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MG Midget and Sprite Technical - Expansion tank. Filler neck depth/rad cap sealing.

I converted my Sprite to a crossflow rad, with an expansion tank. It's a plastic tank, I can't remember what it came off.

The depth of the filler neck is just over 20mm

The sprung depth (not the external measurement) of the correct short reach 15lb radiator cap, is barely 20mm too.

The rubber seal of the radiator cap ( new cap), is barely making contact with the slightly raised sealing ring in the bottom of filler neck (I don't mean the metal ring at the top)

My question is, is there anything missing from inside, at the bottom of the filler neck?





anamnesis

Could it be that there is some corrosion on the underside of the metal ring and this has lifted the metal ring making the depth slightly more than it should be?
Simon
S Holt

Hi Simon. No, I've had it out, it's clean/smooth.

Even if I take that steel ring out, the spring in the cap is barely compressed at all.

Seems to me, the cap is too short.

The long one is a QH 7lbs. The short one is TRE 15lbs short reach. I've got 3 new, all identical.

The original I had, rusted and snapped. Unfortunately I binned it before checking it's length. I have a feeling it must have been a couple of mm longer, in order to compress the spring.





anamnesis

Presumably the spring should be only very lightly compressed when the cap is fitted. The spring then provides the pressure to resist the set lbs rating. If it were already compressed when fitted, then it couldn't move to release any excess pressure.
GuyW

I assume so too Guy. But I feel no compression at all, when pushing down on the cap; -- before twisting/locking it into place.

There's no indication at all, that the rubber seal is even touching the bottom of the filler neck.

Either these caps are a tad too short, or something is missing at the bottom from my filler neck.

Has anyone here got the same expansion tank?
anamnesis

As far as I remember every pressure cap I've come across on cars, boat engines and what have you have all required pushing down against the spring to locate them, some more firmly than others as you would expect with higher cracking pressures.
That said, if you're not losing fluid it suggests that the cap is sealing well enough.
Or perhaps your engine doesn't get hot enough to pass fluid through an unsealed cap, which I doubt given what you have said about your driving style.
I don't think I've ever seen one with any sort of separate seal in the bottom face of the neck.
ETA: I think you need to locate a longer reach cap. I also wonder if 15psi is a tad excessive.
Greybeard

Thanks Grey, that's what I remember with the old cap I threw out. I had to push down against the spring. But not this new one.

15lbs is the book figure for the crossflow rad and expansion tank.

I maybe losing water, I topped up today, and I have noticed since using the new cap, the gauge is reading higher, close to 190 all the time; --- 82 stat (180f).

I wonder if I can raise the seat in the filler cap, effectively making the cap length greater. A rubber washer maybe.

anamnesis

Agreed, Anam. I wasn't saying the spring shouldn't compress when fitting the cap. Simply that the initial shortening of the spring needs to be relatively little, leaving movement available in the remaining spring for it to lift off should the system pressure rise too high.
It does sound like your spring stem is too short.

Just checked the one in my 1275 cross flow (expansion tank). From the face of the rubber seal up to the top SS sealing plate under the cap is exactly 20mm. Its a 15lb cap G1050 I don't know if its right but has been on that car for some years, without any problem.
When fitting the spring barely compresses, or at least I cannot feel it do so. Though there is friction on twisting which feels like the rubber washer and there are indentations in the washer where it has clearly been making firm contact.
GuyW

Thanks Guy.

G1050. That's the same number on my cap(s). Brand TRE. Same 20mm measurement on the cap.

Did you happen to measure the working depth of the filler neck?

If yours works, then I suspect it may be that the seat at the botton of the filler neck in my plastic expansion tank has worn down. There are no marks on my cap seal at all; suggesting only the lightest contact if any.

So I'll either need a new/replacement tank, or to somehow raise the seat. Maybe I can epoxy a washer of some kind in there.
anamnesis

Could you contrive a packer to go above the rubber seal - between it and the sprung backing plate?
Maybe a washer cut from an inner tube with a centre hole large enough to stretch over the suction seal in the centre? Or 2 C shaped bits of something ( plastic?) epoxied into position.

Shouldnt have to though. Either something is wrong with the parts, or there's nothing wrong and you are inventing demons! 😂
GuyW

Some more measurements, taken using the depth pin on a digital caliper gauge so reasonably precise:
Both my cars use the brass expansion tanks. Critical depth of the neck to seal is 19mm on both. Corresponding measure on 1275, 15lb cap is 20mm and on the other 7lb cap its 25mm

The 7lb system is my own modification for the frog. Its a standard vertical flow rad with the overflow taken to a later style expansion tank. Simple flat, non sprung, cap on the rad and this 7lb pressure cap on the expansion tank. Interestingly, though I hadn't realised, the spring in this cap is compressed by 6mm when fitted, whilst that on the 1275 with a stiffer correctly matched 15lb spring only compresses by about 1mm

All works fine but I wonder if I should look for a short spring version of my 7lb cap?
GuyW

Thanks Guy, that clinches it.

Just measured my old vertical flow rad neck depth. About 25mm, a tad under. Correct long reach 7lb cap working length, a bit over 27mm. So about 2mm compression.

I was wondering about the poundage too.

Suppose as a temp' fix, I put the long reach 7lb on my crossflow expansion tank set up, instead of a 15lb short reach. I'll get about 7mm of spring compression

Would that cause a problem? If 7 works to prevent boiling on a vertical, why not a 7 on a crossflow?





anamnesis

My 7lb cap used on an expansion tank gives 6mm 'static' compression, which is clearly more than was intended as the correct 15lb cap for that type of expansion tank seals with just 1mm compression. But as a modified system, it seems to work just fine. I presume therefore that the spring compression is pretty linear, and the pressure required doesn't increase substantially the more the spring us compressed.
If its not linear, then I may be operating my vertical flow rad at something a but over +7lbs but it hasnt leaked, yet!
GuyW

If you run much more than 7psi in a vertical radiator, you will find the top tank will start to bulge - this does not look good, and difficult to fix without removing the top tank from the core.

There are at least 3 different 7psi radiator caps - all with slightly different reach.

Safety Fast

Tony
The Classic Workshop

A L SLATTERY

Thanks Tony, for that advice. So far there is no evidence of the top tank malforming. The cap I am using is rated at 7lbs and I have assumed the spring tension is uniform. But if a shorter version of a 7lb cap is available that would be a safer bet. Do you have part numbers?
GuyW

Ok thanks Guy. I'll try my 7lb long reach in my expansion tank tomorrow. 70 mile round trip on anice sunny day will give it a test. But if you haven't had a problem, I see no reason why I will.
anamnesis

By the bye, don't be tempted to stick spacers in with epoxy. Most epoxies (not all) let go at high temperatures, inconveniently round about boiling temperature.
Ditto "superglue" or cyanoacrylate. They eventually dissolve when exposed to hot water. Standard A&E treatment when people show up with their fingers glued together!
If you want a spacer I'd try a washer cut to fit accurately from, say, 2mm neoprene sheet and pushed into the bottom of the neck. It can't go anywhere.
But a longer reach cap is a better solution. I think I have a couple in the drawer at work that I replaced because boatyard put 7psi caps in the CATs which need 13psi ones. Made a sh*tty mess of bright pink coolant all over my engine room, the rascals.
ETA: Okay I just noticed you have a 7psi cap already. As you were...
Greybeard

2mm neoprene.

Ok thanks Grey.

I'll check the spec on the epoxy I have. But as you say, probably not needed anyway.
anamnesis

Made a couple of washers from tge rear window I salvaged from my old soft top. About 1mm thick. With 2 washers, spring in cap is compressed nicely, and seals tight.
anamnesis

This thread was discussed between 23/06/2023 and 26/06/2023

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