12644A1 4-Inch External Exhaust Flapper Kit · Fits MerCruiser 4-inch OD through-transom exhaust outlets, Replaces 12644Q01, 18-4457, 89250

12644A1 4-Inch External Exhaust Flapper Kit · Fits MerCruiser 4-inch OD through-transom exhaust outlets, Replaces 12644Q01, 18-4457, 89250

GHMarine 4-inch external exhaust flapper kit for through-transom exhaust outlets with 4-inch OD. Replaces OEM 12644A1 (also 12644Q01) and cross-refs 18-4457, 89250, 9-44001, 509495. This is an external flapper on the hull outlet — not a rubber bellow and not a Y-pipe internal water shutter (807166A1 / A2 / A3).

Catalog / service packaging for this family is typically 1 flapper + 1 clamp. Confirm the exact contents against your package photo / packing slip before install — do not assume a multi-pack.

Which flapper is this?

This is the rubber flapper that mounts on a through-transom external exhaust outlet. When the engine stops or following seas push water toward the outlet, the flapper helps limit water backing up the exhaust path.

It is not a sterndrive exhaust bellow (e.g. 8M0124227), not a 3-inch external flapper (34765A1 / 89240 family), not a 5/6-inch external flapper (89260 family — GH 23EF445), and not a Y-pipe internal shutter (807166A1/A2/A3 — GH 17ST00x).

Signs yours is due

• Measure the through-transom exhaust outlet OD at about 4 inches — the external rubber flapper is cracked, missing, or stuck open/closed.
• In reverse or following seas, water burps back into the 4-inch outlet path (not the engine-bay 807166A* Y-pipe shutter family).
• External clamp on the 4-inch outlet is rusted through; the flapper hangs loose or has fallen away from the tip.
• Divert: outlet measures 5 or 6 inches → GH 23EF445 (89260), not this 4-inch kit. You need a bellow, not a tip flapper → leave this page.

Will it fit my drive?

This kit is Compatible with marine through-transom external exhaust outlets where the outlet OD is 4 inch and the old part / catalog number falls in the 12644A1 family. Engine horsepower alone does not prove fit — measure the outlet OD and match the old number.

If your outlet is 3 inch or 5/6 inch, or your old shutter is an 807166A* Y-pipe piece, stop — this listing is the wrong family.

OEM & swap numbers

Primary OEM is 12644A1. Service / packaging parallel number 12644Q01. Cross-ref numbers below identify the same 4-inch external flapper family.

Wrong part? Read before you order

• NOT a 3-inch external flapper (34765A1 / 89240 family)
• NOT a 5/6-inch external flapper (89260 — GH 23EF445)
• NOT a Y-pipe internal water shutter (807166A1 / 807166A2 / 807166A3 — GH 17ST00x)
• NOT a rubber exhaust bellow / transom boot (8M0124227 / 18654A1 — GH 17B1003)
• NOT a default 2-pack — official 12644A1 family is a single-outlet kit

What ships in the box

• 1 × 4-inch external exhaust flapper (12644A1 family)
• Clamp: catalog/service packaging for this family typically includes 1 clamp — confirm against your package before install

Often ordered with related SKUs

• Need 5/6-inch external flapper → GH 23EF445 (89260 family)
• Need Y-pipe internal shutters 807166A3 → GH 17ST004 / 17ST005
• Need twin-engine 4-inch bellows + A3 shutters → GH 18BS002
• Need transom exhaust bellow → GH 17B1003

Install tips on the through-transom outlet

External hull outlet work. If you are not comfortable working at the transom exhaust outlet, use a marine shop. Do NOT use bellows adhesive or a bellows expander — those are for rubber bellow boots, not this flapper.

1. Confirm outlet OD is 4 inch and the old number is in the 12644A1 family (not 3 inch / 5-6 inch / 807166A*).
2. Remove the old flapper and clamp; clean the outlet lip.
3. Seat the new flapper fully on the outlet; install the clamp if supplied in your package; tighten securely.
4. Check free movement of the flapper — it must open with exhaust flow and close when reverse water pressure hits the outlet.
5. Run and inspect for spray / leaks at the outlet before returning the boat to service.

FAQ

Is this a bellow or a flapper?

Flapper only. It mounts on a through-transom external exhaust outlet. It is not a sterndrive exhaust bellow and does not use bellows adhesive or an expander tool.

Will it fit my 5.7 / 7.4 MerCruiser?

Only if the through-transom outlet OD is 4 inch and the old part is in the 12644A1 family. Horsepower alone does not prove fit — measure the outlet.

Is this the same as 807166A1 / A2 / A3?

No. Those are Y-pipe internal water shutters. This SKU is an external through-transom flapper on the hull outlet.

Is this the same as 89260?

No. 89260 is the 5/6-inch external flapper family (GH 23EF445). This listing is 4-inch only.

What comes in the box?

One 4-inch external flapper. Catalog packaging for 12644A1 typically also includes one clamp — confirm against your package photo / packing slip.

Do I need bellows adhesive?

No. That is for rubber bellow boots such as GH 17B1003. This flapper seats on the outlet and is retained with a clamp when supplied.

Please note

Aftermarket replacement part. Not affiliated with engine manufacturers named above. "Compatible with", "Replaces", and cross-ref numbers identify the 4-inch external flapper family — always confirm outlet OD, old part number, and package contents before ordering.