Re: 9.9/15 impeller housing tricks?

[Don]

Well, those exhaust gaskets are tricky, because the inner exhaust cover in not flat inside and is a pain to clean/true up properly.
First, I would recommend that the powerhead be removed in order to be able to get at all the surfaces and do a better job, nearly impossible with the powerhead installed on the midsection. Be sure to true up the flat surfaces, like the outer cover, and the flat side of the inner cover, along with the block. Keep in mind that the water passage walls are kind of thin, so be sure to inspect these surfaces carefully. In other words, you should be concerned about internal water leaks into the exhaust chamber, not just external leaks. Consider replacing the inner exhaust cover if sealing is questionable.
Probably a good idea to remove the plastic elbow while the cover is off, replace it with a new one. The only reason I mention this is that you will be in a real mess if the old elbow snaps off/leaks once the powerhead is back in place on the exhaust housing. Be sure to clean all the threads, and bolt shanks in salt water units.
Please do NOT use silicone or any other goop, use the OMC/Merc perfect seal gasket sealer only.
All that being said, I have never really found a cooling problem related to problems in the exhaust cover cooling passages. So, it might be best to "leave well enough alone". I surely have seen crappy jobs that had to be redone when people pull this cover off though. And, I have opened "Pandora’s box" a few times when I removed these covers.
And, once again, remember that the early 9.9/15s (74-78) had NO upper water tube grommet issues, the problem was introduced in 1979 when the grommet design was changed, the original grommets are no longer available. So, I usually leave an original ones in place, without trying to pull out the water tube. Keep in mind, that the original grommet has probably been replaced with the crappy grommet if the powerhead has been off before.