proposed test. The difference in lengths has been around a long time, but unless you get the coolant up to cap pressure by blanking off the rad you won't know if a 'short' cap (or one that appears to be the correct length for that matter) IS holding the correct pressure.

A few years ago there was discussion elsewhere about this and people were finding caps of 22mm and necks of 26mm, whereas I measured both mine (top fill) at 20mm. Looking at parts catalogues Mk1 radiators and caps were often describe as 3/4" or 19mm although one owner posted that his rear-fill was 20mm. The MGA apparently had a 1" or 26mm neck and cap, and an aftermarket radiator supplier quoted theirs at 7/8".

Another annoyance is the rubber ring that has appeared and fits between the spring seal in the cap and the very top of the rad. This makes the cap extremely difficult to remove and replace on my radiators. It's not needed on our cars as it is intended for coolant recovery from an unpressurised container. But before I started seeing those, i.e. with just the spring seal, when I had a cooling system problem on the V8 coolant recovery worked perfectly well without it. As well as making turning very difficult the excessive mechanical pressure also wears the underside of the 'bayonet' fittings on the neck which will tend to make a given cap hold less pressure, so I discard them.


paulh4

Chris,
The point again is that the cap I took out was so similar in working length to the neck depth that I doubt there was significant contact AND a couple of my "selection" were shorter (although the same part number). Yesterday I got my engineers blue out, guess what? The shorter ones didn't touch the neck seal surface!! You can feel that they're not seating as no vertical pressure is necessary.
Nigel,
"yours is the first I've had back" . How many times have we heard that one. If it's a relatively cheap part, most probably think returning is not worth the effort anyway, but checking out forums often reveals the truth.
Paul,
I am talking V8!!!!
Allan Reeling

V8 or roadster is immaterial, both my 73 and 75 necks measure 20mm even though the cap pressures vary of course.

When I took that rubber ring off it seemed that mine wasn't compressing either, but that was only from comparison with before. By putting variously a pressure gauge and a Schrader valve in adapters I have been able to measure both the running pressure and the cap pressure on both cars.

If some of your caps don't even reach the neck, have you measured the neck?
paulh4

Bit late to this but--
Usually when you sit a cap on a radiator it needs to be long enough to have around 1/8" of compression left in the spring
You should be able to twist it onto the first click and the spring be under light pressure and then tighter when fully twisted on-
Any garage worth his salt should have a radiator and cap pressure tester--
Why not take your caps in and get them tested
-and yes they will need to know the depth of your filler neck to enable them to select the correct fitting for their tester--
To me it sounds like all your caps are too short
willy

Paul
Just to clear things up there are two different methods of setting up a recovery system
#1- Pressurised radiator, sealed cap and recovery bottle vented to atmosphere
#2-Radiator with blank cap and pressurised recovery bottle with the pressure cap on the recovery tank
If you have removed the upper rubber seal on the cap with system #1 it won't work properly as it can't suck the coolant back as it cools off
Removing the seal on system #2 wouldn't effect it's operation
William Revit

Paul, I did point out that I had measured the neck.
Nigel, I've just been to measure the overall dimensions you gave. Seems to me the tolerance shown is down to a) a difference of 0.87mm in the depth of the brass washer and, b) the depth of the recess in the top of cap which varies by up to 0.5mm.
Willy, the requirement of a push against the spring seemed logical to me. Some of the selection (1050) I had "rattled"!
The one I ended up using did, at least require some pressure and this showed by a "pressure" ring on the rubber.
The motto is, you can't rely on a part number for accuracy any more!!
I feel a little project coming on, I think I'll have to fit a valve to the radiator bung to allow me to put some pressure in the system!
Allan Reeling

Allan,
I assumed there would be variations in dimension from different manufacturers. Mine fits the expansion bottle and I've never noticed any difference in feel when fitting two or three different caps that I can remember.

Some posters used to have copies of the old paper cross-reference catalogues with dimensions in for pressure caps (IIRC there are three different to Spridgets).

The "yours was the 1st" reply made me smile, the previous part lasted about 10 years or so unlike others I'd tried so I wasn't expecting issues this time but I've enough experience to know things can change.

Nigel Atkins

William - I do know about the two types. The MGB (V8 and 77 and later 4-cylinder) use the second system you describe, although that is a remote expansion tank system, the radiator being completely filled, not coolant recovery as such.

I was using coolant recovery before ever I saw a cap with the additional rubber seal. With the rad caps I have had the spring-steel seal did the job. That has been a temporary arrangement on both cars at various times - the remote expansion tank on the V8 has an overflow tube just like the earlier 4-cylinder rad neck. But an MGB shouldn't need it, under normal conditions there should be no flow of air or coolant through the overflow tube.

paulh4

I knew you would know Paul, but unsure which system Allan's car has , it's a v8 conversion I think so could be either or-
willy
William Revit

Can of worms.

When returning my car to the road last year, I thought that my cap didn’t feel like it had any pressure on it, when tightening.

I know that I had bought a new one back in the ‘80s, but I couldn’t remember whether I had realised at the time that my car had the ‘wrong’ radiator. It’s a ‘67 Mk1, but with a centre-fill rad.

So, I ordered a cap for a centre-fill, using the part number given in the Moss catalogue. The supplier, however, convinced me that I needed a different number. I didn’t fit this cap, but decided to swap it for what I believed to be the correct cap, on my next visit to the supplier.

When attempting to fit this new cap, I couldn’t tighten it, as I discovered the rubber ring where there hadn’t been one before. Having recently read about removing the rubber seal, I thought about trying the new cap again, but having since moved house, I’m not sure where it is.

When I find out where I’ve put it, I shall give it another go.

I will also take some measurements of cap and radiator neck.
Dave O'Neill 2

FWIW when I measured my cap with the rubber seal I got 18mm whereas the neck was 20mm - doomed to fail, you might think. But when fitted the rubber compressed (you can see the wear/pressure groove in the above image) and the steel seal was pressed back. Rubber seal removed both were 20mm, but again when fitted the steel seal is pressed back which allows the main spring to press the main seal against the lower part of the neck. The litmus test though is to pressurise it and see exactly what it IS able to hold.
paulh4

This thread was discussed between 01/04/2019 and 03/04/2019

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