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7.5hp Mercury lower unit/axel diameter

K

ks72

New member
Hi, I have early 70`s 7.5hp motor without the lower unit or transmission. I have read that those units are same atleast with 4, 4,5 and 7.5hp models only there is two different diameter axels. My engines serial number is 9045609 and it is first non round shaped model and running guite well and I would like to see it on water once more. So all tips and data is valued.. -KS
 
Define axel.----Prop shaft or driveshaft.-----The 9.8 is also the same.-----Your location ?-----Location can determine availability of used ones.----E-bay is also good.
 
Driveshaft. I am writing from Finland and we have some used parts on market but it would be easier to look if I knew exactly what I should get.
 
The driveshaft diameter could be either .456 or .438.The.456 part number is 47-89981,the .438 is 47-89980.These are the water pump impeller numbers if thats what youre looking for.
 
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<p>
there may be a difference in the shift shaft length.------look tight here on this site to see what part numbers fit year to year.------Or there is another parts website that list years that the part fits.</p>
 
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You want a 7.5 or 9.8 lower unit. The 4hp might fit but the propshaft is shorter and you probably won't find a 4hp propeller with enough pitch for your 7.5.

I'm assuming by "non rounded" you have a fiberglass hood, which places your outboard at around 1976 or newer. Any lower unit from a 7.5/9.8 '76 and up would work. I can't recall when Merc went from the very long shift shaft to the shorter one, but likely around '72. So a '72-'75 Blue Stripe lower unit probably will work as well.

The newer ('76-up) lower units have stainless driveshafts which is desirable.

Older ones have mild steel driveshafts and have rust/rot problems, especially when run in salt water. So they will rot around the driveshaft seals and then the gearcase fills with water. Best to avoid them.

Here's the correct parts diagrams for your Belgium outboard:

Mercury Marine 7.5 HP Outboard Motor Parts by All Engine Parts & Diagrams

Lookup Mercury Marine 7.5 hp outboard motor parts by component and buy discount parts from our large online inventory.
www.marineengine.com www.marineengine.com

Hope that helps............ed
 
Just an update; I was checking on eBay and it appears that later 4hp (single cyl) lower units have the same propshaft as the 7.5 and 9.8. So if you were to come across one of those, it should work too. Note I verified the propshaft part numbers were the same.

BTW there are lots of lower units for sale on eBay, not necessarily cheap! But I did see some pretty nice ones.

Do you have a Short (15" transom height) or Long (20") shaft motor? That will certainly matter, too!
 
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