After I posted I realized that's the EXHAUST side of the piston. So it sounds like it needs replacing. I do have a good piston and cylinder I can install.
Stihl MS 390 carnage - disassembly.
- Thread starter MacAttack
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Is there any trick to removing the bearings? 2-jaw puller and a little heat?
If you don't have a hydraulic press available, a 2-jaw puller may work if you have enough clearance between the bearing and crankshaft to get a grip. If not, run down to your local Harbor Freight and pick up these. Heating the bearings will help a lot with removal and installation.
Get everything set up and throw the crankshaft and puller assembly in the freezer for a couple of hours or overnight. Pull it out and apply heat to the bearing. It will slide off easily.
When assembling, freeze the crank, heat the bearing and it will practically slide together. Tap the inner race using a piece of pipe or a punch and hammer to insure it is fully seated against the crank.
Get everything set up and throw the crankshaft and puller assembly in the freezer for a couple of hours or overnight. Pull it out and apply heat to the bearing. It will slide off easily.
When assembling, freeze the crank, heat the bearing and it will practically slide together. Tap the inner race using a piece of pipe or a punch and hammer to insure it is fully seated against the crank.
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The mirror image of the exhaust port on the piston face- never a good thing.
Will I need a piston ring compressor to put this back together or nah?
Yeah....it ran lean I think.
Bottom end looks very nice, very oily, I do not think it was straight gassed.
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Id guess it got VERY hot and stopped suddenly- cooling with the piston stuck- cylinder covered area cooled at a different rate to the piece open to the port- leaving a clear "photo" image.
Very interesting, you may be exactly right. I have no idea what happened with the previous owner, but the cooling fins were completely clean, so it's possible the carb tuning was way off, or it just got worked REALLY hard in hot weather.
Getting some parts on order...they say these Stihls are hard to work on. Little do they know I've worked on Mini Macs, haha.
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Closer look at one of your photos- it looks like something has been caught slightly right of centre top of the exhaust port and possibly caused the stoppage.
Looks a bit like a bearing needle rod imprint, possibly a piece of bearing cage. Be interesting to see if there is a corresponding mark on the cylinder wall.
Looks a bit like a bearing needle rod imprint, possibly a piece of bearing cage. Be interesting to see if there is a corresponding mark on the cylinder wall.
I use two flat heads to get the bearing away from the crank flange then a 2 jaw puller to fully remove it, tap the new bearing on with a 1/2 drive deep socket on the inner race. If reusing a piston get new rings, the bottom of the cylinder is chamfered so you do not need a ring compressor. Simply orient the rings correctly and slide the piston in, I have needed to push a ring into its land in the past to get it slid in. Oem rings are the best choice for longevity, caber are ok in a pinch.
I simply drill a 1/2 hole in the center of the muffler baffle then cut out the majority of the area covered by the exhaust diverter. This greatly increases flow killing the bad restriction these mufflers have.
I simply drill a 1/2 hole in the center of the muffler baffle then cut out the majority of the area covered by the exhaust diverter. This greatly increases flow killing the bad restriction these mufflers have.
For years I have gotten along without a piston ring compressor. It takes some patience and co-ordination, but it can be done bare handed, top down. I made a couple of hardwood supports and shims for the piston to rest on as I work my way along. As you may already know, the Stihl 290, 310, 390, require lots of take down to rebuild the top ends. Good Luck!
It's possibly from a chunk of carbon, but I'm not sure. All of the bearings seem to be in good order, but I'm inspecting everything. I can't find evidence of anything failing except the piston and the flywheel...
Look very closely at the impulse hose. They love to crack and create air leaks at the bend radius. Causes lean condition. See it on most I have repaired.
I will 100% check it over, thanks!
Good used piston installed.... piston rings and oil seals are on order. I'm going to reuse the main bearings, they look and feel absolutely perfect, metal races, I can't justify pulling them off.
Hey...this IS a budget build.
Hey...this IS a budget build.
I inspected the impulse line very carefully…no cracks, and still very flexible.
Thanks!
Good luck with that. The bits of metal had to have come from somewhere and bearing cages are the common suspect. They may turn just fine but I would pull them, clean thoroughly with carb or brake cleaner and inspect under magnification.Good used piston installed.... piston rings and oil seals are on order. I'm going to reuse the main bearings, they look and feel absolutely perfect, metal races, I can't justify pulling them off.
Hey...this IS a budget build.
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jeagan
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Honestly, that is a bit much for my feeble mind to absorb in a fixed period of time.
