Sierra 18-70925 MerCruiser Molded Cooling Hose

When a cooling hose on your MerCruiser starts weeping at the thermostat cover, you don't want to fight a length of straight hose into a tight bend. That's exactly what the Sierra 18-70925 solves. It's a molded hose, pre-formed to the shape it needs on the engine, so it holds the correct curve from the thermostat cover down to the port exhaust manifold without kinking or collapsing. Water flows the way it should, and the install goes a lot faster.

This one fits MerCruiser stern drive and inboard engines running the V6 and V8 small blocks, 305 and 350 cubic inch. If you've got one of those classic MerCruiser powerplants, this is the short molded run that carries water between the thermostat housing and the port side manifold. Get the fitment right by confirming your engine model and year before you order, since the routing on these blocks changed a little over the production years.

Sierra builds these molded hoses from EPDM tubing with a synthetic knit reinforcement. That matters. EPDM handles the heat and the water chemistry well, and the knit reinforcement layer keeps the hose from ballooning or collapsing under the pressure and vacuum swings a running cooling system sees. It's the same construction approach the original hose uses, so you're not stepping down in quality to save a few bucks.

It works on both fresh water (closed) and raw water (open) cooling systems. On a raw-water boat, that hose is moving lake or sea water straight through the engine, so keeping it in good shape is part of keeping the whole cooling circuit healthy. On a fresh-water-cooled setup, it's carrying coolant through the closed loop. Either way, a molded hose that actually matches the bend is less likely to chafe or crack than a forced-in straight piece.

Here's how we'd approach the swap. Pull the old hose and take a good look at the thermostat cover and manifold gasket surfaces. Scrape them clean and flat, because a rough or scaled surface is where a slow leak starts, and it'll show up within a few weeks if you rush it. Slide the new molded hose on dry to check the routing first. It should sit naturally in its curve with no tension pulling it off the barbs. Keep it clear of the exhaust manifold surface where you can, so you're not resting rubber against a hot casting. Then set your clamps snug but not crushing, and run the engine on a set of muffs to check for leaks before you splash.

A cooling hose is a smart thing to knock out while you're already in there. If you're pulling the thermostat cover to replace a thermostat, or you're servicing the water pump impeller, doing the molded hose at the same time means you're not back in that cramped spot again next season. On saltwater boats especially, give the whole cooling side a look every year. Hoses, clamps, the impeller, the thermostat. They're cheap parts that prevent an overheat, and an overheat is never cheap.

Who's this for? Anybody maintaining a MerCruiser V6 or V8 sterndrive or inboard who wants the correct-shape replacement instead of guessing with universal hose. It's a direct, OEM-quality molded piece. Who's it not for? If your engine isn't one of the 305 or 350 cubic inch MerCruiser blocks, or the hose you're chasing is a different run than the thermostat-cover-to-port-manifold leg, check the part number against your engine's cooling diagram first. Molded hoses are shape-specific, so the right number matters more here than it does with straight hose. When in doubt, pull your old hose off and match the bend and the barb sizes against this one before you order. That's the surest way to know you've got the right leg for your engine.