What could cause this?

[trooney]

Just wondering? Intake side. It happened not even running, trying to start...

 

[huskihl]

Just wondering? Intake side. It happened not even running, trying to start...
View attachment 139534

If it's a hybrid, the cylinder probably wasn't lined up perfectly with the case. I've seen it on other saws where either the cylinder bolts were loose or the crank bearings were bad. As soon as something is out of whack, the rod hits the skirt

 

[Wood Doctor]

I lost one this way when the crankshaft bearings wore out and the piston skirt collided with the case and took a chunk out of the case as well. I rebuilt the entire engine with a whole new assembly and whatever else on the old saw that I could salvage -- flywheel, tank housing, muffler, handles, chain brake, clutch, etc.

 

[trooney]

If it must be told, it came off a JMS 034. But I gave it to another member and he said the base was square. The bearings seem alright, gotta check into that though.

 

[Woodpecker]

Go figure another methed up jms saw.

 

[trooney]

I'm more interested in the cause. If I understand the cause then I can avoid it doing this to another piston. The cylinder is not the problem I believe.

 

[redline4]

If the piston whacked the rod, wouldnt there likely be a mark on the rod?
Ive seen the marks little 7mm valves put in pistons when they meet each other. I would think it would have left something?

 

[Redfin]

Lets see the underside of that skirt and the intake on the jug Tom.

 

[smokey7]

It seems the piston had the standard foreign material scoring from dirty parts or not cleaning out chips before assembled. I also would like to see more pictures of the piston cylinder crank base rod you know the works.

 

[cus_deluxe]

Lets see the underside of that skirt and the intake on the jug Tom.

Man, theres gotta be at least two jokes in there...

 

[Bigmac]

It could have gotten a crack or stressed when the top end was off and someone rotated the crank, like trying to remove the clutch with the cylinder off? If it was a runner right before it happened then...maybe excessive intake wear, or someone tried to take the clutch off with a stick in the intake? 46mm piston in a 48mm bore? Lol

This happened form excessive wear!

 

[Redfin]

Man, theres gotta be at least two jokes in there...

We have been waiting 53 minutes?

 

[460 magnum]

I have a 461 that done the same thing but this is on the exhaust side.

 

[Canadian farm boy]

You can see that the piston has been dickered with. Looks like the windows have been messed with. I'm gonna guess that if the windows were opened up that the whole piston has likely been lightened.
I'm wondering what sort of run time is on that piston. If the intake port has been widened and the bottom of the port flattened (maxed out) a piston with excessive wear rocking in the bore could of snagged the port edge on the down stroke.
This is just my guess. I'd love to see the intake port and see a measurement of both piston skirt thicknesses.

 

[trooney]

You can see that the piston has been dickered with. Looks like the windows have been messed with. I'm gonna guess that if the windows were opened up that the whole piston has likely been lightened.
I'm wondering what sort of run time is on that piston. If the intake port has been widened and the bottom of the port flattened (maxed out) a piston with excessive wear rocking in the bore could of snagged the port edge on the down stroke.
This is just my guess. I'd love to see the intake port and see a measurement of both piston skirt thicknesses.

I will get them Monday as I'm working 7-7 tomorrow. I'll also get the pictures that John suggested. My initial thoughts were he took an older piston and lightened it up, not the best combo. But I think its a 290 piston not an 034 because the rings were 1.5.

 

[huskihl]

I will get them Monday as I'm working 7-7 tomorrow. I'll also get the pictures that John suggested. My initial thoughts were he took an older piston and lightened it up, not the best combo. But I think its a 290 piston not an 034 because the rings were 1.5.

A 290 piston would have a full skirt with no windows. If yours is supposed to have 1.2 mm rings, I'd say it's an AM piston

 

[trooney]

Maybe...

 

[huskihl]

Maybe...

Been known to happen...lol

 

[drf256]

Very loose AM piston rocking in bore. Skirt caught intake floor.

I could feel the piston skirt getting caught there on the last JMS066 I fixed. The skirt had .009 clearance.

 

[srcarr52]

This happens when the piston is worn out and rocks the skirt into the intake port.