Stihl ms460 build what happened?

[Mendocino Redwoods]

New member first post, been coming on here a while when I need info. So I got a ms460 arctic for cheap that had no compression and has probably been sitting for 10-15 years with gas in it, it looks like a pretty low hour saw, it’s a Stihl and has all its paint lol. I pulled the jug and it looked bad at first. I used scotch brite and purple power and was able to clean it up to where it looked and felt like new. I couldn’t feel any scoring at all with my finger nail. Replaced fuel line, fuel filter, impulse line, ultrasonic cleaned carb and rebuild kit, piston kit, all OEM Stihl. Pressure/vac test before and after rebuild, thought maybe an air leak blew up the saw in the first place but it held steady at 7 psi for 10 minutes on both. I even already bought seals figuring they’d need replaced. Compression test 165 psi after I got it back together. Fired up in 3 pulls and I shut it off after literally 2 minutes because my new muffler got delivered and I was gonna tune it with new dual port muffler. I pulled the old muffler off and the piston is already scored. Did I miss something on the cylinder? From what I could see of the crankshaft it looked good, a little side to side play but no up and down.

 

[jacob j.]

I wouldn't called that "scored" necessarily - more like wear/witness marks. It looks like there's some residue left around the exhaust port - is that residue from the old piston?

 

[Mendocino Redwoods]

Old piston and new, I forgot to take a picture of the before and after of the cylinder. Looking at the old piston you can only imagine what the cylinder looked like but man the scotch brite and elbow grease had it looking brand new.

 

[Junk Meister]

You were waiting for your new Muffler. How clean was the original. You might have pulled some Carbon or crud back in through the exhaust ..

 

[Mendocino Redwoods]

I wouldn't called that "scored" necessarily - more like wear/witness marks. It looks like there's some residue left around the exhaust port - is that residue from the old piston?

It wasn’t left over, that’s new. I had that cylinder looking and feeling new. I really wish I took before and after pics but I didn’t think about it until after I got the jug back on.

 

[Mendocino Redwoods]

You were waiting for your new Muffler. How clean was the original. You might have pulled some Carbon or crud back in through the exhaust ..

You were waiting for your new Muffler. How clean was the original. You might have pulled some Carbon or crud back in through the exhaust ..

Maybe but it was pretty clean inside, I only replaced it because the outside was rusty, I just took this picture of the old one.

 

[Mendocino Redwoods]

Maybe but it was pretty clean inside, I only replaced it because the outside was rusty, I just took this picture of the old one.

I blew the cases out as well as I could with compressed air, I thought maybe something came up from there.

 

[jacob j.]

I blew the cases out as well as I could with compressed air, I thought maybe something came up from there.

The outer muffler gasket (the for the face plate of the muffler) - was that new for your install or existing? A piece of that may have gotten sucked back into the cylinder. Or something came up from the bottom end...

 

[Mendocino Redwoods]

The outer muffler gasket (the for the face plate of the muffler) - was that new for your install or existing? A piece of that may have gotten sucked back into the cylinder. Or something came up from the bottom end...

Yep that’s the old gasket man I wonder if that’s what happened. I’ve been leaning towards something in the bottom end didn’t get cleaned out or somehow I missed something on the cylinder even tho I looked so close at that thing, especially around the exhaust port. Is there anything else I should look for? At this point should I clean the cylinder again and replace the piston or replace the rings or just clean the cylinder and run it for a little bit and see if it gets worse. Losing a $100 piston effing hurts but I really don’t want to lose the jug. I was about to buy a meteor and at the last minute said nah let’s do OEM haha

 

[jacob j.]

I'd clean up the cylinder again and get all traces of that old gasket out of the muffler. The piston isn't damaged that badly and will run fine. I'd also flush the bottom end out a couple times with mixed gas. Pressure and vac test it again and inspect everything very well and put it back together.

 

[Junk Meister]

My way of thinking is something (carbon) entered on the exhaust port ''cuz the streaks are on the sides of the ex. if it was in the lower end wouldn't the crud enter via transfers and be more random on the piston? Sketchy thinking so chime in if for no other reason I might learn something.

 

[FederalQ]

The marks on the new piston definitely mirror the scoring on the old piston like there was some transfer or imperfections left on the cylinder wall.

 

[Junk Meister]

The marks on the new piston definitely mirror the scoring on the old piston like there was some transfer or imperfections left on the cylinder wall.

Makes a lot of sense. OP needs to triple check that area of the cylinder.

 

[David Young]

I think it was residual from the old scoring. I also suggest going a step or two finer for a final buff on the walls before reassembly. you also have a new piston that may have tighter tolerances.

Always make sure the cylinder is warm to the touch from idling before any throttle is applied. The piston is smaller in relation to the cylinder and is the first to absorb heat. The piston can and will expand before the cylinder. Possibly causing interference especially where there may be residual transfer on the cylinder.

 

[Mendocino Redwoods]

Makes a lot of sense. OP needs to triple check that area of the cylinder.

The marks on the new piston definitely mirror the scoring on the old piston like there was some transfer or imperfections left on the cylinder wall.

Yeah that’s what has me second guessing my cylinder and maybe I missed something but like I said before man that thing looked and felt clean. It’s probably worth mentioning I’m not new to this, I’ve been running saws professionally for 25 years and for the most part working on my own saws. I’ve done a few cylinders and pistons before and have never had this happen. That being said I definitely don’t consider myself a builder lol

 

[farminkarman]

The marks on the new piston definitely mirror the scoring on the old piston like there was some transfer or imperfections left on the cylinder wall.

That would be my guess. When removing transfer, a person has to be very careful to make sure there aren't any high spots left.

 

[FederalQ]

Yeah that’s what has me second guessing my cylinder and maybe I missed something but like I said before man that thing looked and felt clean. It’s probably worth mentioning I’m not new to this, I’ve been running saws professionally for 25 years and for the most part working on my own saws. I’ve done a few cylinders and pistons before and have never had this happen. That being said I definitely don’t consider myself a builder lol

By no means am I discrediting your abilities just trying to help diagnose. My whole introduction into this “world” was finding Mastermind’s video on how to sand aluminum transfer from a cylinder after I lightly scored my 5105 enough to lock the piston ring. It worked and still using the saw. I blame myself for maximum RPM before the engine warmed up and it had the KAT muffler at the time. The new rings on that 460 have probably cleaned everything up now and it’s ready to go. Great saw and heated handles are nice when the weather is not.

 

[Wonkydonkey]

I,d see if those marks on the cylinder fizz when you rub a bit of acid on with a que tip. And I was also wondering if a little transfer was missed and that’s the reason for the new marks on the piston ?

I’ve found this has happened when I’ve cleaned cylinder transfer off before.

And another thing, does everyone else do 3 heat cycles when putting in a new piston ?

 

[59billy]

My way of thinking is something (carbon) entered on the exhaust port ''cuz the streaks are on the sides of the ex. if it was in the lower end wouldn't the crud enter via transfers and be more random on the piston? Sketchy thinking so chime in if for no other reason I might learn something.

That's the value of threads like this everybody learns something.

 

[David Young]

I,d see if those marks on the cylinder fizz when you rub a bit of acid on with a que tip. And I was also wondering if a little transfer was missed and that’s the reason for the new marks on the piston ?

I’ve found this has happened when I’ve cleaned cylinder transfer off before.

And another thing, does everyone else do 3 heat cycles when putting in a new piston ?

well first off there are variables to consider....

I just go real slow with warm up make sure it got a good tune. If I can I like to put in the wood, light cuts at first make sure it is clearing up
and THEN give it the rented pig treatment.