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.
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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
SKF bearings installed! I put the crank in the freezer and heated the bearings with a heat gun, went on super easy.
Am I doing this right? How many thousandths gap should I be looking for in the piston ring end gap? One of the old rings is about .010".
I got new Caber rings in the mail, going to check them later.
I got new Caber rings in the mail, going to check them later.
Toby Smith
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The only problem I can see in leaving the outer bearing plastic seal in place is the crankcase rubber lipped seal will not receive lubrication from the fuel mix and may dry out and fail in time.
That's actually a great thought! I removed all the seals from my bearings.
The plastic or rubber "seals" that pop out are not so much a seal as a shield so they will pass some lubrication along to the lip seals in any case.
There were a number of the large frame McCulloch saws (Super Series engines) that had two seals side by side on the flywheel side of the saw to insure the points box was not contaminated. 37 and 38 are seals PN57897 and 57898.
Mark
There were a number of the large frame McCulloch saws (Super Series engines) that had two seals side by side on the flywheel side of the saw to insure the points box was not contaminated. 37 and 38 are seals PN57897 and 57898.
Mark
If I'm being honest, I can think of plenty of applications where dust seals operate and hold up without any source of lubrication. I'm probably over-thinking this. In any case I'm glad I went through the effort to replace the bearings.
Next step is to check the end gaps of the new piston rings, which it sounds like should be around .005 to .008 thousandths.
Next step is to check the end gaps of the new piston rings, which it sounds like should be around .005 to .008 thousandths.
