1992 OMC 5.8 Cobra Sterndrive Spark plugs don't match Specs.

by Dave
(Chester, CT)

Old & new plugs

Old & new plugs

My 1992 OMC 5.8 Cobra Sterndrive is running great. Although I put very few hours on it each year, it's been about 8 years since I had someone replace the spark plugs. I figured it was about time to change them, so I looked up the plugs for my engine and ordered them online. When I took out the old ones, they didn't match the new ones, so I didn't want to change them until I knew what was correct.

My engine has a sticker that says 584APRAMK. The online sources say the plugs should be RV15YC4. But the old ones are RV8C. The new one is longer and has a projected tip and the old one doesn't. Like I said, it was already running fine. So why are they different than the specs?

Just in case this matters, When I lived in Michigan, I had an engine problem that resulted in a cracked head. The mechanic knew someone with a used set of matching heads that we used to replace my originals. It was an expensive job, but it's run perfectly ever since. Could this have changed the spark plug requirement?
Thanks for your help.
Dave in Connecticut.

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