'78 merc 7.5 hp rebuild

[sledneck22]

Background:
This is my buddies 7.5 hp Mercury. He couldn't get it started this year and it didn't hardly run last year so I volunteered to help get it going. We had to use all new gas and oil to get it to barely run. Then when running at a higher idle, on decel, it would knock and ping. I suggested we take it apart because it sounded horrendous. I could move the pistons in the cylinder about a 1/16".

So I tore the entire power head apart, which wasn't too hard in my opinion, and found it was a good idea we did so. The pistons are pretty much toast and the rings were surprisingly still there but not doing a lot.

I'm to the point where I need to order new parts. I want to try and find a new set of rings, pistons, and wrist pin needle bearings. Those I can find easily enough on ebay and different parts stores online. But what I'm having a problem finding is a power head gasket kit. Does anyone know where I can find this for reassembly?
Also, any tips, hints, or tricks to reassembly? What kind of rebuild lube should I use for this 2 stroke?

His serial number is 5015859.
Any help is kindly appreciated.
Regards,
Kyle

 

[sledneck22]

Re: '78 merc 7.5 hp rebuild

TTT, any help would be appreciated.

 

[Bondo]

Re: '78 merc 7.5 hp rebuild

TTT, any help would be appreciated.

Ayuh,.... Welcome Aboard,... I would Guess Mercury Marine is gonna be 'bout the only place for that stuff,...

How bad do the cylinders look,..?? that piston is awfully scuffed up,...

If there's a 1/16" clearance in the bores, that motor's pretty much a Parts motor,...
It'll have to be bored, 'n new over-sized pistons fitted,...

Really not worth the effort, nor the money involved,..

 

[racerone]

Re: '78 merc 7.5 hp rebuild

Damage ( if any ) from the piston shown will hone out.----Post 3 more pictures showing the other piston and other side of the pistons.

 

[sledneck22]

Re: '78 merc 7.5 hp rebuild

I can get more pictures in the next day or so but I took a picture of the worst place. The other piston isn't too bad. I think most of the play in the pistons came from the rings being shot. It looks like they spun on the piston. The bore actually looks to be in good shape with very minimal scuff or scores. I think that will hone right out. Being that the motor ran and idled fairly well before tairdown, makes me believe a set of rings and gaskets and hope it starts back up. If he gets another year or two out of $100 invested I don't see why not rebuilding it.

 

[racerone]

Re: '78 merc 7.5 hp rebuild

Rings can not spin on the piston as there are little pins in the groove to prevent that.-----------Outboard pistons are tapered to compensate for temperature difference from the top to bottom of the skirt.--Fit one of the rings in the bore and post the measurement of the gap that you find.

 

[sledneck22]

Re: '78 merc 7.5 hp rebuild

So we were able to pick up an early 80s model 110 9.8 hp mercury. The problem with that motor is the lower unit is seized up. What are the odds that the 7.5 hp lower unit will bolt right up with the 9.8 hp motor? I'm having difficulties cross referencing parts. Anyone ever done this?

 

[racerone]

Re: '78 merc 7.5 hp rebuild

The lower unit will bolt up.----But there are some subtle differences in the end of driveshaft.-------Spline length / o-ring groove.--Compare those.