Marine engines live a harder life than anything on the road. Constant RPM under load, salt air or lake humidity, months of storage followed by hard startups — it all adds up. When your inboard finally gives out, you need a marine long block that’s built to survive that environment, not a truck engine with a marine sticker on it.
Here’s what to know before you buy.
What’s a Marine Long Block?
A marine long block — sometimes called a marine base engine — is a remanufactured or new engine built specifically for marine applications. It includes the block, crankshaft, pistons, connecting rods, camshaft, timing set, cylinder heads, and valvetrain. You bolt on your marine intake manifold, exhaust manifolds, and accessories (raw water pump, alternator, starter, etc.).
The key difference from an automotive long block: marine engines use materials and coatings that resist corrosion, and they’re built to handle sustained high-RPM operation under constant load. A truck engine cruises at 1,800 RPM. A boat engine might run at 3,500–4,500 RPM all day long.
Marine vs. Automotive Engines: What’s Different?
Can you put an automotive engine in a boat? Technically yes, and people do it. But you’ll need to swap the cam, add marine-grade exhaust manifolds, upgrade freeze plugs, and address corrosion issues. By the time you’re done, you’ve spent as much as buying a proper marine long block — and you don’t have a marine warranty. (For the differences between long blocks and short blocks, see our long block vs. short block guide.)
Common Marine Engine Applications
Most inboard marine engines are based on GM small block and big block platforms. If you’re replacing a Mercruiser, Volvo Penta, or OMC inboard, the base engine under all that marine hardware is usually one of these:
The GM 350 (5.7L) is by far the most common — it powers everything from runabouts to deck boats to entry-level cruisers.
What’s Included in a Marine Long Block?
A marine long block typically includes:
- Engine block (crack-checked, honed, and decked)
- Crankshaft (reground or new, balanced)
- Pistons and rings (new)
- Connecting rods (reconditioned or new, resized)
- Camshaft (marine-grind profile)
- Timing chain and gears
- Cylinder heads (rebuilt with new valves, guides, and seats)
- Lifters/tappets
- All internal gaskets
Not included: Intake manifold, exhaust manifolds, raw water pump, thermostat housing, distributor, starter, alternator, fuel pump, engine wiring harness, motor mounts, and all external accessories. These transfer from your old engine or need to be purchased separately.
How Much Does a Marine Long Block Cost?
Shop marine 350 5.7L Vortec long blocks at Bishop Car Parts →
Labor cost: Marine engine swaps run $1,500–$3,000+ in shop labor depending on the boat, engine access, and what else needs replacing while it’s out (bellows, gimbal bearing, raw water hoses, impeller, etc.). Limited engine bay access on most boats drives labor hours up compared to trucks.
Remanufactured vs. New: Which Should You Buy?
Remanufactured Marine Long Block
The engine is completely disassembled, every component inspected and measured, worn parts replaced, and the whole thing reassembled to spec. Machining work includes boring, honing, decking, crankshaft grinding, and valve jobs.
Pros: Significantly cheaper than new. Core exchange keeps costs down. Quality units carry 1–3 year warranties. Cons: Requires a rebuildable core return. Warranty terms vary by seller.
New Crate Marine Engine
A brand-new engine, zero miles, typically from the OEM or a licensed builder.
Pros: No core return. Longest warranty. Zero unknowns. Cons: Expensive — often 2x the cost of remanufactured. May have limited availability for older applications.
For most boat owners, a remanufactured marine long block is the right call. The savings over new is substantial, and a quality reman unit will deliver the same service life as new when maintained properly.
What to Check Before You Buy
- Confirm your engine model. Match by the engine serial number stamped on the block, not by boat model alone. A 2003 Bayliner could have one of several engine options.
- Marine cam, not automotive. Verify the long block is built with a marine-grind cam profile. This isn’t optional — a truck cam in a boat will be underpowered at sustained RPM.
- Corrosion protection. Brass freeze plugs, treated castings, and marine-grade coatings should be standard. If the seller can’t confirm these, you’re looking at an automotive engine in disguise.
- Warranty coverage. Marine engines run hard. A 12-month minimum warranty is standard; 2–3 years is better.
- Core return logistics. Returning a marine engine core involves getting a 500-lb block out of a boat and onto a pallet. Know the core return requirements and timeline before you order.
Where to Buy
Bishop Car Parts carries remanufactured marine long block engines built on GM platforms — the 350 (5.7L), 4.3L V6, and other marine applications. Direct-to-consumer pricing, no middleman, with marine-specific builds that use the right cam, bearings, and corrosion protection for life on the water.
Marine 350 5.7L Vortec long blocks in stock now:
