Wow, I didn't expect to see this old thread surface again. Thanks for the replies. I am going to let the motor stay for 1 more year. When it comes out next year, it will be a full rebuild, and set the motor up to run a Whipple... at least that is the way I am leaning towards now. Who knows what next year will hold..

so 12 years later, was there every a resolution to your motor conundrum? there is a 502 boat on craigslist with 500 hrs so now i find myself in need of your help ha

My current 502mpi's have 600 hours on them and they run the same as they did with 250hrs when I bought the boat.
I have no immediate plans to do anything other normal maintenance.

My triple 500 efi are at 1190 hours and still sealed from merc! Nothing, i mean nothing, was opened on these engines! But more babied than hammered

WOW!!! We see 500EFI's with spring issues with as low as 300 hours on them and 525EFI's as low as 250 hours. We recently did a 525EFI with 170 hours that had broken springs but that owner is know to have it on the mat no matter what.

To add to that, we have now seen (3) 525's that have broken a connecting rod and not one of them had made it to 200 hours yet. They all made a complete mess of the block, crank, rods, pan, etc. They all appear to have broken rod bolts. For those that don't know, they use ARP 8740 bolts. It really wouldn't have been much more money at all to go to the AP 2000 bolts. Probably would have saved these.

Every 525 that we have been into have had rocker arm/valve spring issues, and that's over 50 of them. We scan every one of them and most have been fairly babied. Hell, the first one that we saw that had a broken connecting rod only had 1.4 hours over 4000 rpm. This was only the second time that the engine had been over 5000 rpm.

Lots of 500 EFI's also burned the head gaskets between cylinders in the 300 + hour range. Almost 1,200 hours is amazing! I'd say you got your money's worth out of Merc!

I have twin stock 1997 415hp 502 mpi magnums with around 930 hours in a 38 powerquest. I bought the boat in 2006 with about 240 hours and after 10 boating seasons and almost 700 hours latter I noticed no loss in performance with top end speed/RPM or any more oil consumption. These engines have only had tune ups, oil changes, fuel filters,a pair of circulating water pumps, and a set of exhaust riser gaskets. Other than that nothings been apart or touched. I know one Merc mechanic has scanned 502 mpis with 1400 hours still running.