1979 115hp Mercury - Pistons

[Moehme]

Quick scenario:
Have a 115hp inline 6 cylinder with 120 lbs compression on 1, 2, 4, 5 and 6, but only 30-35 lbs on cylinder 3. Tried a couple quick "backyard mechanic" ideas to no avail so I'm down to my last resort - overhaul.

Found a mechanic (my wifes cousins boy) who will do the work for me. Explained what was going on - his thoughts were (once the engine is torn down):
1) Could possibly be as simple as honing the cylinder and installing a new set of rings (standard size).
or
2) Rebore (+0.030") and install OS piston and rings.

This guy lives 500+ miles from me (near our cabins in Minnesota) and I'll be taking the boat up with me the end of July for us to work on, so I'd like to have all the parts we'll need when we I go up.

My questions are:
1) Regarding boring the cylinder, Seloc manual says I should only go 0.015" over - Does it really make any difference (0.015" or 0.030")?

2) Do I order a high dome or low dome piston?

3) Most common piston kit brands are either Mallory or Sierra - any preferences?

4) Do I need to order a complete gasket set or can I order just certain individual gaskets (keeping the cost down)?

Thanks

 

[Yepblaze]

Re: 1979 115hp Mercury - Pistons

Many times there is nothing more wrong with the bore than some aluminum left behind from piston failure. It can be removed with some acid.

What is, or will be very important is that after the repair it survives.

Chances are it either ran too far advanced, too lean, or both for the conditions. If not rectified during service, you will probably experience this malady again.

I believe pistons come in various shapes and flavors, and are somewhat interchangeable. It's not uncommon to bore one hole, but takes a machinist that can do blind hole.

You may find your decisions answered for you as you dissasemble, evaluate, and locate your possible parts sources, to see what's available within budget.

 

[Moehme]

Re: 1979 115hp Mercury - Pistons

Okay, after doing more reading, I think I've answered all of my questions but 1, reboring the cylinder.

Seloc manual recommends that I only go 0.015" over (referencing something about an American Power Boat Association recommendation) - Does it really make a difference (0.015" or 0.030"). and if it doesn?t, I?m half tempted to go with 0.030?

 

[Chris1956]

Re: 1979 115hp Mercury - Pistons

Reboring is necessary to remove the scratches in the cylinder wall. While you can rebore to any size you want, most folks rebore to minimum necessary to get walls clean. That way if it happens again, they have ability to repair again.

 

[corm]

Re: 1979 115hp Mercury - Pistons

Hi,

Low dome pistons. todays fuel is poor at best.
You will need all the gaskets to do it right.
The rest of it you seem have under control or
have recieved good advice already.