'76 Merc 850 - Damaged piston skirt, and other fun...

[BR]

I?ve been working on ?76 Merc 850. Motor would start & run but not idle. I had good spark to all four cylinders. Compression is even on all four 120 psi +/- 2psi. I removed carbs & cleaned, replaced fuel lines, etc. Still wouldn?t idle or even run below 1,000 rpms. I tried the old trick of placing my hand over the carb throats while running. Upper carb had vacuum and would stumble when I did this. Nothing on lower carb. I suspected an air leak into crankcase, maybe a bad reed or a crank seal. Upper two plugs were oily and wet, lower two were clean & dry. Seems the motor?s only running on top two cylinders. So I tore it down & opened up the crankcase to look inside. What I found was a bit of rust stains on the lower end cap and some water in the crank seals leading me to believe that the seals were at least allowing air and possibly some water into the crankcase. I believe this may account for the lower 2 cylinders not running. The lower bearing seems ok ? it?s not rusty and it spins freely. The real surprise came when I inspected the pistons. All seem to be fine except for #4. The piston skirt is gone in the area below the hole for the wrist pin? Isn't the wrist pin 90* from this hole? (see photos). I did not find any fragments of the skirt in the crankcase. Question is where did they go? Into the combustion chamber and out the exhaust? Don?t know how long ago this may have happened or whether it has any effect on current poor running symptoms. Any ideas what may cause a piston failure like this? What should I do to repair? As I said earlier, I have good, even compression even on #4. Should I replace the #4 piston anyway?

 

[emckelvy]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

That isn't a wrist pin hole, it's the remnants of a "power ported" piston. It'll never run right unless you replace the piston.

Be sure to renew the bearings on at least that crank throw, as there will have been all sorts of chunks of metal flying around the innards. Inspect the crank journals carefully for damage/wear.

Can't say I've ever seen damage like that before; it's almost like someone or something was pounding the piston skirt!

You'll have to pull the entire rotating assy out of the block in order to R/R the piston, be sure to check everything for problems.

HTH & G'luck..........ed

 

[BR]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

Ed-
Thanks for the reply. I guess I'll find out when I pull the whole crank out of the block but...is there a lip on the cylinder wall, perhaps the intake ports where the edge of the "power port" hole could have caught causing mechanical interference and breaking the skirt during the stroke of the piston? That's the only thing I can think of that might have caused this kind of failure. Exactly what does the "power port" do? Also, do you recommend new rings and honing all the other cylinders in addition to the new piston?

 

[TheWoodCrafter]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

You would ALLWAYS install new rings and hone if you pull a piston out of the cylinder whether you replace to piston or not.

 

[BR]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

Took the intake port cover off to see if there was any damage / pieces left in there. Things look ok but now I can see why the piston can't be left in. I took a couple more pictures showing the piston skirt hole passing over the intake ports. #4 cylinder is to left in the photos. The port has a little "bridge" across the center which would prevent the kind of hang-up I was thinking about in my previous post. I still have no clue how the skirt broke loose like that. The crank still turns over and there's no interference at all.

 

[Chris1956]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

BR - I did not think the 850 came with the power ported piston as original equipment. It is interchangeable however. it might mean someone worked on this motor in the past. You might have the #4 cylinder measured for excess cleanance. Maybe the piston skirt was slapping around. Inspect the crankcase for chips of the piston, and make sure you put three seals in the lower end cap, as well as a new oring, to keep out that pesky water.

 

[BR]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

Chris-
They are all power ported pistons and I'm pretty sure I'm the first one inside this motor. Unless someone re-painted the powerhead after opening it up, which I doubt. All crankcase bolts seemed like they had never been disturbed. I pulled the crank out of the block. The #4 piston had a stuck upper ring. My guess is this kicked the skirt towards the intake port side of the cylinder wall and it caught an edge, shredding the piston.

 

[Chris1956]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

Not that it matters, but you might check the OEM parts list to see if the powerported pistons were standard. Compare the piston part numbers to the '77 Merc 1500, which did have the power ported pistons. Why did the rings stick? Overheat? lack of oil? You probably need to find out the cause for the piston to stick.

BTW - The power ports do not function, unless the block is machined for them.

 

[Yepblaze]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

In 32 years time it's not uncommon to find almost anything inside one of the older motors.

And yes, if someone intentionally changed out parts with something less than worthy, they may indeed paint the block, and sell a story on how they never touched it.

You could be dealing with something some unscrupulous idiot assembled. And if so, all parts and components would come into question. Meaning perhaps more than the obvious parts need attention. And if so, consider it for replacement rather than repair.

 

[BR]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

Okay, I'm pretty sure the "power ported" pistons are original equipment for this motor. Merc's website says "power ported" pistons were first introduced in 1973. This motor is a '76. Second, all the pistons have the power port and I've taken a look at the cylinders and they have what I think must be the extra machining Chris mentions above - see pics just below intake ports. Third, I compared the Merc part no. for the pistons with the '77 1150 as suggested by Chris and the part nos. are very similar, only the suffix A6 vs. A12 is different. My pistons are 2 ring versions. I think the 1150 may have 3 rings each. Another question: The condition of the #4 piston is suspiciously clean compared to the other three which were pretty coked up with a layer of carbon. Does this mean a.) the cylinder was just not firing or firing irregularly or b.) there was water or steam entering the combustion chamber or c.) both a & b? Having never pulled pistons out of an outboard motor before I don't know, is the amount of carbon on these pistons considered normal?

 

[emckelvy]

Re: '76 Merc 850 - Damaged piston skirt, and other fun...

850's had power-ported pistons and similarly machined block. The 2-ring pistons were an effort by Merc to reduce friction over 3-ring pistons and increase HP (and also decrease cost, no doubt from 4 less piston rings per motor!).

You can replace these with 3-ring power-ported pistons with no consequence. Or re-use any 2-ring pistons which are good. Just be sure to install new rings. Cylinder sealing is critical and you can't scrimp when there's one less ring on the piston to start with.

Some replacement pistons are only 2-ring but they are (at least the top ring) of "Keystone" or wedge-type design which uses combustion pressure to force the ring against the cyl wall for improved sealing. This type of ring is also pretty much "self-cleaning" and has a much less tendency to get carboned-up.

It's not unusual to see light carbon deposits on the pistons. Signs of poor maintenance or use of inferior quality oil are piston tops carboned-up really bad, and/or with carbon-stuck rings.

With today's fuels, you need to run a carbon-cleaner periodically, to keep engine deposits down. Seafoam is real good for this, as are numerous other mfr and aftermarket cleaners.

Your scrupulously clean #4 is very likey a result of water ingestion, as the water blasts all the carbon away. Typically from leaking crankshaft seals, or problems on the exhaust manifold side.

It could also be from an excess of unburnt fuel, from a non-firing cylinder.

But I'd bet on water problems. At minimum pull the exhaust manifold cover and inspect for leaks, inspect the exhaust baffle carefully for cracks or perforations. Renew gaskets and torque bolts to 150 inch-lbs.

Very common to see 850's with bad crank seals, I have one in the garage that had a rusted lower crank bearing self-destruct, with the bearing parts going thru #4 cylinder! Needless to say it beat that cylinder up pretty bad.

And if you are gonna fix this motor, I'd renew that bottom crank bearing unless it's perfectly smooth after you clean it out with solvent and lube with a light motor oil or equivalent. The "6206" bearing is readily available at most auto parts stores or go to your local bearing house.

HTH.......ed