I change my own impellers. I've changed them on OMC engines, MerCruisers, and did a couple Yamahas (my F225 and my Brother's F115) for the first time last spring. Never changed one on a Mercury outboard though. Never owned one and neither has any friends or family.
First off, I recommend you have a service manual. I prefer an OEM manual as opposed to a Selog or Clymer, but either of those two are better than no service manual. One thing I liked about the OEM service manual for my F225 is that it lists the actual permatex part number for all sealants. For instance, there's a thread sealant (not one of the loctites) that I didn't have on hand and having the permatex part number from the manual made it easy to go to the automotive parts shelf and find that exact thread sealant.
Secondly, I use a torque wrench on virtually every fastener.
Like Bobp suggested, remove the prop first, even it it's an aluminum prop because your going to need to clamp / vise (use pads on the jaws) the lower unit, the prop just gets in the way.
To get a very good idea of what your getting into, I highly recommend that you jump on over to THT and review the step-by-step tutorial that shows an impeller change on an F200. Here's the link:
http://www.thehulltruth.com/forums/thre ... 3&posts=42
The procedure shown on THT is the same procedure as my F225 and my Brother's F115, and for that matter, is very similar (at least in the fundamental steps) to practically all the impeller changes I've done. Sure, shift linkages vary (though I've only worked on two Yamahas so I don't know how much they vary on Yamahas) and some motors have water tubes, oil passage seals (MerCruiser), or exhaust ducts, but those are all slight nuances. That tutorial did leave out a step for that F200. The step it omitted was installing the metal collar over the plastic spacer that goes on the shaft. In the manual, it calls for a special tool to press the collar over the spacer. I merely used the old collar (turned upside down) and pressed on it using the old impeller housing base plate. You need to know to slide the plastic spacer down over the tapered part of the shaft before you press the collar onto to it.
One more tip, before re-installing the lower unit onto the motor, even though you put grease on the splines of the shaft, never put a glob of grease on the top of the shaft.
I buy my parts from Shipyard Island Marina. Very knowledgable and helpful staff there. Wish I could say the same for my Grady dealer.
Brad
