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MG Midget and Sprite Technical Radiator cap BBS discussion at MG-Cars.net
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MG Midget and Sprite Technical - Radiator cap
| Hi Chaps:- here's today's conundrum...and yes, l know l need to get out more... The rad cap on my midget is " 7lb"...does this actually mean a lifting pressure of 7 psig when fully tightened down on the flange of the radiator? Thanks. Dave |
| David Cox |
| I believe it does David. Conventionally referred to as "cracking pressure". I've seen them with various different pressure rates and sometimes with SI units (bar or kg/cm2 etc.) on modern aftermarket caps. Also I think that any Spridget engine should have a 13psi cap but I might be wrong about that so I stand ready to be corrected. HTH. |
| Greybeard |
| I believe that cars with cross flow radiators do have 13psi caps, but the earlier vertical flow rads have lower pressure caps, probably 7psi. |
| Dave O'Neill 2 |
| Early cars have the lower 7lb/sq in cap - I think I read somewhere the early rad can't stand the higher pressure |
| Malc Gilliver |
| Yep- If you're using a radiator/cap pressure tester you select the fitting same depth as the radiator neck/cap ,screw the cap on, fit the pump/gauge on the other end and pump him up---If it's a seven pound cap it should crack off the seat at 7lb and should hold 7lb--if the pressure drops off the cap needs replacing-or sometimes it's just rubbish that has got sucked in under the little vac valve in the centre of the cap making it leak,-that can be cleaned out and retest. But yeah to answer your question 7lb is as Grey. says, the crackoff pressure and that's the pressure your system runs at normal running temp. |
| William Revit |
| Thanks for help as ever... I know there is some history about this in the archives but l ask after looking at replacement caps which seem to be different. Who knew that a simple thing like a rad cap could lead to such a rabbit hole๐. Dave |
| David Cox |
| GRC110, is the SHORT NECK (25mm) 15lb cap for the expansion tanks on the later 1275 and 1500 x-flow radiator setups.
-------- For the Vertical Flow Radiator, you should order a LONG NECK 7lb cap, 34mm long, measured from the outside of the cap to the outside of the vacuum relief valve. Some equivalent part numbers to the FC41A are: GRC101 FRC64. CHT105 See picture Here's the archive thread in technical year 2011. Last entry Posted 19 October 2011 at 14:02:12 Title: " 7lb longneck Radiator Cap, part No's " Enjoy. Lol  |
| anamnesis |
| Yes and any downflow radiator car will always boil over at a lower coolant temperature than any crossflow radiator car (all things being equal) because of that lower poundage cap (1.7 degrees C per pound of pressure). |
| Daniel |
| All interesting stuff, thanks...reason l ask is my car burps some coolant after almost every run despite correct filling level and a NOS cap. Investigation shows that the sealing rubber on the cap is not parallel with the inner ridge on the cap meaning that when screwed down on the cam there is an imperfect seal to the rad flange. The rabbit hole just gets darker...๐ Dave |
| David Cox |
| David after some (many) overheating issues I fitted a new radiator from Motaclan (Leaceys) last year but the cap fitment never properly sealed.
I compared it with another at the shop which was the same but the cap bayonet device doesnt like to seal at all on long neck rads. I ended up dropping a large rubber O ring inside for the seal to press against, but however I do it there is always a rusty stain across the rad neck's flat piece. I have just begun removing the engine to replace the clutch and hopefully the gearbox and after its run to Caen the radiator was still full but again with the familiar rusty water stain on top. |
| Bill sdgpM |
| Not sure if I am missing something here ( like a brain cell) but surely on a vertical flow without expansion tank, the sprung cap goes on the radiator. But on a cross flow with expansion tank the pressure cap goes on the expansion bottle My MK1. Sprite is a hybrid arrangement that I put together using a pressurised expansion tank, (7lb cap fitted) and a plain sealing cap on the tank header. I'm not sure, are there pre cross flow arrangements that use an expansion tank as standard? |
| GuyW |
| To answer your first question Guy, yes. And no, you appear to be repleat in the brain cell department. To answer your last question, I don't think so Guy. Not for Spridgets anyway.
I did the same as you, before finding my original vertical core was partially irreversibly clogged. I though used a 15lb cap on the plastic expansion tank. Then I converted to a crossflow rad. Not only much cheaper that recoring, or buying a new vertical flow rad, it results in more room in front of the engine, and is far better at cooling, such that the electric fan was no longer needed. And a bonus, I made an alteration that enables me to have the rad out in about 5 minutes or less. Part of the delay really being the time it takes to drain. Perhaps the problem isn't the caps, it's the poorly made necks in newer replacement rads. But if the cap fits, you should wear it. ๐คฃ๐คฃ๐คฃ. Unless, ---- it's a pain in the neck. ๐๐ |
| anamnesis |
| Ain't nothing so bad as a leaking rad To make me act so funny, make me spend my money When the cap's too loose like a long-necked goose Nah baby, that ain't a-what I like. https://youtu.be/Uk_oJ6mDV3w?feature=shared |
| anamnesis |
| Hi All...l tried fitting a rubber O-ring which stopped the burping but also caused the cap spring to lock out thereby preventing any lifting to vent. Thrre has to be a sweet spot which balances the sealing pressure against ability to lift and vent. I don't want an ICE powered steam powered bomb catastrophy๐ The radiator is the one fitted in the factory in 1966...the filler neck is ptobably the only bit that's not been patched๐... All good fun |
| David Cox |
| 7lb is pretty low. The engine, and heater unit can take more. The hoses and clips can take more. I ran 15lb pressure on my original 1966 vertical rad. It didn't leak. Like Guy, I added a separate expansion tank and used a short neck cap. You need a long neck. Try a long neck 13lb. Perhaps the QH FC64. But double check the cap top fitting flanges and diameter. Plenty on ebay circa 10 quid.  |
| anamnesis |
| Cheers all...if it were easy it wouldn't be half as interesting. Would it?. |
| David Cox |
| Dead right. ๐. Hasn't been easy since bl disappeared, and even then there were sometimes problems. But keeps us on our toes. ๐ |
| anamnesis |
| I get the feeling that this BBS is populated by a contingent of blokes of a certain age who are squirming on the twin tines of Morton's Fork. One the one hand tryng to make sense of a world which seems to be running away from me and growing ever more bonkers; and on the other dealing with the tricks and mischief dealt us through trying to keep, in an electric world, our ever increasingly preposterous machines running.. Or so it appears Dave |
| David Cox |
| Ha ha. Got me for one bang to rights there. ๐. |
| anamnesis |
| I see David is STILL on the ball, bang on with me too. |
| Bill sdgpM |
| I confess to having looked up the meaning of Morton's Fork..... |
| Bill B |
| Me too Bill ๐ค |
| Jeremy MkIII |
| ...every day's a school day๐... |
| David Cox |
This thread was discussed between 02/07/2025 and 07/07/2025
MG Midget and Sprite Technical index
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