'77 Merc 500 lower unit and water pump questions.

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  #1  
Old 08-01-10, 05:32 PM
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'77 Merc 500 lower unit and water pump questions.

I'm replacing the lower unit with a good used unit that looks to be the same, and I was told it would fit. The old unit was casting #1643-6037, the new unit is #1643-6458.

The shaft is exactly the same and I swapped the bolts for my longer ones. Here are the questions... this unit has a plastic barbed fitting in front of the shifter, my old unit dosen't have this. Do I plug it or ignore it? Second question, what is the part number for a water pump rebuild kit and can I be sure that the original part number will fit now that I changed the lower unit? Do I go with a stock replacement or an advertised "improved impeller design"? I want it all new while its apart. Third question, my old prop was a 13 pitch, the new one is a 12 pitch. Do I bother changing them? From what I have read the difference is only 200 RPM on top but the 12 would give me more low end.

The motor number is #4803973 and it is a 1977, 50 hp, 500 Thunderbolt, blue band, 4 cylinder. I'm thinking the pumps may all be the same, but even so, I'm a newbe and I don't know what the part number is.

Sorry for the newbie questions, I am trainable...LOL
 
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  #2  
Old 08-03-10, 07:27 PM
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Location: Montgomery MN
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You're doing a good job on a big one at that. How is the boat coming?

The plastic barb might be a speedo pick up. Is there a small hole on the leading edge of the case? If there is that's the pickup for the speedometer.

I'm not the best at the older motors so what I would do with the impellers is take both of them out and match them up. I'm betting they are the same, I will only use mercury impellers.

Try the 12 and see what your top rpm is with just you and a light load. It's best to prop for top suggested rpm with the lightest load you will carry that way it will be in the middle of the operating range once loaded down with gear and people.
 
  #3  
Old 08-04-10, 05:18 AM
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Samauri, So I would just remove the fitting and plug it?

The boat is coming well, slow, but good. Floor is done, carpet is done, transom is done, lower unit arrived and will go on soon...look for more questions..LOL, rod locker is in, having trouble remounting the fishing platform...rivets are too short, the dash is half done, wiring after that. Livewell pump is 5 weeks late.... thanks Oceanelectronics.com, bilge pump is too new to have its own bracket so I have to fabricate one. Livewelll and tubing to go in yet.

I plan to have it in the water this season with a few lingering projects. I will still need new seats and may want to add a aerator to the livewell.

As far as the impellers, the impeller for my replacement unit is missing. The guy that I bought it from tested the unit and removed it, he was very clear that it needs a new impeller. I guess I can pull the old one and see if it fits.
 
  #4  
Old 08-04-10, 05:48 PM
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Location: Montgomery MN
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Yes you can plug it, it should be a pipe thread.
 
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