'76 650 vs. '78 700 Lower Units

[dtmc78]

I was curious if the '76 650(3cyl) lower end was the same as the '78 700(3cyl) lower unit. In the service manual, the 700 is not in the book. However I did notice that the 700 and 650 is frequently grouped together in certain systems (electrical diagram). Are the parts interchangeable? Thanx for any enlightenment on this issue.

 

[emckelvy]

Re: '76 650 vs. '78 700 Lower Units

I'm thinkin' they should be virtually identical, at least physically. The 700 was an upgraded version of the 650 so the spline dia & count are the same on the driveshaft. You might check the sealing arrangement for the driveshaft where it interfaces with the crankshaft; there may be differences there.

You could also compare part numbers for interchangeability, this site has real good illustrations and current part numbers, and is one of the best sites I've seen since Mercury Parts Express imploded into a useless pile!

http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2

HTH........ed

 

[dtmc78]

Re: '76 650 vs. '78 700 Lower Units

Thank you for the information. I guess what also confuses me is that when you look at a part catalog like the one on this and other websites, it lists a 650(3cyl) and not a 700. Yes I know that it is a 70HP but when things are listed as model numbers? I ordered an impeller rebuilt kit once and the impeller was wrong the seals were right (oem merc parts). Mine had 45 deg bent fins and the replacement was too large and had straight fins. Is the 70 (3cyl) different than the 700 (3cyl)?

 

[emckelvy]

Re: '76 650 vs. '78 700 Lower Units

Don't get hung up on model numbers. Merc used to add a "zero" to the HP rating for many years, guess it made them sound more powerful!

I'm thinking that right about the time the '70' came out, Merc dropped the "zero" from their entire line.

And regarding impellers, the vanes of a new impeller are straight, they will become curled up as a function of turning in the water pump body. After a while the rubber gets old and takes a 'set', then the blades will not straighten-out if the impeller is removed.

It's possible the impeller you got was for a later-model 70 HP, which is a totally different motor, using a much larger water pump.

That's why it's important on Mercury's to not only go by HP but by Serial Number.

If you'll check out this link below from mercruiserparts.com you'll see that there's quite a few different serial number ranges for a "70 (3 cyl)".

http://www.mercruiserparts.com/selectSerailRange.asp?doc_nbr=70+(3+CYL.)

These include serial numbers which will match up to your motor, and also ones for the later-model 70HP 3-cylinder which will hardly have any parts that interchange with yours (I'm thinking fuel filter and spark plugs!).

Anyway, that's how this outboard thing works, serial numbers Tell All.

That's it for Serial Numbers 101, hope that helps "clear the mud"..........ed

 

[dtmc78]

Re: '76 650 vs. '78 700 Lower Units

Thank you for clearing up the model / hp confusion. You were defiantly right about everything being serial number driven. Now I feel like I can safely order parts.

Thanks for the tip on the ?set? impeller and the great web link. - Dave