Need help!

[mortalitool]

Ok so I work on saws for a tree service company. Working on a ms660. Toasted top end. Bad grooving on the intake side of the piston. Deep grooves. I check out the lower end. Crank moves freely. No slop. Bearings feel great.

So I get a stihl piston cylinder kit. New crank seals. I'm thinking I'm doing it up right. Just as I always would/do.

Go to test the saw just now and it pops over right away but then after a few more pulls it feels hard to pull over. I grab my 660 and do a comparison. Yep this rebuilt one seems really hard to pullover. I start freaking out. I immediately tear the jug off and inspect the piston. Wow am I up a **** creek here. Piston shows deep grooves and is cracked. Piston rings are broken. I then look at the jug, deep grooves, both transfer ports are cracked. Wow. I just can't believe what I'm seeing. I paid for the parts out of my own pocket. That's how our relationship works between my tree guy and myself. Always been fine.

This is a stihl oem direct jug and piston kit. No aftermarket stuff here. What the heck is going on? Why would my piston immediately be grooved and jug transfers crack? NEED HELP! what should I be looking for?

I'm thinking offset on the crank? Maybe someone used 2 different cases at some point? Maybe the cases don't mate up correctly?

What do you all think? I'm panicking here. I can't afford to eat these parts. I'm about $360 into this with the jug piston kit and other misc parts it needed.

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[Jed1124]

Wow! That's some nasty damage. Where the circlips intact when you pulled it apart?

 

[a. palmer jr.]

The only thing I can think of is maybe wrist pin circlip came out, piston on backwards (don't know what that would do) or something left in the port during the orerhaul. Just guesses..
At any rate it looks like you're gonna be out another piston/cylinder kit.

 

[mortalitool]

Some nasty damage real quick.

I inspected the piston for the clips. Both intact.

Piston facing correct way.

Piston ring pins both intact.

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[Chainsaw Jim]

It looks like your crankshaft bearing broke and chunks of it are going everywhere. Bearings can feel ok with what feels to be normal slop and still be broken. You need to rotate the crank repeatedly until you see the bearing make an entire revolution so you can inspect the entire cage. I've seen them look and feel good and then discovered a missing chunk after rotating it enough. The damage to the piston on my saw was identical.

 

[Jed1124]

The only thing I can think of then is bad casting caused it to snag a ring. Somebody who know more than me will be by shortly to offer their suggetions.

 

[mortalitool]

I'm going to split the cases on this and inspect. Thanks for the help. Keep the thoughts coming.

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[Jacob J.]

It looks like your crankshaft bearing broke and chunks of it are going everywhere. Bearings can feel ok with what feels to be normal slop and still be broken. You need to rotate the crank repeatedly until you see the bearing make an entire revolution so you can inspect the entire cage. I've seen them look and feel good and then discovered a missing chunk after rotating it enough. The damage to the piston on my saw was identical.

That's my thought. The number one killer of rebuilds are when people don't replace the main bearings or diagnose the big end rod bearing correctly. If the original top end showed damage like that, then new mains and a thorough inspection of the rod bearing is always called for. Never assume anything on used bearings.

 

[BuckthornBonnie]

Did the original scored piston have both clips? Maybe there was something sneaky in the crankcase... Id still assume it's one of the bearings, though.

 

[turtle561]

some how got a ring end in a transfer puting it together and broke it off. put a magnet in there and see if you can find some pieces.

 

[a. palmer jr.]

If everything in the top end is okay then it must either be bearings or something fell into the crankcase during the p/c change. I had that happen this year, a circlip fell in, luckily I saw if fall in and took my trusty magnet and fished it out.

 

[mortalitool]

That's my thought. The number one killer of rebuilds are when people don't replace the main bearings or diagnose the big end rod bearing correctly. If the original top end showed damage like that, then new mains and a thorough inspection of the rod bearing is always called for. Never assume anything on used bearings.

I'm also suspecting the crank bearing Is faulty. But we shall find out after I tear into it and diagnose. I just can't imagine something causing that much damage do quickly.

So the damage is done and what interests me at this point in the rebuild (the unfortunate part )is to learn from this mistake and become a better saw mechanic. If you notice on the cylinder the transfer ports are both cracked. Along with the damage to the piston on the mating surfaces. Also along the intake face of the piston are 2 very deep grooves and a crack.

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[the GOAT]

With a little jb weld that will buff right out.

 

[a. palmer jr.]

I kinda like the clam shell chainsaws because you can pull the engine out and really get a good look at the interior of the engine.

 

[Chainsaw Jim]

If your cylinder isn't scored up above the transfer port then grind the cracked part of the bridge completely away and leave a nub on each side with a proper chamfer for the rings to slide by smoothly. Don't touch the top edge of the upper transfer area or you'll affect duration timing. You can get away with cutting the entire portion of the bridge away that is broken because you won't be widening it beyond the width of the upper transfer port.
Some builders actually port their cylinders this way. While you're at it you may as well take advantage of the burr bit and open up the intake and exhaust a bit to match the port job you'll be forced to do to save the cylinder. I'd even grind the other transfer out to match it.
Watch out, this will create a monster out of your saw.

 

[mortalitool]

If your cylinder isn't scored up above the transfer port then grind the cracked part of the bridge completely away and leave a nub on each side with a proper chamfer for the rings to slide by smoothly. Don't touch the top edge of the upper transfer area or you'll affect duration timing. You can get away with cutting the entire portion of the bridge away that is broken because you won't be widening it beyond the width of the upper transfer port.
Some builders actually port their cylinders this way. While you're at it you may as well take advantage of the burr bit and open up the intake and exhaust a bit to match the port job you'll be forced to do to save the cylinder. I'd even grind the other transfer out to match it.
Watch out, this will create a monster out of your saw.

The cylinder is not scored above the transfers. Can or should I still grind away at the crack if the crack ends up going from bottom to top on my brodge? Obviously I want to leave some what of a bridge up top.

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[Chainsaw Jim]

The cylinder is not scored above the transfers. Can or should I still grind away at the crack if the crack ends up going from bottom to top on my brodge? Obviously I want to leave some what of a bridge up top.

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You can take the entire bridge out as long as you don't go any wider than the upper transfer opening.
Here's a drawing of what I would remove...

 

[mortalitool]

You can take the entire bridge out as long as you don't go any wider than the upper transfer opening.
Here's a drawing of what I would remove...

View attachment 437957

No kidding. Well, I'll have to give it a whirl. I worry about hang up on my piston rings.....something to be concerned about or no?

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[Derf]

Not sure if this was a possibility but
- air leak? Any chance that there is some loose or cracked rubber on the intake, or maybe a bad deal on a gasket or seal?

- straight gas in the fuel tank? Maybe some rookie working for the tree co put it in? Did you change out the gas before starting the saw up?


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[mortalitool]

Not sure if this was a possibility but
- air leak? Any chance that there is some loose or cracked rubber on the intake, or maybe a bad deal on a gasket or seal?

- straight gas in the fuel tank? Maybe some rookie working for the tree co put it in? Did you change out the gas before starting the saw up?


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I did a vacuum pressure test after I put it together.

Saw was toasted when I got it. Tank was empty due to me emptying it when I tore into it originally. Used mix fuel when I test fired it.

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