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Merc 800 lower unit compatability need help

P

pugetsounder

Advanced Contributor
"Here's the story: Bought

"Here's the story: Bought a 1978 Merc 800 80Hp used. The powerhead is in good shape, good compression on all 4 cyls. Took it in to the shop dumped another $500 on rebuilding carbs, new wiring harness, and waterpump impellor. Put it on my boat and couldn't plane it. Turns out it's a 15" shaft! Who woulda thunk it. I still can't figure out what a 15" shaft 80hp would be good for. Maybe racing or a boston whaler. I tied to sell the motor with no luck. Nobody wants a 15" shaft motor that big.

Anyway to save my expenses I thought motors with blown power heads and good lowers are pretty cheap. If I could pick up another late 70s 4 cyl merc with a 20" shaft which is quite common, I could save myself. So what years and HP ratings could I use the mid shaft and lower unit and would bolt onto my current power head?

Also what would be easier, take my power head off to bolt onto another motor or just buy a mid shaft and lower unit to bolt onto my motor? Thanks.

ps I would also like to buy one with power trim as well."
 
"Your asking quite alot here.

"Your asking quite alot here. Your best bet would be to find a run out long shaft and swap heads, its a easy job, worst part is cleaning the gasket surfaces. You can check sites like crowley marine to compare which years would work by comparing which ones use the same gaskets.
I have 2 questions, why would an Advance member take it to a shop to have the carbs rebuilt, and why did you not know it was a short shaft?
Sounds like that engine would be great for a river john boat as is."
 
"Thanks for the fast response.

"Thanks for the fast response. And thanks for the compliment on "advanced" member but I may know my way around an I/O I don't know squat about outboards. We bought it off of craigslist and of course it was sold "for a friend" the guy we paid didn't know much about it. Around here it seems 20" are short shaft and 25" are long shaft, I don't think there is much of a calling for 15" shaft motors here. never dawned on me it could be. Of course when you buy used there is no return policies so now I have to deal with it and make the best of it.

I am assuming that Mercury probably made several power heads that fit the same drive units? But I don't know if the 90-115hp 6 cyl would fit a 4cyl power head. Or if the 800 lower and the 850 lower are the same? I'm hoping someone here knows older mercs like the back of their hand. If so these old mercs are a dime a dozen around here with blown motors. I've seen them go for a couple hundred bucks even with tilt and trim. Just need to make sure my power head will fit. Or I end up with a yard full of junk I don't need.

BTW what's the crowley website you speak of? As far as jon boats you would need a pretty big one for an 80hp, most aren't rated for much more than 45hp with a short transom. Like I said I still don't know what an 80hp 15" shaft would be a good use for. Just didn't dawn on us to check. "
 
"That explains everything, sor

"That explains everything, sorry for the grief. This site you can look up parts by year or serial number if you find another engine. http://www.crowleymarine.com/parts.cfm
I don't know what will work, but there are lots of 80hp.
That engine with a jet drive would make the perfect duck boat on a river.
I hope someone else helps out here."
 
"The short shaft on that big o

"The short shaft on that big of a motor is a bit rare from what I have found out. They are still in a bit of a demand for older aluminum boats around here (west coast Wash state) River runners put jet pumps on them and they work well. I just sold one a few months ago to a guy who thought we would have to build up his transome and use a long shaft (20"). He was looking on ebay for months and was real close to getting a long shaft out of Texas. I had a sign out saying outboards for sale and he stopped in. Man was he tickled. I had the short shaft and the jet pump that I wasn't going to use. He couldn't believe it. He was looking in Texas and I was 2 miles away from him all along.

Anyway, they are rare and there is a demand.

Thats my story. Merry Christmas.

BB46"
 
We had the motor for sale on C

We had the motor for sale on CL for a month in the summer with no bites. Maybe I can trade or sell the lower once I get the motor working with a longer shaft on the boat.

Thanks JB that website is exactly what i was looking for short of somebody actually telling me. i plugged in several hp ratings and years and it looks like all of them similarly use the same mid shaft casing and lower casing. good stuff.
 
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