Diagnose my failure

[phantomyamaha]

Anyone ever seen this before, striped my 2.6 200 yam after it melted a piston on the bottom cylinder and found this major crankshaft failure, 2 cylinders have worn badly on the guide part of the crank number 3-5, the rods are equally worn, the motor runs on pre mix at 50/1 and ran perfectly until the moment it melted its piston, the motor runs .020 overbore with pro marine pistons and pro marine con rods, all the bearings are fine not showing any signs of wear or skidding etc the pistons are all fine not showing any unusual wear etc [apart from the one that melted obviously].
Has anyone seen this issue before? or even something similar? any input would be greatly appreciated.
cheers.

[Dave S]

Sounds like the bores are not 90/00 to crank..... bad bore job. Unless top guided.....then sloppy bores..... If ya need I have cranks and rods.

[H2OPERF]

Agree with above ... probably bored off the deck su
rface.

[Dave S]

I have also seen mismatched rod caps.... Check them out.

[Jay Smith]

Dunno what happened BUT I will suggest from get go IMO your not running enough oil ! I NEVER premix a motor any less than 40:1 your asking for lubrication failure !!

Jayb

[ChrisCarsonMarine]

Dunno what happened BUT I will suggest from get go IMO your not running enough oil ! I NEVER premix a motor any less than 40:1 your asking for lubrication failure !!

Jayb

Not looking for an argument,but I generally disagree on this.The OP did not mention the rpm the motor turns,but I'm guessing this is a more or less stock motor turning less than 6500.If so,in my opinion(shared with Yamaha Factory)50:1 is called for,recommended,and quite adequate.Until recently Yamaha sold large "C Service"customer value motors that were premix at 50:1.and smaller ones,40 hp and down I think premixed at 100:1...does make me a little nervous.
In 1963 OMC(24:1) and Mercury(20:1) both announced their motors should be run at 50:1 as long as a TCW rated oil was used instead of 30 wt. non detergent motor oil and the motor(OMC)was equipped with roller and needle bearings,not babbet.
We have converted hundreds of all makes and sizes of carburetor and efi motors to premix at 50:1 with NO lubrication issues.
I fully agree with you in the case of high performance high revving motors,usually 40 or 32:1,depending on rpm and bore type...Chris

[Motv18]

Tough one. Almost looks like machining marks, notice how it’s not polished. It should in my opinion be shiny if was wear. It could have happened in the failure a meltdown will stress the crank hard and push it places it shouldn’t go. Check the races for out of round from the pressure.

[Chaz]

Anyone ever seen this before, striped my 2.6 200 yam after it melted a piston on the bottom cylinder and found this major crankshaft failure, 2 cylinders have worn badly on the guide part of the crank number 3-5, the rods are equally worn, the motor runs on pre mix at 50/1 and ran perfectly until the moment it melted its piston, the motor runs .020 overbore with pro marine pistons and pro marine con rods, all the bearings are fine not showing any signs of wear or skidding etc the pistons are all fine not showing any unusual wear etc [apart from the one that melted obviously].
Has anyone seen this issue before? or even something similar? any input would be greatly appreciated.
cheers.

Was that picture from the movie ... Now you see me ?
Hard to figure out what happened when you show what is quite possibly the end result of a mistake made during your .020 rebuild. Or the problem fixed that caused the need for the .020 rebuild in the first place ...
As far as you pointing out the technicolour rod journal (inner race). The soldier
(piston) in that hole couldn't keep up. Put excess load on that assembly. The heat produced is your witness.

[phantomyamaha]

Firstly thanks for your replies.
Ok as for 50-1 pre mix I'm fairly confident its fine to run this ratio, I've another yam v6 I've been running for 11 years at 50-1 and it never misses a beat so I'm confident it ok. i rev the motor to 6k running the stock yam rev limiter, i am however running 28* advance.

As for a bad bore job it had crossed my mind, the machinist who did it has a good reputation here in the uk, what can i do but trust the reputation of a well recognised engine builder, obviously theres nothing to say he didn't make a mistake on my motor though, also if the bores were far enough out of 90* to the crank do you not think the soft alloy pistons would be showing signs of unusual wear as theyd of been running up and down the bores at an angle so would show heavy wear, the pistons however are fine.

As for technicolour rod journals, i agree it looks like bluing from excess heat, however this colouration cleans of easily with some spray oil and a scotch pad so i think its just a build up of oil, all 9 journals on the crank are the same colour, the crank also had this colouration when i initially removed it from the donor engine, and again it very easily cleaned off, also the damaged big ends are not the cylinder witch suffered piston failure, they are both however on the same side of the engine, weather theres anything in this???

[Chaz]

If it's not from the movie ... how did you get it to levitate with one finger ..




Make up my mind ... does is have melted pistons .. or are they prestine ?

How does the outer race look ?

How bout them little round guys that roll around tweenst the two ?

The partitioning device for said round guys ?

Condition of gudgen pin and bits ?

The brown gelatinous easily removed goo, would that happen to be some of Jays special ... nevermind .

So you see, without the rest of the picture .. it's hard to tell who's hiding the Ace of Spades ...

[Dave S]

I have a rod out of a HP merc 2.4 that looks like it was on that crank....big rod SPS bolts..... non matching cap.....same wear.

[flabum1017]

If it's not from the movie ... how did you get it to levitate with one finger ..




Make up my mind ... does is have melted pistons .. or are they prestine ?

How does the outer race look ?

How bout them little round guys that roll around tweenst the two ?

The partitioning device for said round guys ?

Condition of gudgen pin and bits ?

The brown gelatinous easily removed goo, would that happen to be some of Jays special ... nevermind .

So you see, without the rest of the picture .. it's hard to tell who's hiding the Ace of Spades ...

I seen that happen to a propshaft when fishing string got trapped on it.............

Do those Yammis have top guided and bottom guided rods like the mercs do? possible go two different pistons or two different rods mixed up in the order?

[Motv18]

Ohh your pointing to the spot where the journal isn’t round, not the scuffs.

Oil/gas just build up there are gum varnish up a sludgey goo fairly easy to clean.

Its bad, but probably not what took it out.

28* is probably what did it. Compression plus fuel and peak pressure is hot and even steals power as you squeeze the explosion. That said port timing, fuel, ect actually effect what degree you can run.

Trying to think if if yours is a low compression # 5-6. Yamaha does a thing sometimes with 4 cylinders higher then the other two. Making it 6 even compression messes a bit with the bearings sometimes.

[phantomyamaha]

The bearings are fine, as are the cages, I’d clean them up and run them again, the big end journal in the con rod is fine, the end play on the big end is fine when measured on the dial gage, it’s only the bottom guide that is worn, both damaged rods are only worn on the bottom not top and bottom wich leads me away from thinking this was caused by the rod over heating and jamming in the crank as that would damage both sides of the guide I would expect, 5 pistons are fine I will re ring them and run them again, again the melted piston was on a cylinder with no rod damage.

28* degrees advance yes I suspect it may be a bit much, I’m running 140 psi compression on this motor, would you not expect damage on all 6 rods if it was caused by timing ?
My motor ran 140psi on all 6 cylinders.

[Motv18]

It could, if split compression as those would also be lean when raised.

Would have to check it even is one though. Nm that’s on epa power heads

You’ll find a cause eventually, could be as simple as went lean from a random air bubble.