I hit 300 hours on my HP500 efi's this past season, what rebuild should I be doing on them and what should I be looking for that may have been noticed by others.

I know that the valve springs should be replaced, is valve job needed as well?

Please offer your thoughts and suggestions, thanks in advance.

The best thing to do is pull the heads and have the valves and guides checked and springs replaced. The head gaskets can sometimes be an issue at 300-400 hrs so that takes care of that as well.

While you have the intake off go ahead and put in some fresh roller lifters. I have heard over and over 200+- hours is about the life expectancy of factory lifters.

God, I sometimes read some things here on these threads that get me shakin my old head! I can personaly as an engine builder attest to the fact that GM factory hydraulic roller lifters if undamaged can go as over a thousand hours without replacement in lower spring pressure engines like the 500efi, the various 496's and 454-502 MPI-EFI engines on the billet hardened camshafts these engines come with.
When you get to higher spring pressures in some engines , boost pressures and higher rpm operating ranges the life of the factory roller lifter can be lessened and problems with low oil pressures or oiling issues, mechanical debris or damage and overheating events can lessen their life even quicker.
Its my simple old belief that you should apply the thought of the KISS (keep it simple stupid)theory and the old addage somewhat "If it ain't broke, don't fix it" Most in the sport and industry know that Mercury recommends checking and replacing the valve springs on the Racing engines such as the 500efi and HP525efi every 250 hours or so, but this not mean there are other cylinder head issues like valve and seat leakage or such.
My recommendation would be to do the simple compression and leakdown test and based on those results look at cylinder head removal only if the compression and leak down numbers indicate valve leakage, hjead gasket leakage or possible ring and psiton issues.
I say this because it is very easy to remove the valve springs, replace them as well as the seals and check valve stems for unusual wear, check roller rockers, pushrods and valve adjustment and not tear down the top of the engine by removing the complete efi system,exhaust systems, heads and accessories to take the heads off just for a valve spring replacement. It's done fairly easily on a nearly fully assembled engine in the boat with compressed air and a proper valve spring compression tool.
Tearing down the top of the engine, removing heads and replacing head gaskets with all the associated clean up re-torquing and re-assembly can sometimes lead to errors or problems with all the addtional work required. This also gets more difficult if not sometimes almsot impossible to do in the boat and requires engine and drive removal. This work being done by a good Mercury Racing engine expierenced mechanic or shop is less likely to see these kinds of problems, but they do happen as the work gets more involved and complicated.
If the leak down and compression tests show other issues with rings, pistons and such then head removal and furthur inspection and repair is a must.
My bottom line is follow Mercury Racings recommendations for spring replacements, but test first to see what type of teardown it should be done with before you tear into that nicely assembled engine.

Again, just my tow cents and expierence here.

Best Regards,
Ray @ Raylar

Anyone ever put a new part on and have IT break? Do some checks. If it's all good put springs and a blower on and call it good.

GM factory dogbone lifters last quite well, but last time I looked, 500EFI's were running Crane tie-bar lifters. I'd swap them for new ones..
MC

thanks guys for the input and especially raylar. Compression and Leak Down were going to be the first thing done once it comes out of storage. hopefully those numbers look good, really don't want to that deep or have to take the motors out.

Maiden America, would love to put a set of a Whipples on these engines and wake them up.

Buddy had his 500 EFI's done last spring by Brad Smith. 2003's with 300 hours. Compression and leakdown numbers were good. Head gaskets were all but gone when the heads were pulled.

Should I stay with the Merc Springs or are there better/more reliable ones out there?

Where's the best place to purchase them?

Are you going to do anything with the throttle body? I make 58MM twin bore and a mono-blade throttle body for them.