1979 Mercury 90HP Inline 6 One Cylinder Dead, Needs Rebuild. Please advise.

[jimmbo]

The Crankshaft is in the engine block and is coupled to the driveshaft via splines. The driveshaft remains in the midsection/exhaust Housing.
It pretty much lifts off, once the nuts are off the studs, which are about 5 or 6 inches long

 

[sludgeguy68]

I just went through a similar situation, did my first rebuild on a 1979 115hp. I think I spent $1500 in parts and boring all cylinders, they were mic'ed and all were out of round as Faztbullet mentioned they might be. I opted to rebuild since I already replaced the stator, trigger, and switchboxes (ADI ignition), which wasn't cheap and I love the TOP motors. I was lucky to find a blind borer who also had the merc piston installation tools - even with both of us wrangling it in, the dressed crank was a bear to get back into the block.

It is time consuming, and yeah a single sentence could equal hours of work for a novice. A factory service manual might be better than the Clymer when doing all this work. It also doesn't hurt to read ahead and get a feel for what's happening. Good luck!

 

[elr]

I have two questions... I bought a flywheel puller/lift eye deal and it has a little metal crankshaft protector. One side is flat, the other is concave... which side goes against the crankshaft?

2nd question is regarding the following pictures. I'm seeing a fair amount of corrosion under some of the covers. Is that normal?

 

[racerone]

Corrosion looks normal.

 

[elr]

Take a look at the damages...

I have the block at a shop that say they can fix me up but they need the pistons in hand. Planning to bore all 6 cylinders and go .015" over on all. Can anyone tell me anything good or bad about Pro Marine? They have their brand pistons in stock and I'm thinking to get those unless someone can tell me a good reason not to.

 

[elr]

This is cylinder #1, the bad one, but a couple others also had some of this flange eaten away as well. Is that simply from overheating?

 

[elr]

Forgot one last thing. This pic is from the clymer manual. I’ve seen other faulty info in manuals before... are these numbers correct?

 

[jimmbo]

More like Detonation/Preignition. Then again... Strange only the upper(when in the cylinder) portion of the Deflector is melted...
Are any of the rings only partially there?

 

[racerone]

I would say those numbers are correct in the manual for a standard bore / piston.----You are correct O/S numbers are WRONG.

 

[elr]

#1 had all complete rings. #2 had a broken ring plus small chunks of piston missing right on the edge of the middle ring groove.

So what causes preignition or detonation? Isn’t that at least partially caused by overheating?

racerone, if my manuals standard numbers are correct, then the oversized numbers are good also, not? Just +.015” added to them...?

 

[racerone]

The 2.890 " is correct !----Not the skirt diameter for the O/S piston !!.-----Wrong prop / plugged or partially plugged carburetor / wrong timing / old or cheap impeller / wrong sparkplugs / poor oil / wrong mix all causes these issues.------------Some folks still think that as long as a motor starts there can be " nothing wrong ?------An inline 6 needs a new impeller every 2 years.

 

[Faztbullet]

All the above plus bad trigger if distributer style or turning too many RPMs(4500-5200 max)

 

[elr]

Previous owner (actually original owner) told me I didn’t have to worry about rpms with this motor because the 115 and 130hp used same block just higher revs and maybe different reeds, etc. Boat never had a tach in it but I plan on putting one in if I actually manage to get it resurrected.

Boat is 16’ alum Starcraft with a 13 1/4 x 22 stainless prop.... I don’t know much about props and I know this isn’t a prop thread, but does that sound like a good setup?

 

[jimmbo]

All the above plus bad trigger if distributer style or turning too many RPMs(<a href="tel:4500-5200">4500-5200</a> max)

The Rpm range is 4500 - 5000. Motor makes its best power between 4700 - 4800, while the engine can spin way past 5000, it will be making less than 90 hp quite rapidly

 

[elr]

Can anyone give a recommendation for or against Pro Marine and their parts? Or are all pistons created equal?

 

[Faztbullet]

Pro is good stuff...they going to sell to you direct or thru a dealer?

 

[elr]

Well that’s what’s got me confused... I called an 800# and they say they have in stock and they’re 77.94/each and they can ship out ups and i’ll receive in 2-3 days.

Later I read people saying they don’t sell direct, need to buy thru a dealer/distributor.... guess it’s the same guys?

 

[jimmbo]

If you called their 800# and they said they have them and can ship, it sounds like Direct Sales to me

 

[Faztbullet]

ProMarine used to sell to dealers only unless their policy has changed in last couple of years.

 

[elr]

Well I hope their policy has changed cause if not then i’ve been had... Will find out hopefully in a day or so when a box of new pistons shows up at my doorstep.