I just bought a 2003 Nitro with a 115 Mariner 4 cyl. 2-stroke. I was getting it cleaned up and doing some maintenance on it, including changing the lower unit oil. When I pulled the drain plug, a bunch of water came out followed by a small amount of black sludge. Alarmed, I contacted the previous owner who told me that he had the lower unit serviced recently and there were no issues with it he was aware of. Of course.

I wanted to take it out anyway for the first time, so I refilled it with gear lube and took it out on the lake yesterday and ran it around for a bit. After a while, I noticed the boat was behaving strangely when accelerating so I pulled the motor up and lo and behold a large chunk of the fin-like part at the very top of the lower unit was missing a huge chunk. I know we didn't hit anything, so it must have fallen off. But why? I'm thinking the previous owner must have struck something and cracked that upper fin on the lower unit and also caused some kind of leakage allowing the oil to escape and water to get in.

Now I'm faced with replacing the LU. From my research, a new Mercury LU runs about $1800. A new 3rd party LU from SEI is around $900. The other option is to purchase a used one on Ebay for about a grand. I'm not sure which option I'm going to pursue. The OEM is expensive. The SEI is not OEM but I've heard good things about them on this forum as well as others, but there have been some sob stories mixed in. A used LU means I don't know the history of it. I need some guidance on this. I've been doing some reading on some of the forums on the Internet and opinions are all over the place.

We have seen some problems with SEI on higher horsepower motors. For your application it should be fine.

Thanks, I was wondering if y'all install a lot of SEI units on outboards around my size?

that piece can probably be welded b+b propeller in n.richland hills can probably fix it. as for the water just needs to be resealed

Sorry to hijack I was going to post this myself
I'm in the same boat now, but I have a 200 3.0 merc.why the bad rep for higher hp motors

I went ahead and pulled the trigger on an SEI unit. Don't want to spend all of the fall season with my boat in the garage, hopefully it won't. The customer service guy for SEI was very helpful and was able to match up the LU using my serial number. Said it would arrive in about a week. $915 shipped with 2 quarts of lube. Includes shaft and water pump. 3 year warranty. Going to roll the dice based on the good reviews I've read.

Prop shafts break

If you seen how some of those guys run their boats you would understand. Why do the high HP boats have an SEI lower on them? Because they tore up the factory unit. We are talking 250 up hp. Hot foot`s and lowers do not get along. I have installed a lot and have had 2 that were returned for warranty. A 225 Yamaha broke the prop shaft and a 225 Evinrude was thrown into reverse running over 20mph. SEI fixed them. Actually they just sent another fresh lower.

This is seen a lot. Also if you fish tournaments then the warranty is voided! We have had to send about 8 back for customers that bought them and had failures. Remember if your propshaft breaks they will give you another lower unit but not a new stainless prop!

8 out of how many? 10? 12? 1000? Just curious about the rate of failure. I haven't heard about propshaft breaks on SEI units in other reviews, mostly leaking seals, blowing out the gears, paint cracking... I wonder if all people who replace their lowers with these follow the stringent break in period, or do some folks strap them on and run them wide open from the get go?

I have had 2 out of 34. The factory new ones are blowing too. A lot of bad lowers are replaced by insurance claims.

You already got it now, but next time have seals checked and weld it up, cheaper that way. Worked in a shop for a while, had one sei come in bad but it was abused and propped wrong. At $915 bucks you cant beat it with a 3 year no fault warranty.

My concerns with reworking the current unit are, how long had the previous owner been running it around, unaware there wasn't any lube in the gearbox, how much damage had the gears already sustained, and the cost of a potential complete rebuild, which could amount to about as much as a new unit, right? Add on the cost of welding the broken piece. But then, how many other points on the unit have sustained some kind of similar stress injury? Too many variables, I just made a decision to give the aftermarket a shot. OEM replacement would have been optimal, but too expensive.

Heck just get the old when fixed when you get the coin and you got a spare

You should be able to sell that old one as salvage then.