1977 7.5hp lower unit compatibility?

[BanjoSHOES]

My dad has a 1977 or 78 7.5 hp mercury outboard that I can have, but the lower unit is bad. I don't have the motor here to look at it, and I don't have any serial numbers at this time.. Is there a chart somewhere that tells what lower units are interchangable? I heard the 7.5 and the 9.8 units are, I don't know if that information is correct, and I don't know what years are compatible. Any help would be wonderful. Thank you.

 

[Texasmark]

Re: 1977 7.5hp lower unit compatibility?

If you go back to the old OMC(.de) sight you will see that in the lower hp ranges OMC had separate sized LU's for the different HP's offered.

I have a '94 sales brochure on Mariner/Merc's 6-8-9.9 hp engines (7.5 no longer made) and from all available statistics, the only differences are in fuel/air availability, hence different carbs and maybe timing tweaks. So there you have a wide range to choose from if you had that vintage.

I had a 7.5 of somewhere around that year but do not recall if it's partners on either side shared it's components. Possibly it and the 9.9 did. Probably the smaller 5/5.5 would be different and would co exist with the little 4.5 single cylinder they made then of which I had one of them also. The 4.5 was a full blown engine with all the amenities of it's larger brothers and was a super engine for a small boat with one aggravating exception......THE LITTLE SUCKER WAS SINGLE CYLINDER and it drove me nuts!!!!! To the point I sold it just because of that. Having run the OMC alt firing twin 3 hp (a little sewing machine) this thing was prehistoric!

Best I can do.

Mark

 

[emckelvy]

Re: 1977 7.5hp lower unit compatibility?

Any 7.5hp or 9.8hp lower unit will bolt up to your motor. The biggest difference you might find with older L/U's is shift shaft length. This can be changed without disassembly of the gearcase if you're careful.

From around '76 and up, Merc used stainless on the driveshafts so these years are better choices. Earlier models have mild steel shafts which rot away around the seal area, this leads to water leaks and lotsa other problems.

The later model 6-8-9.9 etc are totally different designs and won't interchange.

Older-design 4/4.5hp Singles or 6hp Twins might have the same size gearcase but the propshafts are different and take different props which will be too small in pitch for a 7.5. You might find a 6hp lower & prop suitable for a 7.5 kicker on a larger boat, but not for propelling a small runabout. And no 6hp will have a stainless shaft either, being from the 60's.

Best bet is to stick with lower units having stainless shafts. Seen some on eBay and you might also check your local www.craigslist.org you may get lucky and find a parts motor with a good L/U.

Here's a few examples of lower units with stainless shaft in the water pump/seal area, on eBay:

http://cgi.ebay.com/ebaymotors/MERCURY-7-5-HP-9-8-HP-LOWER-UNIT_W0QQitemZ230436388789

http://cgi.ebay.com/ebaymotors/Merc...hort-Shaft-1630-7360A12_W0QQitemZ360234334434

HTH............ed

 

[BanjoSHOES]

Re: 1977 7.5hp lower unit compatibility?

Thank you for your help. Good to know about the 76 and up shafts