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MG Midget and Sprite Technical - Radiator filler car references

Can someone with access to a suitable parts numbers catalogue help with this please.

What I am after is a radiator filler cap that is just a simple seal to the top of the filler neck. I have added an expansion tank to my system so it is now the expansion tank that has the 7lb pressure cap. The radiator itself just needs a sealing cap, not one wth a spring loaded seal to the base of the filler neck.

I have been searching for filler caps but all the illustrations appear to be of the spring pressure seal type, although I know that standard flat ones do exist! What I need is a part number to search with.
GuyW

Austin/Morris 11/1300 used a flat seal cap with a spring cap on the expansion tank. I don't know the part number, unfortunately.
Dave O'Neill 2

GRC126
William Revit

Thanks Willy. I knew they existed but my search efforts just came up with the spring loaded ones.
I have been using an old 7lb cap with its guts ripped out but the upper washer isn't sealing as well as it should.
GuyW

Gates for quality usually - Gates Sealing Cap, radiator RC117 (5 year warranty)

GRC126 is also known as GRC201 for other models and tractors.

And our old friends First Line do a B&B - Borg & Beck Radiator Cap Part No - BRC71

Also cross referenced as Nissan 214309C002 which might get better prices (Blue Print: ADN19913, First Line: FRC71, QH: FC57).

'Buyers Guide' here gives loads of possible alternatives - https://webcat.firstline.co.uk/PartDetails/BRC71/#partInfoBRC71

MGB Hive - https://mgbhive.co.uk/product/5-mgc-filler-cap-grc126-grc201/

Mini Spares - http://www.minispares.com/product/Classic/GRC126.aspx

I think of GRC as general rad cap and then see what the differences are on the numbers that follow, though I never take any alternative codes or cross references as gospel but they can be useful in alternative searches and often pricing.
Nigel Atkins

Thanks Nigel. I only need one cap ;-)
Ordered one for an MGA.
GuyW

Yes but it's good to have alternatives in case you've ordered one and it turns out to be piss-poor quality as is the potential for many parts now.

Plus the info is now here for future reference.

What annoys me is with say MGOC on their parts website they don't show a photo of the part yet on ebay they do, their photo on ebay shows a cap that *looks* of reasonable quality - but I'd still go for Gates.
Nigel Atkins

I seem to recall when I did that conversion, I just cut the spring pad and seal off my existing rad cap.
anamnesis

Talking of poor parts, I've just seen that a B owner has just got a horn slip ring without the hole drilled through the plastic for the horn pencil contact, rendering it useless without removing the metal ring and drilling through the plastic. He's sent it back for a refund.
Nigel Atkins

You can also use a 13psi cap on the radiator so that the 7psi cap on the expansion vessel is still the primary relief, but the stiffer one on the radiator acts as a backup to it.
I did this on my Diahatsu camper.
Usefully, the pressure rating is usually stamped on the top of the cap and they're pretty cheap from the likes of Halfords or many local garages.
Greybeard

I use a 7lb pressure cap on both the radiator and the expansion tank, works just fine.
Dominic Clancy

I don't understand how the rad, as it cools and contracts again, then draws coolant back into the main system if you have a pressure cap on the radiator?
GuyW

Guy,

Some pressure caps have a valve in the centre to allow coolant back into the radiator when the system cools.

https://www.allpar.com/threads/cooling-systems-the-surprisingly-wide-range-of-radiator-caps.229050/
David Billington

Thanks David. I think I knew that, just wasn't sure how the recovery valve actually worked, which is what I was meaning to ask. But that is an interesting read. Well written in a nice informative style without being too heavy! Thanks.

From that and greybeard and Dominic's advice, it seems I butchered a perfectly good 7lb rad cap that was probably perfectly compatible with my modified system!
GuyW

Guy

Coolant contracts when cooled, creating a pressure drop in the rad. The secondary vac valve opens up - the small thing like an engine valve at the base of the cap - and pressure difference draws the overflow back into the main system.
Oggers

Yes Oggers, that is why I have adapted my Frog system. The MK1 cars didn't use the coolant expansion tank like the later cars. I just wasn't sure of how the secondary valve worked. It seems that there are two types - spring loaded and gravity. And apparently some pressure caps dont have them anyway if intended for a non recovery system.
GuyW

A bit late Guy but my '78 Jaguar xj6 also has a non-pressure cap partnr.: C.28365
A de Best

This thread was discussed between 10/05/2021 and 12/05/2021

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