Funnily enough, I have just done this on my 2004 245 5.7GXi. One riser to manifold gasket was weeping, leading to some external rust on the manifold. I decided to change them both before they got any worse ( and potentially serious). I decided to remove the manifolds as well whilst I was at it, to give them a good clean out and to replace the manifold gaskets ( figuring that if a riser gasket was on the way out, the manifolds ones were possibly not far behind). The water galleries of the manifolds and risers were remarkably clean, maybe the "Neutrasalt" I have on my boat actually does something!
In my case, the riser bolt threads were not rusty and they came out easily. If your's are rusty, then a squirt of "plusgas" or similar the day before should free them up.
I found VP only sell a "kit" comprising of 2 riser gaskets and an instruction booklet.
re the bolts, I reused mine as the threads were clean ( I put a little bit of colloidal nickel antisieze on them. to keep them that way). Or rather, I re-used 6 and bought 2 new ones, as those 2 were quite bent ; no idea why, they were the original ones! I cut the heads off/ slotted the ends of those 2 bolts and used them as manifold to head gasket locators, it made refitting the manifolds very easy.
It's vital to ensure the mating surfaces are clear of all traces of old gasket and that they are flat/ not pitted ( and of course than no bits go into the engine!). I used a gasket removal liquid, it made it much easier than just scraping manually. Let me know if you go for aftermarket gaskets and need any details from the VP instruction booklet.
Graham
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