AC Delco Spark Plug 41-104

Sold individually. 

the ACDelco 41-104 is an ACDelco/GM iridium spark plug commonly used in GM-based marine engines — most notably Indmar engines and some Ilmor-specified plugs (Ilmor’s KT5505 is the same OEM plug).

• Indmar (GM-based inboard engines) — Indmar lists the 41-104 (or its ACDelco/GM equivalent) as a service spark plug for certain LSA / GM-derived marine V8 engines. If you have an Indmar 5.0 / 5.7 / 6.2 family engine, 41-104 (or closely related ACDelco numbers) is commonly the specified iridium/platinum replacement. 
• Ilmor — Ilmor’s KT5505 iridium plug is cross-referenced to ACDelco 41-104 (users and parts listings show they’re the same item). 
• Other GM-derived marine MPI engines — many parts vendors list ACDelco 41-104/41-101 as the OE/service iridium plug for various 5.0 / 5.7 / 6.2 MPI engines used across marine brands that use GM blocks (Indmar, some PCM/Mercruiser/Volvo-based repower kits reference similar ACDelco part numbers). Always confirm by engine model/serial.