rma121

I've got a few radiator caps that I believe are correct for Mustangs. I'd appreciate help identifying the proper application for these caps.


Thanks.

bop11

They all say 13# so if they fit they are usable, just different manufacturers. Filler neck depth is 3/4"

Mark Hone

The middle one is correct for /66 mustang. M.

rma121

Thanks for the response. Would you happen to know if the center cap is correct for 66 Mustangs only, or did it come on other years? Also, a knowledgeable friend said the first cap came on 69 & 70 429 Boss Mustangs. Anybody know for sure? There is one on Ebay that appears identical, asking over $1,000.00.


Thanks.

Mark Hone

I can only speak for /66 mustangs, but I would bet that it would be O.K. for a year or two either way. M,

barnett468

.
there is no such thing as a $1,000.00 rad cap.

there is no reason to run a used 30 - 50 year old rad cap on a car.

buy a new repo cap.

the caps you have are worth around 5 cents.

Mark Hone

There's nothing like having a original /66 rad cap on your car (as long as it's working)

bop11

Barnett is right. Radiator caps do wear out. The relief spring gets either brittle from work hardening or loses all tension. In the first case the radiator pressure can go way up, in the second you lose coolant and the boiling point of the coolant goes down. The gasket at the bottom also goes pad as most are only a modified paper gasket material. If the car is a show car only, keep the original, but if it is a driver or a driver/ show, sooner or later you will have coolant all over that nice looking engine bay.

69mach1377

Why 13 lb? Will a higher rated one cause damage?

bop11

Increasing pressure increases the boiling point of the coolant



The boiling point of the coolant is also raised by the concentration of antifreeze.



Adding antifreeze lowers the heat capacity (cooling ability) of the coolant



The engine runs more efficiently when it is hot and the air charge is cold. With a 190° thermostat, some of the coolant will be running much hotter near the cylinder walls. To keep this from vaporizing the coolant, the coolant pressure is increased. This increase comes from the coolant expansion and the air expansion as it heats.

The pressure is a trade-off. A typical radiator is about 20 inches wide and 4 inches deep giving a surface area of about 80 square inches. At 13 psi this is a force of about 1000 pounds trying to push the top tank off and the bottom tank off. Since these tanks are soldered or brazed on we have to keep the radiator cap relief pressure to a safe point where the radiator keeps together but the coolant boiling point is as high as possible. Ford chose 13 psi.
The cap must also allow for air to go back into the radiator as it cools or the radiator and hoses would be sucked flat as the pressure went down. So the cap's ability to hold pressure and leak back are important. Either system can go bad, so it is good to replace the cap periodically.