i have a 496 mag efi the hour meter says 430 hours but the original owner said he would leave the ignition on sometimes while playing the radio, iv since put the radio to another switch so i can have it on when the boat is off, he said actual engine hours are closer to 350-360hours. At what point should i be thinking about a rebuild? What do i look out for? I was thinking maybe pulling it out this winter and freshening it up a bit but wasn't thinking of a complete rebuild. What would a rebuild cost roughly with a little added power?

thanks!

well... this is raylar's territory but in general you need to know a lot about where the motor came from and how it was used. a home built motor run on the limiter for 300 hours is a different animal than a professional motor thats been well treated and well maintained. so... do a compression test. do a leak down maybe have the oil analysized ... that will give you a base line as will the maint records of the prev owner...

then you sort of make a decision. if the temps and pressures are good, power seems good, plugs look clean , comp and leakdown in the ball park then maybe all you are looking at are specifically lifed parts that ray will know about...or maybe nothing at all...

remember rule number 15. never fix anything that isn't broken...

main record show it was clean previous owner spent 2-4gs a year at prestige marine in havasu maintaining it so i know the baot has been well cared for. Doesn't have any power issues starts first turn never left me in trouble. knock on wood!

maybe raylar could chime in...?

Have the ECM scanned. It will tell you the actual hours and what rpms the engine was run at and for how long.

A 496 should be good for 800 or more hours if it was run normally.

Like Stevexm said here to be sure do a compression and leak down test and that will show if there are any obvious ring or valve issues. If the compression is around 145-155 psi in all cylinders with less than 10 psi variance between any two cylinders and the leakdowns are less than 8-10% and its not using oil or showing a lot of blowby then I say ot should go between 700-1000 hours maybe more with good care. This is the benefit and beauty of todays precision parts, machining, fitment. Lets all sing the praises of closed cooling, controlled operating temperatures, large oil capacity systems, good oils and electronic efi engine controls. They do make marine engines last longer.
Remember always - "IF IT AIN'T BROKE-DON'T FIX OR FOOL WITH IT!"
Just pay attention to minor problems and properly care and maintain it!

Best Regards,
Ray @ Raylar

i have near 700 HRS on my 496 HO 2001,i buy it new and it still run like the first day.

I have 700 on an 02 that also runs as good as the day I got it.
Relax and quit wanting to tear apart a motor that has low hours. Go have some fun. I'm shooting for the 12-15000. JC

Hundred not thousand, Wouldn't that be nice.

Would the same numbers apply to an HO engine? Compression and Leak Down and should the leak down be done with the engine hot or cold?