Piston / cylinder feels very nice. I'm waiting for the rings to be delivered so I can check end gaps and file them if I need to. Thanks!
Stihl MS 390 carnage - disassembly.
- Thread starter MacAttack
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Great input, thanks!
I had read about the old bearings coming off with just a little heat, mine were on much tighter than that!
Check the P/C with feeler gauges at the skirt so you know it's tight or loose. Mics are better.
How many thousandths of clearance should I be looking for at the skirt? I have feeler gauges and a digital caliper, I don't have micrometer though.
New P/C ~0.002-0.0025". Double that and it's getting sloppy, much less and it may seize.
Many thanks! I will check later.
Reason I mentioned is Stihl sized P/Cs. Should be stamped on top of Cyl and crown of piston. A, B, or C.
Later replacement pistons were stamped A/B and were a compromise to fit A or B.
I can't remember if A or B is a bit larger?
Meteor stamps their sets too.
My good used piston is a Mahle, I'll look and see what's casted into it.
They are available without seals, but sealed bearings are more commonly available and usually cheaper than the open bearings from the same manufacturer with the same specs (C3). It's no big deal to pop out the seals, we do it all the time.
"Why would a proper bearing come with double seals and grease?"
Many times those bearings are installed in equipment that is "permanently lubricated" and makes no provisions for lubricating the bearings. I remember very specifically a gearbox in the 90° elbow of a screw auger conveying silage or cracked grain.
Bearings with metal shields are not sealed at all, but the metal shields help keep some larger bits of debris out.
Bearings with one or two plastic seals are not actually sealed either, but the plastic seals do hold the grease in the bearings and also serve to keep debris out of the bearings. Those plastic seals are not intended to hold the kind of pressure and vacuum you would experience in the crankcase of a chainsaw.
I have put a few saws together leaving the outside plastic seal in the bearing along with the rubber lipped seal on the crankshaft. I don't know if it really accomplished anything but the saws all worked fine for as long as I was running them.
Mark
Many times those bearings are installed in equipment that is "permanently lubricated" and makes no provisions for lubricating the bearings. I remember very specifically a gearbox in the 90° elbow of a screw auger conveying silage or cracked grain.
Bearings with metal shields are not sealed at all, but the metal shields help keep some larger bits of debris out.
Bearings with one or two plastic seals are not actually sealed either, but the plastic seals do hold the grease in the bearings and also serve to keep debris out of the bearings. Those plastic seals are not intended to hold the kind of pressure and vacuum you would experience in the crankcase of a chainsaw.
I have put a few saws together leaving the outside plastic seal in the bearing along with the rubber lipped seal on the crankshaft. I don't know if it really accomplished anything but the saws all worked fine for as long as I was running them.
Mark
