Radiator Cap Lbs Of Pressure???

[Powerhouse]

Anyone know what lb rating a 218 needs for a rad cap????? I have some aftermarket thing on there rated for 17 lbs....

[james curl]

0 lbs pressure

[greg g]

Doesn't matter the radiator vent is seperate from the filler and cap. Through 49 they were unpressurized.

[olskulow]

6 lbs of pressure.

[Powerhouse]

I should mention I have a 39 plymouth...the overflow tube is on the radiator cap neck....I also do not have a bypass water pump...

[55 Fargo]

Okay, but is this a pressurized type rad, say from a 1951 and later Mopar.

If so, you can use either a 4 lb or a 7lb cap, if it is a non-pressurized desgn, you can use whatever, because there won't be any pressure to build up.

Is this the original engine, or is a 218 from the early 50s, with an internal by-pass water pump and t/stat, if that be the case, and the rad is also a pressurized type, go with a 4lb cap. Good Luck............Fred

[Powerhouse]

well...1939 radiator...the engine is a 218 from a 54 dodge truck...I think that was the last year they had 218's...Anyway...It has an older water pump with the external hose port on top but it's capped off. I'll have to post pics so you can understand...

I really would like to know if I need a pressure cap or not...

[Normspeed]

For comparison I have a 53 Plym 218, recored radiator, I drive in the southwestern U.S in pretty extreme temps. I took out the rubber washer on my 7 lb cap so I run zero lbs pressure. I think the oldies are made to run with no coolant pressure as long as they are in good working order.

[Powerhouse]

I'm planning on driving to redlands from vista(north cnty san diego) soon...about an hour and a half drive. I wanna make sure she's gonna be runnin as cool as possible. The overdriv should help some.......

[Don Coatney]

A pressure cap used in a system designed for pressure does nothing to reduce engine temperature. It simply raises the boiling point of the water or coolant.

A pressure cap used on a system not designed for pressure (such as a top vented P-15 radiator) does nothing to reduce engine temperature. It simply keeps things from falling inside the radiator.

A pressure cap or non pressure cap does not care if the system has an internal or external by-pass.

[Powerhouse]

Here's what is in the car...same setup since i bought it...it's a 54 dodge truck 218 in a 39 Plymouth...and yes I LIKE silicone...

[billwillard]

The radiator on your car is not designed to carry 17lbs presure. You are lucky you haven't had a leaking radiator or worse. One thing for sure ,your head gesket is in good shape. If your car overheats with no presure look for other problems.

[Powerhouse]

OH...well off it comes tomorrow! I'll get a non-pressure cap from NAPA! I haven't had any real overly hot engine temps yet..but I just moved to San Diego from NEW YORK...so there will be a noticeable temp difference.

[james curl]

Does your pressure cap seal above the overflow tube? If it does you do not have any pressure in the radiator, if the rubber seals below the overflow tube then you do have pressure on the radiator.

[Lou Earle]

Most, and I believe ALL cars, are designed to run below 212 or boiling point at sea level. All a pressurized system does is increase the boiling point- that is increase the margin between the thermostat setting(160 to 190-maybe even 195 ) and the boiling point so it is safer to run higher temp thermostats.

Many a time I have had a leak develop in a vehicles water system and loosened the cap (so it had no pressure) and driven many miles- like 400 by just adding water. Just last week loosened cap on 83 Chev Pu because hose began leaking- ran fine no overheating.

If I were u I would drain the radiator put in a cup of powdered dishwasher detergent- electra sol etc- fill with water stuff a rag in the filer neck and drive it 15 miles or so. If it overheats u have problems other than cap- if not then drain and refill- you now have a fairly clean radiator and very little cost.

I suspect your 17 pound cap is not pressurizing your system because I think that much pressure would have blown a leak somewhere in your radiator.

Your engine and cooling system is designed to run with a 160 to 180 thermostat and NO pressure. Someone modified your tank by installing a new neck- I had 48 Dodge that way.

Put a 5 pound cap on it and have a good trip!!

[55 Fargo]

This my friend is the first of the pressurized rads that Mopar used, and should have a pressurized rad cap.

Go to NAPA, and buy yourself a 4lb rad cap, that should take care of business just fine.

This rad is not designed to run with a non-pressurized rad cap, this is a 50s engine and is designed to run with a 4 lb, and someitmes up to a 7 lb cap and system.....................Fred

[55 Fargo]

1951 and later rads, look just as this rad in the picture, a pressurized design, there first models of this kind.

I had one on my 47, it was from a 1951 Dodge, I eventualy replaced it with the non pressurized rad from Tim Adams, the pressurized rad is in Norway, in Kingsway 52s car..............Fred

[BobT-47P15]

The non-pressure cap I recently bought from NAPA cost about $2.69

plus tax. This is their stock number. They had to order it for next

day delivery to the store from the warehouse.

This is the underside of that cap.

[Lou Earle]

This my friend is the first of the pressurized rads that Mopar used, and should have a pressurized rad cap.

Go to NAPA, and buy yourself a 4lb rad cap, that should take care of business just fine.

This rad is not designed to run with a non-pressurized rad cap, this is a 50s engine and is designed to run with a 4 lb, and someitmes up to a 7 lb cap and system.....................Fred

I am assuming his radiator is in a pre 50 car-his profile show a 38 or39?

Anyway someone changed his radiator neck to supposedly a pressure cap type neck. But his radiator was NOT designed to have any pressure originally.

His the later 218 and 230 engines were not- to my knowledge re designed to run on a pressurized system. The water pumps and bypass was designed to save money and steel and probably iron- Korean war remember? The blocks are all interchangeable as are rods cranks pistons etc or example I have 230 crank and rods in 218 block. Cars started going to pressurized systems

so there would be higher boiling points especially in the mountains- here boiling can occur at 180. The higher boiling point allowed for higher thermostats and therefor warmer interiors. If I am wrong pleas explain what changes were made in engine itself that necessitated a 4 pound pressure system - other than installing a 180 thermostat??

The fact that someone modified the neck does not mean it won't run non pressureized it just allows overflow to go out that pipe rather than one in the center as orginal

Lou

[Powerhouse]

OK..so the top tank neck was converted to a pressurized type? Weird...I had no idea...they did a good job. So I guess the rad cap isn't pressurizing then...I would think at 17 lbs it would be a BIG problem. I'll change the cap anyway to a non pressurized one since it's only a few dollars...and it will look more stock.

[55 Fargo]

if it is a converted non-pressure rad to a pressurized, where is the top overflow tube?

I am not sure why this may haqve been done, please post a pic of the entire top of the rad, Mopar rads 1950 and later were pressurized, they still looked fairly similar to the non-pressure rad.

[Powerhouse]

the top tank has nothing else on it but the filler neck with the overflow tube coming out of it....

[55 Fargo]

If the rad is also from the 50s engine ,it was a pressurized typee, if this be the case, a 4 lb cap or maybe a 7lb cap should be the correct application.

The pre 1950 rad had an overflow tube on the top tank, so in the event the cpoolant pukes up, it will exit the top overflow tube.

In the pressurized rads, the overflow tube is in the filler neck, when the cap seal rises up, under presure the coolant can then exit out the overflow tube.

Not trying to be condesending to anyone, this is how these rads are designed to function.

I would also advocate the "Lou Earle Electrosol Internal Scrub", this works great, and does no harm to the engine, it does a lot of good actually.....................Fred

[Powerhouse]

OK..here is apic of the top tank...

and here is the numbers on the top tank...

ANY clue as to what model this is from???

[55 Fargo]

This is an early pressurized rad, I think, see how the top tank is flat on the top. The non-pressurized rads are sloped on each side rising to the center for those rads, the top overflow tube is on the top, above the filler neck level.

On yours the filler neck is the highest point, as it should be, it will overflow at the filler neck if it needs to.

I would go with a 4 lb cap, or a 7lb cap max, that should take care of that matter.

With 50/50 antifreeze and water, and a 4 lb cap you should be good up to about 230 degrees or so, before it would boil over, but I would not let it go over 200 on a hot day, before shutting this engine down to cool, if you are running 175 to 190, that should bef ine, any more is getting hot, any less is an invitation for sludge. Only my inexperienced opinion however......................Fred