looking at a boat with 450hrs or so on the motor. It doesnt have a hr gauge on this boat? 98 donzi 33zx, anyways i noticed some rust on the exhaust manifolds near the block. It looks like its around the gasket area. Should i be worried about anything? The rest of the manifold looks good and solid, nice black paint. Also is there a service life on the valve train before you replace the springs? Motors seem to run great and very strong should i stop worrying.

A resonably maintained stock motor should last 1000 hours at least, as long as corrosion does not kill something. I would have an engine survey done, and perhaps negotiate new exhaust manifolds and installation as part of the price. Look carefully at the underside of the exhaust manifold where it bolts to the heads- make sure all of the bolts are in decent shape and can be removed easily. If not, it points to manifolds being in bad condition, and this type of repair can be a major pain, sometimes requiring the engine to be removed. Ask me how I know....twice.

A compression and/or leakdown test would be a good thing too.

Stock magum cams are very mild - only about .483 lift. They do not generally require replacement of the valve springs like the HP500's do, unless it was overheated. Stock 330's commonly go 800 hours, and the mag's should be in that ballpark too if well taken care of. However, there have been a lot of motors killed by leaking manifold/riser joints. Riser gaskets are cheap - I would at least pull the risers and take a look at the inside of the manifolds.

Hours are ok. You can have a compression test and or a leak down test done for some insurance on the condition of the engines. As far as the exhaust is concerned, if you are concerned about the rust (some is normal) You can pull the exhaust yourself ($40 per eng in gaskets) or have them pulled and checked. At minimum I would have them compression tested. new glm cast iron exhaust is around $500 per motor. Hope this helps

I have almost 600 hours on stock 1997 502 mpi magnums and they run as strong and use no more oil than when I got them and the boat at 200 hours. I do change oil every 40 hours and run em' in the 2800-3200 rpm range 90% of the time and make sure there fully warmed up before putting the coals to her. I have heard 1000 + hours are not uncommon for a well maintained not abused 502 mags. I would change the exhaust riser gaskets, and make sure any shutter valves in the exhaust are fine too.

Have the ECM's scanned on the engines to find out the correct hours and what rpms they were ran at. I really wouldn't worry about a maintained 502mag with 450hrs. That is what mine have on them and I have no plans to touch them for quite some time.