Radiator caps , anyone know why so many?

Anyone know why there is so many diffrent pressure rated Radiator caps

4lb , 7lb , 13lb , 15lb , as the XK engine’s in diffrent car’s are more or less the same , I would have thought they would have had the same pressure cap !

After all it’s just a basic valve right , to let off too much pressure .

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The higher the pressure the higher the boiling point. As systems and components improved manufactures were able to raise the pressure caps. If my memory serves me correctly modern cars run sealed systems and only have an emergency type PSV to relieve extreme pressure. I also seem to remember that the drive for higher pressure operating systems stems from environmental and engine efficiency goals.
Far more knowledgeable chaps (the various Paul’s) will correct me if I am wrong (again)
Best regards

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Your explanation looks good from my house!


To add to Phil’s explanation, Jaguar used different pressure ratings depending on whether or not the car was equipped with air conditioning. Seven PSI without and thirteen PSI with, IIRC.

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You need to consider the whole system, and not just the radiator cap in isolation.
Once upon a time, all cars ran unpressurised cooling systems, but as the engineering improved, pressurised systems were introduced to increase the boiling point of the water/coolant. Initial efforts were usually limited to 4 psi, as with XK cooling systems, but that involved upgrading the entire cooling system to be reliable and not subject to failure at 4psi. Thus radiator cores, all hoses, hose clip design, pipe design etc had to be improved to withstand 4 psi. The pressure cap ensures the cooling system doesn’t exceed 4 psi, and thus exceed the design of the cooling system. Later on, as there were greater and greater demands on the cooling system, all aspects were improved, allowing higher operating pressures without cooling system failures, thus the 7, 13 and 15psi caps that limited the operating pressure to 7, 13 and 15psi. Its not an exact science, and levels of confidence come into it. I personally will always stay with a 4 psi cap on my XK140, albeit at a touch would probably be able to get away with 7psi, but I wont. I definitely would NOT ru with a 13 or 15psi cap, as that would be extremely high risk of cooling system failure.

Many years ago now I had an XJ40 saloon, one weekend fitted new top hose from a local Jaguar supplier, who had sourced 1/2 price top hoses of same shape/size ex Volvo. Car started up and ran OK, so drove around to a friends, parked car on his drive, went inside and came back to see a gushing pool of green coolant all over the drive way. The new Volvo hose had split along the entire top, hose-clamp to hose clamp.

Seems the Volvo hoses were speced/made for their 7 psi cooling system, and not strong enough for the XJ40s 13/15 psi designed system.

So unless you are prepared to risk all aspects of your XK cooling system, or prepared to upgrade the pressure rating of everything pressurised in system, stick with the original 4psi radiator cap, and hope that any aftermarket hoses etc you buy are well made/good for 4psi. With XKs one of the first places an over pressurised system will fail, apart from cheap hoses, is in the heater radiator core. These heaters were not designed/built for much more than 4/7 psi.

If your XK is overheating, don’t blame the radiator cap - find the real problem and fix that.

Thanks for that Roger
I don’t think Jaguar knew what cap was best , not sure what it was at first on the MK2 , but they changed it to 9lb , then after a few months down to 7lb , then down to 4lb , so I have read !

I will stick with the 4lb .

I can understand diffrent pressures with diffrent manufactures , but not why so many on the XK engine ,

I know a Engine will do more work with Air con on , so may get hotter , but can’t see every hose and heater components being diffrent

I would say Jaguar knew best , but the more I read the less I think they knew :grin:

Data point: my 3.8 E Type, with an after market radiator, ran a 13 psi cap, most of its life.

Not that it was really needed (except when it was being raced) because, with said radiator, the trmp rarely exceeded 90C.

No other issues were ever noted.

My earlier post referred to what was commonly found on E-types. I don’t think Jaguar changed anything but the pressure cap when they went to a 13 lb. cap for AC equipped Series II cars.

I’m hoping my XK120 will be safe with the 7lb cap. I have a very well-made looking NAR alloy radiator, all new hoses and clips, plus the block has just been cleaned out and new core plugs fitted. A slight worry is the heater - a new one was fitted in 1998, though Jaguar Cars fitted the previousl one when the car was just under 2 years old in 1952.

the other issue re radiator cap over 4lb on the XK120 thru early Series 1 EType is the thermostat : XK120 thru early Series 1 E, have a Smiths bellows sleeve type thermostat, which can be damaged if over 4 lb cap, and will be damaged .,.if over 7lb. …when,the coolant does reach a high enough temp to pressurize the system. (WHEN it does…,not if)
see recent post on XK Forum: Radiator Cap for XK120 early 140,

The actual original cap used on a 3.4 litre XK150 is a C.10289 which is
rated at 4 psi, and originals were made by AC (UK) See recent posts on the XK forum,"Thermostats for the XK120 a study,

There various reasons for the variation in coolant pressures. One being it raises the boiling point of the system, the other coolant system design.
For early Jaguars it was a design restriction based on the presiding orthodoxy of the period.
S1 and S2 E’s had a water pump seal that was spring loaded carbon faced and as such coolant pressure was limited by the spring pressure. Any pressure higher than 7lbs. could overload the seal spring and dump the water thru the pump.
When the USA introduced it’s emission requirements in the early 70’s, to pass them, one of the requirements was higher a engine operating temperature (82C), thus the introduction of 11lbs. plus coolant pressure.
Further tightening of emissions required higher operating temps. (88C) with a further increase in coolant pressure as you see today, with 15lbs. + the norm.
Whilst the 69 on Jaguars still used a carbon thrust water pump seal, it had a nuch higher spring pressure to accommodate increased cap pressures.
Just as aside, the original cooling system temp. of 76C. was based on achieving the best wear rate for bore and piston rings.
Though the operating temp. that provides the most efficient fuel burn is 82C., and this is what I recommend as part of my cooling mods.
For those that have to pass an annual or regular smog test, you may have to install the 88C. thermostats to pass.


just to emphasize…the XK120, 140, 150, and early Series1 3.4 E Type, and the then current saloons use a bellows sleeve thermostat which will self destruct if much over 4PSI; even the later Super stat thermostat replacements with the sleeve at the top are for maybe 7lb max. For this reason do not exceed the original factory fitted 4lb caps. You don’t need to anyway, as with anti-freeze 50 50 the boil point at sea level is 223F. (106C) A 4lb cap adds about 3 degrees…but elevation takes away 3 degrees per 1,000 ft. Anyway…223F (106C) with 50 50 anti-freeze…at sea level…so lets say you are at 3,000 ft: …well you can make your anti-freeze 60-40. (Do not exceed 70-30.) Jaguar Factory Manual and Service Bulletins for the XK120-14-150 state 70C as the normal op coolant temp at the inlet to radiator (before coolant is cooled). This is where your temp sensor is for the gauge. We all know that only on a very cool day will our XKs ever run at 70C…and op temp depends what thermostat is installed. The 70-74C was standard, if you have a 74C…guess what …your coolant will run at 74C minimum. See my post in the XK forum on thermostats. The later non bellows stats for later cars, can operate at higher temps…so the later E Type caps are higher psi AND the rest of the cooling system is designed for that. But, in the XKs, stay with a 4 lb cap and keep your temp under 100C for a safe boil margin to the 223F, and you are fine. ( Sorry to mix C and F, but the shop manual, SBs, thermostats deal in C, and the anti freeze products in USA list F.) As Roger P. and others said…the 4lb system has other 4lb design areas as well in the core plus, water pump and radiator construction. Don’t use radiator cap psi as a means to “fix” a hot running coolant… The problem is elsewhere anyway. And…by the way…the cap should not be compressed to release coolant until and unless it reaches the 4lb, or whatever point…exception is that if you fill the radiator all the way to the top…about 1.7 pts of coolant WILL be expelled…this is normal, and the level then will be at what is the normal fill level. This per Service Bulletin 121 which also states a new water pump impeller. As is well stated by Norman, for later E Type…the thermostat (not a bellows sleeve type) and the normal op temp, and the radiator cap psi are all increased. Too cold a thermostat…thus maybe on cold days, too cold coolant creates more wear, poor combustion. For these reasons for the XKs Jaguar SB 167 recommends a switch from the 60C stat to the 74C stat if carb spit back occurs and that the factory made that change in production

.( I guess I could have said …just use the cap psi as was fitted new at the factory…but the topic is…“WHY”.)

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Thanks to everyone for contributing such useful comments on cooling systems. Great to learn these details.

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I have a later, replacement cap, for the XK120, the AC RC5, 4lb. round, 72mm top , smooth-no tabs on the top.two lock tabs on underside lip of the top, it is 23.9mm (61/64in) on this one, from top of cap, to bottom of seal. The Rubber seal is fixed with in the cap base. Visible spring. AC Delco, Made in Dunstable Inland, on the top “Turn Down Tight with arrow beneath, and Remove Slowly” with arrow beneath: also on the top: RC5 K7 AC 4.
Important Note: some replacement caps seem to fit,…but are not deep enough to contact the neck seat. A deep reach (apprx 1 in) cap is needed. Some were 1 1/4 and likely too much. Do not exceed the 4lb. The thermostat can be damaged. If you want to have a higher boil temp just make the anti-freeze a 60-40, or 70-30…but keep the 4lb cap. Boil temp.it is quite high (223F 106C at sea level) at 50-50. add 3C for the 4lb cap…but subtract 3C for each 1,000 ft elevation. Just stay under 100C and you are OK.