The small center vacuum valve is intentionally NOT held closed by a light spring.

When the cap is handled or shaken, the center valve may hang open, drop, move freely, rattle, appear loose, or “flop” back and forth.

THIS MOVEMENT IS NORMAL AND CORRECT—IT IS NOT A DEFECT.

If your vehicle requires a full-pressure cap with a spring-closed center vacuum valve, this is not the correct cap for that application.

The phrase “springless center valve” does not mean that the entire cap has no spring.

• Large main pressure spring: This cap has the large spring that works with the main pressure valve to regulate cooling-system pressure at 16 PSI.
• Small center vacuum-valve spring: This cap does not have the light spring used to hold the center vacuum valve shut on a full-pressure or constant-pressure cap.

The springless description applies only to the small center vacuum valve. Please see the comparison diagram included with the listing photographs.

UPPER FILLER-NECK SEAL (UPPER SEALING GASKET)
Seals against the top of the radiator filler neck. This upper seal is needed for proper coolant-recovery operation. As the cooling system cools and develops vacuum, it helps the system draw coolant back from the recovery reservoir rather than drawing outside air into the radiator.

LOWER SEALING GASKET
Seals against the lower seat inside the radiator filler neck. Together with the main pressure valve and large pressure spring, it allows the cooling system to build and regulate pressure at the cap’s rated 16 PSI.

SPRINGLESS, WEIGHTED CENTER VACUUM VALVE
The small center valve is not held shut by a light spring. It can hang open and move freely when the cap is handled. That is the correct open/weighted vacuum-valve design for this partial-pressure cap.

1. Cold start and light operation: The springless weighted center valve can remain open, allowing the cooling system to operate at atmospheric or minimal pressure.
2. As temperature rises: Coolant or vapor lifts the weighted center valve against its seat. Once seated, the cooling system can build pressure and the cap functions through its large main pressure valve and 16-PSI spring.
3. As coolant expands: When system pressure reaches the cap’s relief point, excess coolant can pass through the radiator overflow connection and hose into the recovery reservoir.
4. As the engine cools: Pressure falls, the weighted center valve opens, and cooling-system vacuum can draw coolant from the recovery reservoir back through the overflow hose and into the radiator.

This design allows the system to operate at low or minimal pressure during light conditions and at regulated pressure when hot, reducing continuous strain on cooling-system seals, hoses, the radiator and heater core.

This cap was designed to work with a properly connected coolant-recovery system. The radiator overflow hose must be connected to a functioning recovery reservoir.

If the overflow hose simply points toward the ground, discharged coolant can end up on the ground, garage floor or carport floor.

The cap offered in this listing is stamped E5 in the center. Under the date-code interpretation commonly used by collectors for these period caps:

• E represents May
• 5 represents 1975

E5 is therefore generally interpreted as May 1975. Buyers seeking a particular production date for an original or judged vehicle should examine the close-up photographs and confirm that E5 is the date code required.

NOS (New Old Stock)—new and unused without original packaging.

I purchased these caps in bulk in 1989. They were supplied loose in buckets rather than in individual Mopar retail packages and accumulated storage dust over the years.

Each cap offered in this listing is carefully cleaned, visually inspected and sealed in a protective plastic sleeve. The caps selected for this listing pass my cosmetic inspection and look like the examples shown in the photographs.

The photographs show actual examples from this E5 inventory. The cap received will have the E5 date code and will be comparable in appearance to the examples shown.

The large internal pressure spring may show slight natural variation in the color or coverage of its original protective finish. Some appear more coated while others appear more bare. This is normal manufacturing and long-term storage variation.

I did not originally purchase these simply for resale.

Beginning in the late 1980s, I operated a radiator-repair business out of the back of my father’s shop. I worked on everything from ordinary automotive radiators to large bulldozer radiators. My work included locating and soldering leaks, repairing damaged components, re-coring radiators, cleaning and servicing cooling-system components, and supplying radiator caps and related parts.

I purchased this inventory in 1989 for use in that radiator business. These are part of the remaining unused inventory stored since that time.

I have sold more than 220 of these same NOS Mopar 3781830 radiator caps through my previous eBay date-code listings. The part number, construction, 16-PSI rating and partial-pressure operation are the same; only the stamped production date code differs.

This new listing is specifically for the E5 date code shown in the photographs. The previous links are provided as references for documented sales history and buyer feedback.

Vehicle application charts and interchange information can contain errors, especially when dealing with original cooling systems, replacement radiators and vehicles modified during previous repairs or restorations.

Before ordering, verify:

• Mopar part number 3781830
• Required 16-PSI pressure rating
• Partial-pressure/open-weighted center vacuum-valve design
• Upper filler-neck sealing surface and lower filler-neck seat
• Properly connected overflow hose and coolant-recovery reservoir
• E5 production date code, if date-correct restoration is important

Do not order this cap solely because it appears dimensionally similar to another cap. If your application requires a spring-closed center vacuum valve, you need a different full-pressure cap.