questions about water pump and lower unit fitment, '77 Merc 500

[hotrod53]

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? 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? What is the explanation of pitch, which way is power and which way is top 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

 

[Wingedwheel]

Re: questions about water pump and lower unit fitment, '77 Merc 500

You should find your parts here. http://www.boats.net/parts/search/M...PLETE (SERIAL GROUP NO. 6)(PAGE 1)/parts.html

 

[hotrod53]

Re: questions about water pump and lower unit fitment, '77 Merc 500

Its all greek to me, different model numbers for the same year. How do I determing model number, its not the same as my serial number or referenced to a serial number range. And now that the lower unit is not the original, would I still use the original part number? How do I know if the new lower unit takes a different pump or the same?

I think that I have the prop question figured out, I have a 12 pitch prop and a 13 pitch prop. The 12 should give me higher RPMs and the 13 should give me higher speed. Sounds like the 1" difference will only take a 200 RPM difference.