I have a 99 Powerquest 280 w/the 502 MPI and have a few questions:
1) I'm reading about a lot of people replacing the manifolds due to failure or for performance. At what point should the manifolds be replaced and why?
2) I also am hearing a lot about exhaust manifold gasket failure causing some big issues. At what point should the gaskets be replaced? I don't think they have ever been done on this boat (I just got the boat a couple months ago). Engine has about 380 hours on it.

Is it hard to replace the gaskets? Any tips, tricks?

The bottom of the manifolds look pretty rusty (maybe not rusty, but the paint mostly gone). Should they be repainted? Replaced?
thanks!

Exhaust manifold gaskets are not to bad to do. The problem is the manifolds are heavy bastards!! (like 80 lbs. each) Then to hold the manifold with gasket in place and get a bolt started is the next challenge. You can do it your self in a couple hours or less.

Make sure you have buddy there to help lift and wiggle the manifold in place and at least a 6 pk of cold beers.

Tip: I found it the easiest to get to the bolts and still see what the hell is going on, was to lay down across the intake manifold with a couple towels over the intake for a little cushion for the chest. Other then that, you just need a 3" extition on a ratchet and 9/16" socket IIRC. Gaskets are cheap, I picked up my new Quicksilver gaskets off ebay for $12.50

i just got done putting mine back on. Make sure you get the top of the gasket, on the top. They can be installed inside out and upside down and still fit/bolt up correctly... If installed backwards you will fight putting the sparks pluge back in as the gasket will be in the way.... Dont ask how i know.

The actual gasket failure is on top where the riser bolts onto the manifold, and I'm pretty sure they have been changed. Otherwise you would see water trails(stains) going down the sides of the manifold. When it starts leaking on the outside you can bet it's leaking water on the inside also, and that water can wipe out the exhaust valves or hydrolock the motor. Altough I wouldn't talk you out of replacing them, without looking at them you can probably get more time out them by replacing the riser gaskets every year or 2. It is not what I would call a fun job, but it is on top and fairly easy to get to. I would ask the guy you bought the boat from when he last had them changed, or just change them for piece of mind. Paint on the bottom of the manifolds being gone is normal, so don't worry so much about that, as long as they are not rusted through and leaking water which I doubt they are.

I have a set of Mercruiser 4" stainless steel risers . Came off a 502 mag These will not rust and will fit your standard center riser mercruiser manifold

Part #'s
816900A5
816900A9 250.00 for both + shipping
Let me know if interested

Thanks, I'll let you know if I'm interested.

Thanks for all the info. I didn't realize that the common problem was the riser gasket vs. the gasket where the manifold bolts to the engine block. It looks like it would be fairly easy to replace the riser gaskets.
Actually, looking at this parts diagram, I'm still confused. Would it be the #8 gasket or #21? thanks[IMG][/IMG]

The standard would be #8 unless yours has the riser block in the #18 picture. Take this picture and compare with what you have. The key to sealing these is to make sure they get cleaned up real well, and I have used high temp permatex around the water ports if there was a question of sealing.

If you want to eliminate the potential of the riser / manifold joint leaking water back into the engine, keep an eye out in the Swap Shop section or eBay for someone selling a set of the Merc 496 manifolds and risers. Those have a jumper hose, much like many of the aftermarket manifolds, instead of transferring water from the manifold to the riser via water passages or a "wet joint" like the 502 / 454 manifolds. These are a virtual bolt-in to replace the ones you have now, and I think they are lighter and actually flow better to boot. This is a good, moderately priced upgrade as far as exhaust is concerned on a stock big block. You should be able to locate a good used set of these for around $500.

If you are running in fresh water, and you replace your gaskets on a somewhat regular basis, the ones you have on there should last for a good while. It is when they are run in salt water that the real problems arise.

All the 502 mags I had had the 21 gasket with 4 holes. Don't buy the the 8 and think you can cut holes in it like the other because your wife wants to go out that weekend. I promise it will turn out bad.

I just looked at pictures of my old manifolds and they did have the #21 gasket even though none of them had the #18 riser setup.

Although we are talking about riser gaskets if you have to pull the actual manifolds a nice simple trick is getting short pieces of 3/8 bolt rod or (all thread to some) cut pieces 4" or so thread them in hand tight in 2 of the holes slide the gasket on and the rod will hold it in place and hold the weight of the manifold while you put the other 6 bolts in. Then remove the all thread and install the last 2 bolts.

Doesn't work well on close twin engines though.