78458A1 MerCruiser Sterndrive Wet Exhaust Tube

78458A1 Open Style Exhaust Tube — MerCruiser Sterndrive Wet Exhaust

Match 78458A1 on your parts diagram · Replaces 78458A1 · Cross 18-2760 · 89150 · REC78458A1 · CEF500520K · 9-72810

This is the open style exhaust tube on a MerCruiser sterndrive — the straighter rubber piece on the drive-end wet exhaust run, not the accordion bellow (8M0124227) and not the riser boots in the engine bay. Parts books call it 78458A1; a lot of guys search 18-2760. Read the number on the piece you pull off.

Also crosses to 89150, REC78458A1, CEF500520K, 9-72810.

Where does it go?

Same parts diagram page as the U-joint boot and shift cable boot — but its own row. Straight exhaust tube at the drive end, not the accordion 8M0124227 bellow on that drawing.

Stamp should read 78458A1 or 18-2760. Row says 8M0124227? That's GHMarine 17B1003. Wrong row or wrong spot? Use Wrong tube? below.

Replace it if…

·       Exhaust tick or rasp at the stern at idle — a split or loose clamp on the open style exhaust tube usually shows up as noise before you see water. Same sound at the bell housing boot points to 17B1003 instead.

·       Water drip at the clamps where the tube meets the drive — wet exhaust runs through this straight rubber section. Dry soot or water at the standard bellow row on the same page? That's the accordion 8M0124227 boot, not this one.

·       Soot ring on the tube body or clamp area — blow-by at this exhaust segment. Soot at the 4" riser-to-Y section further upstream → 17B1001; soot above the elbow → 17B1014.

·       Smell exhaust at the swim platform or cabin after you shut down — fix the leak before the next trip. Pull the tube and match 78458A1, 18-2760, or 89150 on the rubber.

·       The rubber is hard, checked, or burnt after an overheat — even if it still looks attached, the heat cycle has cooked it. Quick check: stamp reads 8M0124227 → 17B1003; 70981A3 → 17B1014; 32-44348T → 17B1002; U-joint boot row → different SKU entirely.

Will it fit my drive?

HP on the engine tag won't tell you. Match the serial on the drive or transom and confirm 78458A1 / 18-2760 on the old rubber — not every Alpha or Bravo uses this tube.

Drive / engine	Year range	What to check	Part #	Notes
R / MR / Alpha One	Match stamp 78458A1 / 18-2760	Drive-end exhaust tube row on your diagram	78458A1	Listed under R/MR/Alpha in parts books — not every Alpha boat
Alpha One Gen II	1983–up when diagram specifies	Stamp on removed tube + exploded view	78458A1	Open style tube — not accordion 8M0124227 bellow
Bravo I	1988–up when diagram specifies	Bravo diagram must list this line item	78458A1	Typical Bravo SN 0B664190 & up — still verify diagram row
Bravo II	1988–up when diagram specifies	Bravo II exhaust drawing only	78458A1	Do not assume all Bravo — diagram row beats drive badge
Bravo III	1988–up when diagram specifies	Bravo III exhaust drawing only	78458A1	Same part number when drawing calls for it
Blackhawk	1994–1995 when diagram specifies	89150 or 78458A1 on old part	78458A1	Blackhawk SN 0F186717 & up — narrow 1994–1995 window
454 MAG / 7.4L big-block	Part-number driven	Diagram or kit BOM must show 78458A1 / 18-2760	78458A1	454 MAG / big-block exhaust context — match stamp on old part
Jackshaft / performance exhaust	Part-number driven	Old part or kit BOM must list 78458A1 / 18-2760	78458A1	Specialty setup — do not order from a drive photo only

 

Part numbers

Primary OEM: 78458A1. Common crosses:

Type	Part Number	Notes
OEM	78458A1	Open style exhaust tube — primary reference
Cross-ref	18-2760	Aftermarket tube — replaces 78458 / 78458A1
Cross-ref	89150	Aftermarket catalog index → 78458A1
Cross-ref	REC78458A1	Exhaust tube / exhaust bellow tube cross-ref
Cross-ref	CEF500520K	Kit index tied to 78458 family
Cross-ref	9-72810	Tube, exhaust — replaces 78458A1
Legacy	78458	Short index — confirm A1 suffix on order
Related	54-815504348	Clamp reference on the same diagram — verify package

 

Wrong tube? Check before you buy

Guys mix these up every week — need part # + diagram row:

·       8M0124227 / GHMarine 17B1003 — standard accordion-fold exhaust bellow on the same diagram (different row).

·       70981A3 / GHMarine 17B1014 — 3" upper riser-to-elbow boot in the engine bay (not at the drive end).

·       32-44348T / GHMarine 17B1002 — 4" upper tube at the inner transom plate (both ends 4").

·       32-14358T / GHMarine 17B1001 — 4" riser elbow-to-Y boot farther upstream in the bay.

·       32-90949T / GHMarine 24B1052 — 3" lower wet boot below the elbow on the exhaust run.

·       U-joint bellow or shift cable bellow — separate rows on the same diagram, different housings.

In the box

·       1 × open style exhaust tube (78458A1 / 18-2760 family)

·       Hose clamps — only if they're in the listing photos

·       Not included: bellows adhesive, expander 91-45497A1, U-joint boot, shift cable boot, other exhaust SKUs

While you're on the exhaust diagram…

Replacing this tube? Compare the neighbors before you checkout:

·       Standard accordion exhaust bellow — GHMarine 17B1003 (8M0124227)

·       3" upper riser boot — GHMarine 17B1014 (70981A3)

·       4" riser boot — GHMarine 17B1001 (32-14358T)

·       4" upper transom tube — GHMarine 17B1002 (32-44348T)

·       3" lower wet boot — GHMarine 24B1052 (32-90949T)

Install (shop tips)

Let the exhaust cool. Confirm the diagram still shows 78458A1 before you glue. Access varies — some boats reach the clamps from below; others pull the drive for room per the shop manual.

Grab before you start

·       Marine bellows adhesive (3M Bellows Adhesive or same type — not RTV)

·       Expander tool 91-45497A1 if the stubs are tight

·       Hose clamps, wire brush, rags · good ventilation

Steps

1. Loosen both clamps. Pull the old tube; wire-brush the mating flanges.

2. Apply adhesive inside both ends; wait until tacky (about 10–20 min).

3. Seat the tube on both flanges; expander if needed. Clamps in the grooves, even torque.

4. Run at idle on muffs — sniff at the stern and watch the bilge for water.

Questions

Straight tube or accordion bellow?

78458A1 is the open style straight tube. 8M0124227 (17B1003) is the deep-pleat accordion bellow — different row on the same diagram. Read the stamp; don't guess from the picture.

Where does it go on the boat?

Drive-end wet exhaust — same parts page as the U-joint boot and shift cable boot, but its own row. Not the engine-bay riser tubes (17B1001 / 17B1014 / 17B1002).

Fits every Alpha or Bravo?

No. Alpha Gen II (1983–up), Bravo (1988–up), and Blackhawk (1994–1995) only when the diagram lists 78458A1 or 18-2760. R/MR/Alpha and 454 MAG — match the stamp. Decal on the engine cover doesn't prove anything.

18-2760 vs 78458A1 — same part?

Same tube. Shops search either number. 89150 is just how the catalog indexes it.

Do I have to pull the drive?

Sometimes yes, sometimes no — depends on access at the transom. This is a drive-end job, not a hatch job like 17B1014. Bell-housing bellow (17B1003) is the one that usually pulls the drive.

Glue, clamps, or soap and water?

Glue both ends, hose clamps in the grooves — same as most exhaust boots. Tight stub? Expander 91-45497A1. No RTV. The 4" riser boot (17B1001) is soap-and-clamp only — not this one.

What's in the box?

One open style exhaust tube (78458A1 family). Clamps if shown in the photos. No adhesive, no expander, no U-joint or shift cable boots.

Note

Aftermarket replacement — not made by the engine manufacturer. "Compatible with" and "Replaces" mean built to work like the OEM part — still match your diagram row and the number on the rubber you removed.