K-Seal ST5516 Heavy-Duty Multi-Purpose One Step Coolant

K-Seal ST5516 is a one-step, heavy-duty 16 oz coolant leak repair that mixes with any antifreeze to quickly seal radiator, head gasket, freeze plug and water pump leaks without flushing the system.

If you need a fast, temporary-to-long-term fix for coolant loss in large engines, K-Seal ST5516 is formulated to circulate with coolant and locate holes and cracks, creating an internal seal while preserving cooling efficiency. Designed for commercial, agricultural and heavy-duty vehicles, this pour-and-go treatment works with all types of antifreeze and doesn't require draining or flushing your cooling system.

How quickly will it stop a coolant leak?

Most users see reduced coolant loss in minutes after adding the product and running the engine to circulate it. Complete sealing times vary with leak size and pressure, but the formula actively searches out leaks and adheres to metal and composite surfaces to form a durable repair.

• Starts working within minutes of circulation
• Effective on hairline cracks, small holes and weeps
• Works under system operating pressure for faster sealing

Which types of leaks and components can K-Seal ST5516 repair?

The formula is multipurpose and targets common failure points in water-cooled engines, including radiators, freeze plugs, water pumps, head gasket weeps and heater matrix leaks. It’s optimized for heavy-duty applications, so it’s suitable for larger engines with higher cooling demands.

• Radiator and hose connection leaks
• Freeze plugs and block casting defects
• Water pump and heater core weeps
• Minor head gasket seepage (non-comprehensive head gasket replacement alternative)

Do I need to flush the cooling system or change the antifreeze?

No flushing or coolant change is required before adding K-Seal ST5516; it is formulated to mix with any antifreeze and operate within an existing system. For best results, add the product to a warm, circulating cooling system so the sealant can distribute evenly and locate leaks.

• Compatible with all antifreeze types and concentrations
• Pour directly into radiator or overflow bottle per instructions
• No pre-flush saves time and reduces downtime

How long does the repair last and is it safe for my engine?

When used as directed on compatible leaks, the repair can be long-lasting, but longevity depends on leak size, location and operating conditions. The formulation is designed not to impair cooling efficiency and is safe for use in heavy-duty engine systems when used per label instructions.

• Engineered to maintain coolant flow and heat transfer
• Suitable for commercial and agricultural heavy-duty engines
• Not a guaranteed permanent repair for catastrophic failures

Who is this for: K-Seal ST5516 is aimed at fleet managers, equipment owners, farmers and professional mechanics who need a rapid, on-site coolant leak solution for large engines and industrial vehicles. It’s ideal for operators looking to minimize downtime and avoid immediate disassembly, while still preserving cooling system performance until a scheduled repair or replacement can be completed.