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Can anyone tell me the proper radiator cap to use on a stock 100E/300E radiator and where I might be able to get one in North America? I see references to a "long reach" type cap, so I assume a typical modern cap won't work. Mine has a very worn lower (rubber) seal and a cracked upper (metal) seal. Thanks.

I don't know off the top of my head but the cap is definitely longer, a standard reach cap will not seal against the bottom of the well. Maybe you can cross reference this number to something in North America?

https://www.smallfordspares.co.uk/produ ... essure-cap

Measure yours and compare, there is another that is 1/4 inch longer.

https://www.smallfordspares.co.uk/produ ... sure-cap-1

Looks like mine measures 1 1/4" from the top of the cap to the bottom of the rubber seal. But I'm not sure if that's where they measure it from.

I only see Small Ford Spares listed for those numbers, but I'll keep checking around.

Thanks.

Correct cap Stant 10206 7 lbs long neck or NAPA 703-1411.

50's / early 60's EnFos used 4 lbs and 7 lbs long neck caps. 4 lbs caps aren't used anymore.

They are available on ebay ~ $10

Fred,

Just to clarify. Some 100E/300E models used the 4 lbs 1" reach caps EOA-8100A.

peteleo wrote: ↑Mon Oct 05, 2020 12:09 pm Correct cap Stant 10206 7 lbs long neck or NAPA 703-1411.

50's / early 60's EnFos used 4 lbs and 7 lbs long neck caps. 4 lbs caps aren't used anymore.

They are available on ebay ~ $10

Great. Looks like that NAPA one is in stock at my local NAPA store up the street. (Says it's manufactured by Stant.)

I went ahead and bought the NAPA one. It doesn't look like the original (nor the photo in the NAPA ad, which looks like the original), but it'll work for now. Fits great. Thanks again.

4lb was always the 100E cap, but 7lbs would do fine. All the pressure rating does is limit the blow-off pressure, it doesn't mean that's what the engine runs at, and it shouldn't reach 4lbs, whichever cap is fitted.

4 lbs caps were sufficient when only water was used as coolant and compression ratio ~7:1. Nowadays all engines use mixture of water / coolant which increases thermal pressure and prevents rust accumulating in the system and lubes the w/pump seal. Though 4 lbs cap would still be ok on a standard 100E engine with a coolant mixture.

That's a new one on me, Pete! Antifreeze does a number of things. Its primary purpose, of course, is to substantially lower the freezing point of the water with which it is mixed, but unfortunately it is itself corrosive, which is why you shouldn't spill it on paintwork. It does though contain additives which more than neutralise this corroding tendency inside the engine, which is why it is recommended to keep it in the cooling system all year round, especially in alloy engines or with alloy heads. But it also raises, although not by a lot, the boiling temperature of the water containing it, perhaps by a couple of degrees Celsius. Now water, like almost everything else, expands when heated so you have to allow an air space at the top of the system to accommodate this: whatever rate cap you have, expanding liquid will force it open if filled to the top, but the fact that the coolant would boil at a higher temperature gives a greater safety margin and allows for a higher rated thermostat, in turn leading to greater efficiency and, especially if you live in - say - Canada, a better heater! But I've never heard of antifreeze otherwise having any effect on the system's pressure.

Hi Jim,

Boiling point of water is 212F for every 3 psi is raised 1 degree F of additional pressure. Typical modern car normal cap is ~15psi will hold water in the system to ~260F. Adding ethlene or propylene glygol to water based coolant has to withstand temps below 32F or when its boiling point has to be raised.
What i meant by thermal pressure increase, i left out also increasing the thermostat range, will increase system pressure.
Using ie Evans waterless coolant lowers the temperature of the coolant system but is known to increase thermal pressure around the cylinder head causing blown head gaskets.

Ah, understanding dawns!

The concentration of A/F in modern cars is very high: it was 50-50 when I was teaching Motor Vehicle. I wouldn't try that level in a classic, though.

JAN wrote: ↑Sun Oct 18, 2020 12:36 pm Ah, understanding dawns!

The concentration of A/F in modern cars is very high: it was 50-50 when I was teaching Motor Vehicle. I wouldn't try that level in a classic, though.

So what mixture of antifreeze/water should I be using in my stock radiator/engine? Still 50/50? (My rad is currently getting some structural repairs and will be back soon.) And I'll next be onto trying to figure out what top & bottom hoses I can get in the USA that fit.

I'd stick with 25/75% A/F /water unless the temperature drops well below -10C, about 16F.