The Right Way to Drain Manifolds and Risers for Winter

If your boat will be exposed to freezing temperatures, draining the raw water from your manifolds and risers is essential. These heavy castings hold water in low spots, and if that water freezes, it expands with enough force to crack metal. Repairs are costly. Prevention is simple, if done correctly. Here’s how we do it in the shop — and what you need to check before calling your engine winterized.

Why Draining Matters

Manifolds and risers are part of the raw water loop. Even on closed cooling systems, the raw water still passes through the risers, and often the outer jacket of the manifold. These are the most common failure points after winter if not properly drained or filled with antifreeze.

Step-by-Step: How to Drain Manifolds and Risers

Let the engine cool Never remove drain plugs on a hot engine. Wait until it reaches ambient temperature to avoid burns and pressure release injuries.
Locate the drain plugs Most manifolds and risers have brass or stainless plugs at the bottom corners. Some use petcocks, while others require a wrench to remove. Check both sides.
Open all drain points Remove the drain plugs and let water flow out fully. Use a stiff wire or pick if the port is blocked. Sometimes sediment or scale prevents full drainage.
Remove hoses if needed Some systems trap water due to air locks or poor routing. If you suspect a low point is still holding water, remove the hose to check for standing liquid.
Tilt the trailer or boat slightly if possible Elevating the bow can help drain low areas in the riser or muffler, especially on certain hull designs.

Common Mistakes That Lead to Freeze Damage

Only draining the seacock or raw water pump, but not the manifolds or risers
Leaving hoses connected that trap water inside the casting
Assuming a closed cooling system protects the risers — it doesn’t
Not checking that all plugs were actually removed and cleared
Replacing drain plugs while water is still dripping out

Alternative Method: Pumping in Antifreeze

If you prefer to fill the raw water side with antifreeze instead of draining, run non-toxic pink marine antifreeze through the system until it exits the exhaust. This method works, but only if done thoroughly. The antifreeze must displace all the water in the risers and manifolds. If mixed or diluted, it may not provide full protection.

Inspect While You Drain

Winterization is also the perfect time to look for signs of corrosion, leaks, or damage. Check for rust streaks near gaskets, flaking paint, and uneven drainage between banks. If one side drains slower or less, it may already be restricted internally. That’s worth checking before the next season starts.

Conclusion

Draining your risers and manifolds for winter is simple — but it has to be done with care. One missed plug or blocked drain can cost you an entire exhaust system. We see cracked castings every spring from engines that were “winterized” but not fully drained. Follow the steps, double-check every fitting, and take the time to do it right. Your engine will thank you when it starts up clean next season.