Sierra 18-1986-2 Cast Iron Exhaust Manifold, GM V8 305-350
Sierra 18-1986-2 Cast Iron Exhaust Manifold, GM V8 305-350 — Product description
The Sierra Marine 18-1986-2 is a cast iron exhaust manifold that replaces a worn or corroded manifold on a GM small block V8, anything from a 305 to a 350 cubic inch marine engine. It bolts to the factory head pattern and meets or beats the original part it replaces.
The Sierra Marine 18-1986-2 is a cast iron exhaust manifold built to replace a worn or rusted-out manifold on a GM small block V8, anything from a 305 to a 350 cubic inch marine engine. If you've got water where it shouldn't be, or you're chasing a rough idle back to a leaking exhaust, this is the part that gets your engine breathing clean again.
Here's the thing about marine manifolds. They live a hard life. Raw water runs through the casting to keep exhaust temperatures down, and over enough seasons that water eats the iron from the inside out. You often can't see it coming until a chunk of the water jacket lets go or you start finding water in a cylinder. When that happens, it's not a maybe. It's a replace-now job, because a failed manifold can dump water straight into the engine.
This one is a straight cast iron replacement for the factory GM small block layout. It bolts to the same head pattern the original used, so the swap is familiar territory for anyone who's done exhaust work before. We've handled plenty of these on sterndrive and inboard rigs, and the fitment is what you'd expect from Sierra. It meets or beats the original part it replaces.
A couple of honest notes before you order. This manifold ships without a gasket and without mounting hardware. That's on purpose. Boaters like to match the gasket set and bolts to their exact riser stack and cooling setup, so Sierra leaves those choices to you. Plan on grabbing a fresh marine exhaust gasket set and the right stainless bolts before you start the job. Reusing an old gasket on a new manifold is asking for a leak.
Fitment covers the GM small block V8 family from 305 to 350 cubic inch. Before you pull the wrenches out, double-check your exhaust port spacing against the manifold, especially on later blocks where the port layout changed. A quick measurement up front saves you a second gasket and a wasted afternoon. If you're not sure what you've got, send us your engine model and we'll help you confirm the right part.
Install-wise, this is bread-and-butter exhaust work, but marine exhaust doesn't forgive shortcuts. Snug the manifold down evenly in a cross pattern so the casting seats flat against the head, and make sure both surfaces are clean and true before the gasket goes on. A warped or pitted head surface will leak no matter how good the new manifold is, so give it a look while you're in there. Take your time on the water ports and use fresh sealant or plugs where the original had them.
One more piece of dock wisdom. Manifolds and risers tend to age together. If this one rusted through, the riser feeding it has probably seen the same water and the same years. A lot of guys replace them as a set so they're not back in the bilge next season doing half the job again. It's not required, but it's the move we'd make on our own boats.
Saltwater crews, take care of this thing and it'll take care of you. Flush the raw water side with fresh water after every trip and it buys you seasons of extra life before corrosion sets back in. Freshwater boats generally get more years out of a standard cast manifold, but the flush habit is cheap insurance either way.
Bottom line, if you're staring down a leaking or corroded exhaust manifold on a GM-based marine V8, the Sierra 18-1986-2 is a solid, no-drama replacement. Match it with a fresh gasket set and the right hardware, and you're back on the water.
Key Features
- Cast iron construction, a direct replacement for the factory GM small block marine exhaust manifold
- Fits GM small block V8 engines from 305 to 350 cubic inch
- Bolts to the original GM head pattern for a familiar, straightforward swap
- Meets or exceeds the OEM exhaust manifold it replaces
- Ships without gasket so you can match the correct marine exhaust gasket set to your engine
- Ships without mounting hardware, so you can spec the right stainless bolts for your riser stack
- Suited to both sterndrive and inboard marine engine applications
- Sierra quality parts engineered for the harsh marine exhaust environment, backed by the manufacturer warranty
Why Buy from NVN Marine
- Authorized Sierra Marine reseller, full manufacturer warranty
- NMEA member and ABYC certified, advice from real boat techs
- Same-day shipping before 3 PM ET on in-stock items
- NY headquarters and Fort Lauderdale flagship retail store
Technical specifications
| Title | Sierra 18-1986-2 Cast Iron Exhaust Manifold, GM V8 305-350 |
|---|---|
| Brand | Sierra Marine |
| Manufacturer Part Number | 18-1986-2 |
| UPC | 808282391232 |
| Material | Cast Iron |
| Product Type | Marine Exhaust Manifold |
| Engine Fitment | GM Small Block V8, 305 to 350 Cubic Inch |
| Gasket | Not Included |
| Mounting Hardware | Not Included |
| Application | Sterndrive and Inboard Marine Engines |
| Length | 19 in |
| Sold As | Each |
Frequently asked questions
What engines does the 18-1986-2 fit?
It fits GM small block V8 marine engines from 305 to 350 cubic inch. Before ordering, confirm your exhaust port spacing, since the port layout changed on some later blocks. Send us your engine model if you want us to double-check the fit.
Does it come with a gasket?
No. This manifold ships without a gasket. You'll want a fresh marine exhaust gasket set matched to your setup before you install. Never reuse an old gasket on a new manifold.
Are the mounting bolts included?
No, mounting hardware is not included. That lets you spec the correct stainless bolts for your riser stack and manifold-to-head fit. Measure before you buy so the bolts thread in fully.
How do I know if my old manifold needs replacing?
Common signs are water in a cylinder, a rough idle that traces back to the exhaust, rust staining around the casting, or visible cracks at the water jacket. On a raw water cooled engine, internal corrosion is the usual killer and it's not always visible from outside.
Should I replace the riser at the same time?
We usually recommend it. Manifolds and risers age together on the same water and the same seasons. If the manifold rusted through, the riser has likely taken the same beating, so a lot of boaters replace them as a set.
Is this the same as an automotive manifold?
No. Marine manifolds route raw water through the casting to control exhaust temperature, which an automotive manifold does not do. Never substitute an automotive exhaust manifold on a marine engine.
How do I make the new manifold last longer?
Flush the raw water side with fresh water after every saltwater trip. That single habit slows the internal corrosion that ends most marine manifolds. Freshwater boats still benefit from the flush.
Any install tips?
Make sure the head surface is clean and true before you set the gasket, torque the bolts evenly in a cross pattern so the casting seats flat, and use fresh sealant or plugs on the water ports where the original had them.
Is it covered by warranty?
Yes. It carries the full Sierra manufacturer warranty. We're an authorized Sierra reseller, so any warranty claim runs through us back to Sierra.
