I have a 496HO @ 275 hours and still running strong without any issues so far. I wonder when to replace my original AC Delco plugs 41-983. When I ask around I get all kinds of advice from right now to wait until the motor reach at least 1000 hours.

Stock motor with decent fuel I did mine at 300 hours.

275 hours on mine with no issues. Will probably change them out at the end of the season or beginning of next though.

We run hard at mostly 4500+ RPM. Our plugs Autolite AR 3911plugs are toast after 15-20 hrs. They are worn and would need to be re-gapped. We change plugs annually. If you mostly cruise, plugs will last a long time.

In Europe you tend to have a higher octane and better blend of fuel than we have here in the USA. The platinum tip 41-983 plugs are a little sensitive to fuel and the quality of the burn in the cylinder, but they should give you a 300-hour service life and if you develop a slight loss of power or a had starting or definite misfire issues then change them at that time no matter what. Spark plugs are relatively moderatly priced replacement part but still remember my favorite saying here" If it Ain't Broke, Don't fix it!"

Best Regards,
Ray @ Raylar

Got 330 hours on the stock plugs in my 2003 496 MAG. Boat starts and runs like new. Might consider changing them out at 400 hours. Or maybe 500...

I changed plugs on my stock 502 mpi mags after 350 hours and really didnt notice a difference, probably could have went to 400 hrs.

Thx all for Your replies!
Looks like I´m getting close to the limit for new plugs. And yes, as Ray wrote, we have a higher octane level here in Europe, I use 95 octane. Perhaps it gives my plugs a bit longer lifetime, so far no loss in performance.

Scandinavian, this is where it gets confusing ;-)
Our 95 octane (RON) equals 91 octane (PON) they have in the states.
PON=(RON+MON)/2

Regarding the plugs I have noticed they can run several hundred hrs and then fail completely.
The only way to notice is at full rpm when you will loose a couple of hundred's.
At idle it is very hard to notice since the PCM maintains preset rpm by opening up IAC to give remaining running cylinders more air.