99370A2 Exhaust Water Shutter / Flapper Stops Water From Backing Up Into the Motor Compatible with MerCruiser V6 V8 3 Inch Y-Pipe 1982-1987, Same Numbers as 18-2733

Exhaust water shutter (rubber flapper) for MerCruiser sterndrive applications
that use OEM reference 99370A2 — the early 3 inch (76 mm) Y-pipe era, roughly
1982–1987 on V6/V8 wet-joint packages. It sits in the exhaust Y-pipe path to help block
seawater from washing back into the riser and manifold when you reverse, launch on a steep ramp,
catch a following sea, or slam the throttle shut (hydrolock risk).
This is NOT the later 4 inch Y-pipe shutter 807166A1, NOT offset
807166A3 / 18-2732, NOT 807166A2, and NOT tip-family
60930.

Which shutter is this?

OEM 99370A2 for the early 3 inch Y-pipe water-shutter era.
Same numbers as 18-2733 / 89160. Package on this SKU:
1 × shutter (single piece). Catalog “pack of 2” on some 18-2733 rows is a
packaging style — not the same as our pair SKUs for later shutters.

Signs yours needs replacement

·       Cracked, missing, or stiff rubber flapper — water can wash back on reverse or steep launch

·       Idle clatter that goes away when you add throttle — often blamed on lifters or the drive first

·       Fragments of old rubber (or an old hinge pin) in the Y-pipe bottom or drive exhaust passage

·       Hydrolock or a “gulped water” scare after reverse, ramp launch, or hard throttle chop

·       Power feels choked / won't climb RPM — debris from a broken flapper can restrict exhaust

·       Found during water-pump, hose, or elbow service when the exhaust path is already open

·       Old part stamped 99370A2 or cross-ref 18-2733 / 89160 / 92802 / 92852 / 9-42801

Transom bellows leaking at the drive? Different part family (e.g. 17B1003) — not this shutter.

Will it fit my engine?

Horsepower alone does not prove fit. Match the old part number or confirm a
3 inch Y-pipe (not 4 inch). Catalog context: MerCruiser V6/V8 ·
1982–1987 · OEM 99370A2. Verify serial before order.
*Years from aftermarket catalogs — confirm SN.

OEM & swap numbers

Replaces ref 99370A2 · cross-ref 18-2733 · 89160 · 69370P ·
REC99370A2 · 9-42801 · old 92802 / 92852.

Wrong shutter? Read before you order

·       NOT 807166A1 — later 4 inch Y-pipe shutter (GH SKU 17ST002 / pair 17ST003)

·       NOT 807166A3 / 18-2732 — offset / center-rise (GH SKU 17ST004 / 17ST005) — one digit off from 18-2733

·       NOT 807166A2 — one-piece GEN+ manifold shutter (GH SKU 17ST007)

·       NOT 60930 / 18-2727 — tip / 4-cyl / jackshaft family (GH SKU 17ST001)

·       NOT 9-42800 — that cross is the 60930 family, not this SKU

·       NOT a transom bellow — no bellows adhesive / expander job

What's in the box

·       1 × Exhaust water shutter (99370A2 equivalent) — single piece

·       Catalog “pack of 2” (e.g. some 18-2733 listings) is a packaging style — this SKU is one shutter

·       Hose clamps / tip hardware — only if shown in photos (sold separately otherwise)

Related GHMarine SKUs

·       Need 4 inch Y-pipe shutter 807166A1? → GH SKU 17ST002 (single) / 17ST003 (pair)

·       Need offset 807166A3 / 18-2732? → GH SKU 17ST004 / 17ST005

·       Need tip / 4-cyl shutter 60930? → GH SKU 17ST001

·       Need GEN+ 807166A2? → GH SKU 17ST007

Install tips

Work in a ventilated area. If access is poor or you’re unsure, use a marine shop.
This is a Y-pipe flapper job — not a drive-off bellows adhesive job.

1. Confirm the removed part or manual shows 99370A2 / 18-2733 / 89160 family — measure a 3 inch Y-pipe, not 4 inch.

2. The shutter itself is cheap; stuck exhaust hose and access are usually the hard parts.

3. If pieces are missing, dig the Y-pipe bottom and the drive exhaust passage before buttoning up.

4. Upper / easier-access setups are often DIY; older bottom-of-Y layouts may need more labor.

5. Do NOT use bellows adhesive or expander tools — this is a flapper, not a transom bellow.

6. If your old part is 807166A1 / A3 / A2 / 60930, stop — order the matching GH SKU.

FAQs — the bros want to know

Is this the same as 807166A1?

No. This listing is OEM 99370A2 / cross-ref 18-2733 for early 3 inch Y-pipe packages (roughly 1982–1987). 807166A1 is the later 4 inch Y-pipe shutter (17ST002).

Can I run without the shutter?

Some owners run a while on calm water. Reverse, steep ramp launches, following seas, and hard throttle chop raise seawater backflow / hydrolock risk. Replace a cracked or missing flapper.

Why does the catalog say pack of 2 but you sell one?

Some aftermarket rows price 18-2733 as a package of two. This GHMarine SKU is one shutter (company single-piece definition). Order quantity for how many pieces you need.

Does HP alone prove fit?

No. Match the old part number or confirm a 3 inch Y-pipe. A 5.7 can be 3" early or 4" later.

I hear a clack at idle — is that the shutter?

Often yes when the noise is mainly at idle and fades with throttle. Still rule out debris stuck in the Y-pipe or drive exhaust if pieces broke off.

Do I need bellows adhesive?

No. That is for transom rubber bellows. This is a water shutter flapper in the Y-pipe path.

Please note

Aftermarket replacement part. Not OEM. Not affiliated with engine manufacturers referenced above. "Compatible with", "Replaces ref", and cross-ref numbers identify fitment — always confirm with your serial number, Y-pipe size (3" vs 4"), and old part number before ordering.