Merc 115 6 cyl 2 cycle - impeller won't fit on shaft

I bought the only kit available for my model outboard. everything seems to fit except the impeller will not go over the shaft right at the spot where the impeller fits at the pump. The shaft seems to get a bit thicker there at the pump housing area. The impeller will not even go on without the retainer pin, it won't go down. I've tried cleaning the shaft good, even lubing the impeller a bit (I know I shouldn't but I wanted to see if it would go on). Even tapping with a hammer - another thing I should not do - won't get it over the slightly thicker area. Is the shaft supposed to get thicker at that point or has something happened to the shaft? I have to believe I have the wrong impeller since it came with the kit and everything else looks exactly right. I can't compare the impeller to the old one as it required cutting off the shaft, it was 'seized' on.
Any help would be greatly appreciated.

what is your serial number on the motor, and what is the part number of the water pump kit?

The serial number is not there. I know where the plate should be but it's not. And I'm told that the SN is not embossed or engraged in any way on the engine itself, so with the plate missing there's no way to know. However, I've also been told by several parts houses that for the water pump it doesn't matter, there was only one made for that engine. And from some research I've done, that seems to be true. Even for places that ask for the serial number, regardless of what number you put in it comes up with the same part number. I bought the kit from Wholesale Marine, their part number is SIE-18-3314 [TD]WATER PUMP KIT[/TD]
[TD]- this is a Sierra part number. [/TD]

Thanks for the input. I guess I'll try buying another impeller. Don't really believe that it's going to be any different than the one I got with the kit but I guess it's worth a try. Thanks again Coueytwo.

Try cleaning the shaft with rubbing alcohol or mineral spirits. if that doent help use steel wool to clean it up. Be gentle

There is always the chance that some yahoo could care less worker put the wrong impellor in the kit

The old impeller didnt come off easily. If the new impeller slides most of the way down the shaft, the shaft is supposed to be the same size the entire lenght, so corrosion or debris may have raised the surface just a hair, which is a hair too much.

the shaft on a 115 is wider at the bottom the the top. see the link.
http://www.marineengine.com/parts/mercury-outboard/mercury-outboard-parts.php?modelId=6230&model=115+%286+CYL.%29&serialId=6231&serial=5314656+THRU+5600966&componentId=5063&component=GEAR+HOUSING+ASSEMBLY%2C+COMPLETE+%28PAGE+1%29

the shaft on a 115 is wider at the bottom then the top. see the link
http://www.marineengine.com/parts/m...nent=GEAR+HOUSING+ASSEMBLY,+COMPLETE+(PAGE+1)

Huh, how bout that. Just changed mine too, didnt notice the taper. But, like the OP I had to cut off the old impeller, and had to steel wool the shaft where the old one was stilling before the new one would slide fully into place. Sorry for the misdirection.