I'd expect to get more than 300hrs outta a black merc motor.
A 525efi gets a rebuild at how many hours?

Blacks should be twice as long min imo.

It all depends on how hard your run your engines. Yes, a 1000 hours is quite possible if you don't run them hard. If you run them flat out, I'd only expect 200-400. GM to my knowledge doesn't test any automotive engines beyond 400 hours at full throttle cycling between peak torque and peak horsepower. Running full throttle in a boat at full horsepower is tougher than the GM test. I don't know what the marine division uses for a durability test.

Michael

Though I would put my two cents in here since we kinda see a lot of 496's.
First a normally aspirated 496 mag , HO stock or with Raylar kits that is run with good oils, regular service intervals and not abused in normal recreational use should see 500 to 1000 hours before it should require any real engine mechanical service. What do I mean about abuse and normal recreational use.
Abuse is mis-proping the boat to lug the engine or put in on the rev-limiter all the time, overheating the motor, not paying attention to beeping on the guardian system, letting oil levels and oil condtion go to low or spent oil, excessvie idling, SUPERCHARGING STOCK 496's, racing stock 496's, etc., etc.
My professional opinion on normal recreational use is spending approximatly 10% of engine hours at idle to 2500 rpms, 10% of engine hours or less at 2500 to 3500 rpms, 75% of engine hours at 3500 to 4000 rpms and about 5% at 4000 to 5000 rpms.
One of the big keys is freshwater closed cooling, it keeps the motor at a pretty constant operating temperature, which eliminates "cold -hot shock", condensation, and corrsion, rust and scaling problems.
We have received a few 496's for overhaul that have seen 1000 hours or more with this type of service and use and they really were in pretty good shape on teardown. Most of the bottom ends looked great and most of the issues or needs were cylinder head work, valve job, guides, etc. We don't service the iron heads, we throw them away and install our aluminum heads which will go far longer than stock heads beacause of the quality of valves, seats, guides, etc.
I've seen a few practically new 496's have issues and go BOOM! right out of the box and thats because they are mass produced by GM Powertrain at Tonawanda and then shipped to Mercruiser, Volvo, Crusader, etc. and we all know these are not blueprinted motors and sometimes new parts slip through inspection as well as friday afternoon slipups on the line and such and that happens on all mass produced motors.
As a percentage though this seems to be under less than 1-2% of all 496's manufactured which is better than any big block GM has ever mass produced.
If you own a 496 take good care and use and she'll take good care of you!
Want more power reliably?
Then talk to Raylar and we'll try and help!

Regards,
Ray @ Raylar

Rebuild a Bravo1X at 150 hrs!!! I don't see the need....unless you are using yours to mix concrete!

The most posts I read from you, the more I like and respect you.
Ed Cozzi

ditto.

Thanks Raylar,
I'll bring it to you when I hit the 1000.
I put about 6 more hours on St. Clair yesterday and never went over 45 mph, about 3000-3500 rpm. Too rough in a little boat. Didn't know I was actually being nice to It. Good to hear from the guy who knows. JC

Thought it would be fun to bump this one up. I'm sure there's some with 1500 plus hours. I'm at 800 and just leaked down great.

Good read Raylar! Thank you.