I am about to buy a 2000 donzi 33zx with twin 500efi's that have 324 hours on them. should i be conserned about haveing to rebuld the topend sooner then i would like? the compreshion test came back good.

I just rebuilt my 500 EFI s in my 02 Top Gun with 375 hrs

I have someone check spring pressure on them. You may get a few more hours out of them.

The top ends are about due. The longer you wait, the more risk of a failure that could cause more severe damage.

Can someone explain why @ 3-400 hrs. these motors need rebuilding? Lets say that you drove the boat at WOT (5000rpms) for that 400 hrs that would still only be around 24,000 miles on a car. I have built a couple of hi-perf motors before, I just don't get the low hour rebuilds.

Think a car engine running at 5,000 RPMs will last 24,000 miles??

one word - load
boats put 10x the load on an engine than cars

Not rebuilds, top-end freshening up. Heads rebuilt (valvesprings, guides, seats, etc), check pushrods/lifters, and they should be good for awhile.

I did a set last year that had 450 hrs, and the thing
that was the most obvious was the head gaskets.....they
were pretty much toast between the cylinders.

Congrats on the boat, I would definitely consider budgeting for an upper Valve Train - Refresh, its much cheaper to do it "ProActively" then after words... I had a roller lifter crack in mine and it locked up the cam shaft and basically I had to rebuild the whole motor - needle bearings went everywhere... mine was a freak thing...and only a couple hundred hours...but they are great motors....

New motor is running strong and back in the little Donzi to get me to the Reindeer Games later this season....

At least get a leak down and compression check on the motors as is...and have an experienced Merc or Motor Guy look them over... its worth it...

Those are great motors in that boat...really wake it up compared to stock offering of that vintage...

Good Luck

Danny