reassembling a Merc lower unit need a proceedure


Old 08-04-10, 05:29 AM
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Join Date: Dec 2002
Location: Pittsburgh, PA
Posts: 553
reassembling a Merc lower unit need a proceedure

Working on a '77 Merc 500 50hp....

My Merc is one of those that has an extension piece between the lower unit and the motor. This is my first rebuild so I want to be sure it goes together right.

I heard that it is easiest to put the lower unit in a gear and make the shifter match before I mate them.. is that the case? I heard that if this isn't done correctly I will be missing a gear when it's all together. I also heard something like it has play and can flop back and forth if its not in a known gear and it won't line up right.

How do I insure that the exhaust extension piece and the water line mates up or is it a no brainer? Currently it looks like it was gooed up with high temp silicone, is that standard practice?

I have to find the water tube, if I recall it was just a copper tube with an O ring on each end, is this right? My motor has been apart since last fall, my memory is a little foggy.

There is a round plastic piece that fell out when I removed the lower unit, I'm guessing that it either goes over the shifter shaft somewhere, or on top of that extension piece somewhere.
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Old 08-04-10, 05:56 PM
Join Date: Jul 2008
Location: Montgomery MN
Posts: 541
If the extension is a 5" aluminum part that makes the mid section from a 15" to a 20" I find it easier to put it up on the mid then put the gearcase on.

I keep the gearcase and shifter in neutral. The only problem with this is lining up the splines from the drive shaft to the crank shaft. To overcome this have a helper with you to rock the flywheel back and forth and it will go right in. Once it is in with the splines lined up you can drop it down a couple inches and get the shift shaft and water tube lined up.

I think the white plastic piece is to help align the water tube, but it could be for the shift shaft.

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