Stihl MS 660 Big Bore First Video and Post Mortum

[Terry Syd]

If the rod broke first, I'd expect the top of the rod to still be sitting in the piston. IMO the piston failed first, once one side had broken away, the twist on the rod caused it to snap.

A close up examination of the broken surface on the wrist pin end of the rod may give an indication of the forces that snapped it.

 

[mtngun]

So what was your squish?

I aim for 0.020" - 0.025" on an engine that size.

Squish that is too tight can cause detonation, even before the piston contacts the top of the jug.

But what could I do differently?

Always leak test a new engine.

Get a tach and tune the H screw to 12,500 rpm. Some people are good at tuning by ear, others like me are somewhat tone deaf. A tach is a good sanity check.

It may well be that the piston was flawed and would have flown apart no matter what. But you'd feel better about it if you knew that the saw had been leak tested and tach'd.

The video looks like your saw was cutting well and not bogging easily, so I'm not sold on the too lean theory. If it had really been leaned out to 16,000 rpm, I don't think it would have much grunt in the cut.

 

[mtngun]

If the rod broke first, I'd expect the top of the rod to still be sitting in the piston. IMO the piston failed first, once one side had broken away, the twist on the rod caused it to snap.

Can't argue with that logic, Terry.

My rod was bent when the piston broke. I figure the crankshaft inertia kept the rod going up and down for a split second after the piston broke, rod was slammed into piston.

It's not a good thing for pistons to break like that. :msp_confused:

 

[bayard]

clearance

i was thinking that .020 was good for o64 - 066.is .025 much safer? .k

 

[parrishturf]

The Pictures you requested.

If the rod broke first, I'd expect the top of the rod to still be sitting in the piston. IMO the piston failed first, once one side had broken away, the twist on the rod caused it to snap.

A close up examination of the broken surface on the wrist pin end of the rod may give an indication of the forces that snapped it.

Here are the pics...



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[Terry Syd]

The lower part would be too damaged from thrashing around to give you much information, there may be a bend it in, but hammering away at the cases wouldn't help it to remain straight either.

The top part should be in pristine condition after the break. It appears it may have snapped sideways and left a bit of 'tail' on the side away from the breaking side of the piston. Take a closer look at the break, it may give you more information.

 

[Grande Dog]

The saw definately wasn't ready to cut wood. Gregg, what are your thoughts on the piston hitting the head?

Howdy,
In most cases if the piston is pecking at the cylinder, the upper ring will get seized in the groove. I wasn't hearing it in the video but, if the squish is to tight the shock waves can still be damaging.
Regards
Gregg

 

[mtngun]

i was thinking that .020 was good for o64 - 066.is .025 much safer? .k

0.020" is adequate. The OP had 0.016".

Bear in mind that it is difficult to dial in squish exactly, even if you make a custom gasket, the gasket may compress a little, while the sealant may add to squish a little. Plus it is hard to get repeatable readings if the squish band is tapered as many are. Hence the 0.020" - 0.025" window.

 

[parrishturf]

The lower part would be too damaged from thrashing around to give you much information, there may be a bend it in, but hammering away at the cases wouldn't help it to remain straight either.

The top part should be in pristine condition after the break. It appears it may have snapped sideways and left a bit of 'tail' on the side away from the breaking side of the piston. Take a closer look at the break, it may give you more information.

Here is a close up of the granulation in the forging.

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[gink595]

Pictures are always deceiving but that piston does look like the cast had a flaw in it. Whatever that dark spot is, isn't right! I don't think the piston was hitting the squish band either.

 

[Terry Syd]

That's not a forging, its a cast piston. There does appear to be imperfections in the cast piston. Note the different colours and the granulation. It doesn't appear that the piston was cast with a consistent material or pouring.

The material used for casting has a lot to do with the quality of the piston, I've seen after market pistons that were so soft (a lack of silicon) that they could be scratched with the thumbnail. OEM pistons can also be junk, I remember a failed piston out of a Suzuki that had been poured into a cold mould. The imperfections on the surface of the piston were enough that it should have been easily caught by quality control. If a factory such as Suzuki can't catch visual imperfections, then obviously internal imperfections (such as your piston) would be impossible to catch without some sort of x-ray/ultra sound analysis.

 

[parrishturf]

Pistons...

So I want to build a 660 big bore that can push limits, versus a work saw with more cc's.

Should I go to Weisco for the hotter build and stay with the cast piston for the work saw?

I have no clue where to start looking for a forged piston...

 

[Terry Syd]

You don't need a forged piston. Cast pistons are used in all the factory bikes (and saws). It comes down to the material used and the casting procedures. By the 1960s cast pistons were of very high quality. In your case, it may be that the cast was poured from a ladle (rather than the bottom of the pot) and some of the slag from the top entered the cast.

Perhaps some other member of the forum can recommend a piston that they have confidence in.

 

[AUSSIE1]

The video looks like your saw was cutting well and not bogging easily, so I'm not sold on the too lean theory. If it had really been leaned out to 16,000 rpm, I don't think it would have much grunt in the cut.

Lets put the lack of four stroking aside. Three turns out on both and you can hear a lean condition above idle.

I noticed it cut well considering also but that sound is a dead give away.

I agree the piston had a flaw.

With everything as it should be, this saw would have been a real runner!

 

[barneyrb]

Pictures are always deceiving but that piston does look like the cast had a flaw in it. Whatever that dark spot is, isn't right! I don't think the piston was hitting the squish band either.

Bingo, the light indicates a fresh fracture and the dark indicates an old break that has had the time to get impurities or contaminants in it.

Not saying this was the cause but it is where I would start looking.....

 

[blsnelling]

My 066 only has .017 squish with a factory topend and gasket. .016 squish didn't kill this saw. I've seen tighter from other builders with no problems at all.

 

[wigglesworth]

My 066 only has .017 squish with a factory topend and gasket. .016 squish didn't kill this saw. I've seen tighter from other builders with no problems at all.

But why push it? That extra .003-.006 ain't gonna gain u much comp wise, but might just save ur slug from damage. I feel anything under .020 is too tight. Heck, I've seen the combustion chambers have some really bad dips and humps in em more than .020.

 

[mtngun]

Dark spot looks like porosity to me, and looks very similar to my broken BB piston. Not to say there weren't some other things going on with the saw, but now that we've seen this picture, my money is on the porous casting.

 

[blsnelling]

But why push it? That extra .003-.006 ain't gonna gain u much comp wise, but might just save ur slug from damage. I feel anything under .020 is too tight. Heck, I've seen the combustion chambers have some really bad dips and humps in em more than .020.

I'm simply saying that didn't cause this saw to grenade.

 

[splitpost]

Bummer,looks like piston failure then rod failure,double bummer:msp_sad: