Don’t let a flimsy rubber cap become the weak link in your engine’s cooling system. HPS High-Temp Reinforced Silicone Caps are purpose-built for enthusiasts who demand reliability. These caps are engineered to withstand extreme heat and pressure where ordinary vacuum caps would fail, providing a permanent, worry-free solution for sealing coolant bypass and vacuum ports.

Each HPS coolant cap is constructed just like our performance radiator hoses – made from premium silicone reinforced with a 3-ply polyester mesh. This robust build ensures the cap won’t crack, harden, or blow off even under prolonged high-temperature, high-pressure conditions (rated from -65°F to 350°F). In other words, it’s designed to last for years in the harsh under-hood environment.

HPS Reinforced Silicone Caps are incredibly versatile. They come in multiple sizes to cap off a variety of connections: coolant lines, heater core outlets, turbocharger water ports, throttle body coolant ports, vacuum nipples, and more. Whether you’re doing an engine swap, upgrading your turbo, or fixing a leaky factory cap, this is the go-to solution. (Note: Not for use with fuel, oil, or other petroleum products.)

Key Features & Benefits:

• One-of-a-Kind Solution: Not an ordinary vacuum cap – this is the only silicone cap on the market with multi-layer reinforcement, purpose-built for coolant and high-temp applications.
• Handles Extreme Heat: Outperforms rubber with a wide operating range of -65°F to 350°F (177°C), making it ideal for hot engine bays and race conditions.
• 3-Ply Reinforced Strength: Three layers of polyester reinforcement prevent bulging or splitting under pressure, so the cap holds tight even in high-boost or high-pressure cooling systems.
• Permanent & Reliable Seal: Solves leaks for good. Unlike cheap caps that dry out and crack, the HPS silicone cap provides a long-term fix for coolant bypass ports and vacuum line stubs.
• Resists Aging: UV and ozone resistant silicone construction means no premature dry-rot or brittleness – it stays flexible and secure over time.
• Multiple Sizes & Colors: Available in a range of inner diameters (from 1/4" up to 1-1/4") and three colors (Black, Blue, Red) to suit your project and personal style.

Technical Specification:
Product Type: Reinforced Silicone Coolant Cap
Temperature Range: -65°F to +350°F (−54°C to +177°C)
Reinforcement: 3-ply Polyester
Available Inner Diameters: 1/4" (6.5mm), 5/16" (8mm), 3/8" (9.5mm), 7/16" (11mm), 1/2" (13mm), 9/16" (14mm), 5/8" (16mm), 45/64" (18mm), 3/4" (19mm), 25/32" (20mm), 7/8" (22mm), 1" (25mm), 1-1/16" (27mm), 1-1/8" (28mm), 1-3/16" (30mm), 1-1/4" (32mm)
Colors: Black, Blue, or Red
Material Resilience: UV & ozone resistant

Remark: Silicone caps are NOT compatible with fuel or oil.

Size Chart:

• If one of your stock coolant bypass caps fails and you’re thinking about replacing it with a standard vacuum cap from the local auto parts store, keep in mind that’s only a temporary fix. Although they might look similar, generic vacuum caps are made of plain vinyl or rubber with a low heat rating – not nearly enough to handle actual coolant temperatures.

  The HPS Reinforced Silicone Coolant Cap, on the other hand, is a permanent solution for those leaky coolant bypass and vacuum ports. Don’t wait until it’s too late. Preventative maintenance with high-quality parts will save you time and money in the long run.

  Automakers often use one engine across many different models, adding or blocking off coolant ports depending on the vehicle’s needs. For example, the GM LS/LT V8 platform uses a port on the cylinder head to feed the heater core in some cars, while in other vehicles that same port is simply capped off at the factory.

  
  The same engine from two different vehicles with entirely different coolant line configurations. The engine on the right supplies hot coolant to the heater core via a port on the head, whereas the engine on the left uses a cap to block off that port.

  From the factory, these unused ports are sealed with simple rubber caps – one of the weakest links in any cooling system. Unlike a radiator hose (which typically has fabric reinforcement), those rubber caps have no internal reinforcement at all. As the rubber heats up and ages, it hardens and loses its ability to hold pressure. After years of heat cycles, it’s not a matter of if the stock cap will fail, but when.

  Recognizing this flaw, we engineered a far better solution: a reinforced silicone cap that can handle the job while also taking advantage of silicone’s superior durability.

  The construction of our Reinforced Silicone Coolant Cap (RSCC) is what truly sets it apart. We use premium silicone because it has a much wider operating temperature range (-65°F to 350°F) than standard EPDM rubber (-40°F to 275°F). On top of that, each cap is strengthened with a 3-ply polyester core (versus zero reinforcement in the factory cap). This multi-layer design prevents expansion under pressure and makes the RSCC significantly stronger and more reliable than any traditional rubber bypass cap.

  
  Close-up cross section of the 3-ply reinforced HPS Reinforced Silicone Coolant Cap.

  From engine swaps to pressure testing and race prep, we’ve found countless uses for the HPS RSCC. If you’re deleting a heater core or re-routing coolant lines during an engine swap, our caps let you securely block off those ports without resorting to a looped hose. It’s also perfect for performance mods – for instance, bypassing the throttle body’s coolant heating for lower intake temps is simple and safe with an RSCC in place.

  Even turbocharged setups can benefit. If your turbocharger is oil-cooled and doesn’t need coolant, you can cap the unused coolant feed/return ports with an HPS RSCC (just like we did on our own 2JZ-GE turbo project). And beyond the cooling system, these caps are great for replacing old, cracked vacuum line caps throughout the engine bay – preventing vacuum leaks with the same high-temp durability.

  
  Our 2JZ-GE test engine runs a Borg Warner S300 oil-cooled turbocharger. It doesn’t require coolant, so the factory coolant port on the engine block is now safely capped off with an HPS RSCC.