Leaking Radiator Cap

[J Witt]

My '36 has had a leaking radiator cap since I bought the car. It came with a cap, without the wire bail, plus one of the, I think, Drake caps that has the bail. Both leak around the seal surface of the radiator neck. The leakage appears to occur mostly when the car is driven faster than 30-35 mph. Leakage has occurred with the original radiator and its replacement which is a new Brassworks radiator. The system has a Haney 3 psi valve on the overflow tube. I normally filll the header tank until fluid is about 1 inch or so down from the filler neck. Fluid is 50/50 antifreeze, engine temp about 190F.



Does anyone have suggestions about getting the caps to seal, or a replacement cap that works well.


Thanks to all,


John

[1931 flamingo]

Increase the size of the gasket.......................
Paul in CT

[Kube]

Originally Posted by J Witt

My '36 has had a leaking radiator cap since I bought the car. It came with a cap, without the wire bail, plus one of the, I think, Drake caps that has the bail. Both leak around the seal surface of the radiator neck. The leakage appears to occur mostly when the car is driven faster than 30-35 mph. Leakage has occurred with the original radiator and its replacement which is a new Brassworks radiator. The system has a Haney 3 psi valve on the overflow tube. I normally filll the header tank until fluid is about 1 inch or so down from the filler neck. Fluid is 50/50 antifreeze, engine temp about 190F.



Does anyone have suggestions about getting the caps to seal, or a replacement cap that works well.


Thanks to all,


John

Typically the fluid will be fine just above the top core plate. I'd start there - reduce the amount of coolant a bit and see what happens.

[Steve in Denver]

Don't put such a tight cap on it that you keep it from breathing. These are non-pressurized systems. I had a 7 pound cap on mine and blew the seam at the bottom of the reservoir. Does anyone know of a correct aftermarket radiator?

[V12Bill]

X2 # 3 You have too much coolant in the radiator. Increased speed means increased flow of water and backing up of coolant in the upper tank

[Henry Hopper]

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Originally Posted by V12Bill

X2 # 3 You have too much coolant in the radiator. Increased speed means increased flow of water and backing up of coolant in the upper tank

Interesting....I have similar problem with my 36.Would fitting thermostats help, ie slow the flow rate down?

[Reconnoiter]

FWIW i had a similar problem with my ‘34, would puke coolant from the cap and overflow once I hit about 50 mph, no overheating. Installed 180 thermostats in my hoses and that brought it immediately under control. Runs great, and no more drooling....

[J Witt]

I do have a pair of 165 thermostats in the outlets from the heads (59AB heads). If driven very briskly, these t'stats appear to be wide open, but of course they still have a relatively small flow area. This is in Georgia summertime 90-95 degrees. The problem may go away in the fall when it cools off some.


I have one more parade to do in a couple of weeks and it will barely stay below boiling running 1-2 mph with frequent stops. Hoping for a little cooler weather for that, or may pass it up.


As I mentioned the overflow has a pressure valve that opens at 3 psi. Coolant isn't coming out the overflow, but around the seal on the cap. Appears not to be a pressure issue, but one of high flow velocity.


I'll drop the fluid level some and test again.

[petehoovie]

Originally Posted by J Witt

As I mentioned the overflow has a pressure valve that opens at 3 psi. Coolant isn't coming out the overflow, but around the seal on the cap. Appears not to be a pressure issue, but one of high flow velocity.

Seems to me that if it's leaking around the cap you are not only losing coolant but pressure as well - the pressure valve will not work under this scenario. Do as 1931 flamingo said in his #2 response above....

[J Witt]

Steve, I have installed a Brassworks radiator my car. It fits the mounting holes and has more fin area than stock. The header tank is shaped approximately the same as stock, but doesn't have some of the small features such as stamped in ribs that are on the stock unit.


My original radiator core just got too thin to patch the fins anymore. It was springing pinholes as fast as they could be fixed and I finally got tired of taking it out of the car

[G.M.]

Originally Posted by J Witt

I do have a pair of 165 thermostats in the outlets from the heads (59AB heads). If driven very briskly, these t'stats appear to be wide open, but of course they still have a relatively small flow area. This is in Georgia summertime 90-95 degrees. The problem may go away in the fall when it cools off some.


I have one more parade to do in a couple of weeks and it will barely stay below boiling running 1-2 mph with frequent stops. Hoping for a little cooler weather for that, or may pass it up.


As I mentioned the overflow has a pressure valve that opens at 3 psi. Coolant isn't coming out the overflow, but around the seal on the cap. Appears not to be a pressure issue, but one of high flow velocity.


I'll drop the fluid level some and test again.

Don't drop the coolant level you had it where it should be. Go to the plumbing section in the hardware store and get a sheet of the red rubber
gasket material which is about 1/8" or more thick. Drill the rivet out
of your cap with a 1/4" drill. Get a round head slotted SS 1/4-20 screw
1/2 or 5/8" long, flat washer, lock washer and nut. Cut the round gasket
to fit and install. Use a little Loctite on the screw and it won't leak. G.M.