89 Mariner 115hp L4 rebuild

So picked a boat up for basically the cost of the trailer... Had a bad #4 piston and sleeve (piston is more dome top then a OE flat top, and the intake and exhaust ports on #4 sleeve are broken away). Did some searching for a "new" motor and I have one on retainer for $600 shipped with 130-135 compression on all. Also found a bare block for $50 with a guarantee the cylinders can be cleaned up. It's showing up on Thursday and will have it verified to be a viable block. I have 3 quotes on 2 cycle motor machining bearing from $25 a hole to $100 a hole. $25 a hole is my buddy that bores and hones jugs for snowmobiles and the other 2 are close in price at boat repair shops in my area.

Question is... For this year motor promarine offers a full rebuild kit for $875 (pistons, all bearings, rod bolts, and full gasket set) but unknown on if there pistons have an issue with this. There pistons are ~$60 cheaper then Merc. Quicksilver pistons which is the concerning area. Hoping for input on quality of pistons for Promarine or any other brand out there. I'm new to marine motors so don't know the difference between looper or cross flow or any other terms thrown out there from my searches.


Electrical harness question... Spending money wisely for as reliable of a motor as possible... Would it be wise to invest in a new harness from controller to motor, and all the switch block wires even if they appear to be "good" some wires are getting fragile but don't know if new harness is required or if I can remake them/update the terminal ends to make sure there is a quality contact.

End budget for running motor is currently $1,500 and I can do all mechanical/electrical myself. Harness's do not account for motor budget, just makes it more reliable.

Make sure your buddy can do a blind bore motor as that motor does not have a head. WSM makes a good kit for that motor.

WSM is very good too. You should be able to upgrade to the Wiseco pistons also. Wiring will continue to deteriorate, so a new harness is a good consideration. If original ignition components are still in place, I would consider total replacement as well. Might run closer to 2000 bucks but you will have a sweet machine.....you built it and you trust it. I am facing the same with my 1989 115 Mercury, however all electrical components and ignition components were replaced within the last 5 years. Wiring seems to last longer up here in the cold as we only use our equipment for about 6 months a year. You must also determine what caused that cylinder/piston failure and repair that as well.
2 more posts, Mr. Scott and your an "Outstanding Contributer" class. Congratulations!

Thank you guys for the responses. Coils, plug wires, and starter have been replaced last year from prior owner while was was trying to diagnose the issue (it has a dead stop at TDC on #1 so they did nothing but assist updating stuff for me lol). I have checked all electrical components based on factory service manual and all checks out positive but will be double checking all again at final build up.

Areas I still need to review is the oil injection system and carb system. I have pulled the powerhead and placed it on my work bench but have not went any further then pulling plugs and exhaust cover plate. #1 & 2 plugs appeared normal with minimal carbon, #3 had crazy carbon build up, and #4 was crazy burnt up.

Once I know I have a viable block, I will rip down the original powerhead and be looking at those to areas.

Not sure why folks would replace " solid state " components because they are old.-----The new replacements may have been on the shelf for 15 years already !!

Only "solid state" component I would replace is wires... Like switch box to coil packs... If I can source the terminal ends then I'll make the wires myself to eliminate this concern.of this. My thought is... If wires are borderline breaking when disconnecting how much longer will they last before I'm trouble shooting a broken wire.

Good point, Racer. Just that "solid state" components age even faster when exposed to huge temperature differences, vibration, moisture, etc. Maybe I'm stupid but my advice is out there if anyone wants to take it.

How many times do I read-----" replaced the powerpack and motor still runs the same "----Most folks do not know what the part does or how it works !----Cheap to try it as last gasp effort to get a motor to run.-----Folks are afraid of the $100 +/hr shops as you well know.