Hey all. Thanks in advance for your input. I am a Stihl guy, but all self taught (Youtube, Arboristsite, etc....) Since I retired I have been buying a few saws and my grandson and I repair them and resell. We are both learning so much. I took in a 391 with a lot, and it is all there, looks decent, but the piston is all scoured up. The cylinder does not look that bad. Appears to just have some discoloring. Question on honing. I have zero experience in the machine shop field and have seen these honing devices (that do not appear to be that expensive) If I honed the cylinder lightly, and it cleaned up, do I then take a micrometer and measure the inside diameter of the cylinder and then buy a new piston kit, with oversize rings? How would I know what to order? Or, do I just bite the bullet and buy a new cylinder and piston kit? I couldn't afford OEM, that's for sure. What's your thoughts. Something I should even play with or is it not worth it. Thanks so much !… (**** I grabbed the wrong cylinder for the photos below… photos are of 034av, but the 391 cylinder looks about the same.) thank you all for your comments. have a 391 p/c kit for the 391 on order!
To hone or not to hone...
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If the piston is torn up, you’ll probably be sanding off the rough spots that are catching and galling. Have fun!
That said, most pistons aren’t that expensive and decent to excellent aftermarket options are available. You might even throw in a pop-up piston and get some extra grunt out of the saw.
That said, pay special attention to the rest of the moving parts of the engine. Seals, bearings, pressure test etc. This saw has been run rough and it’s smart to have your eyes open while you have it open.
That said, most pistons aren’t that expensive and decent to excellent aftermarket options are available. You might even throw in a pop-up piston and get some extra grunt out of the saw.
That said, pay special attention to the rest of the moving parts of the engine. Seals, bearings, pressure test etc. This saw has been run rough and it’s smart to have your eyes open while you have it open.
Of the top ends I have rebuilt, 9 out of 10 have been complete P/C kits. Only on a few were the cylinders worth saving and the parts for the kits always seem to fit better and work. One time I bought only the piston and rings because the cylinder was priced sky high and still looked OK. By doing so I saved over $100, and that saw (MS 441c) runs fine today. Of couse, it took me 12 hours to rebuild it, but I got to keep the saw.
Thank you for your wisdom. I do have a vacuum and pressure gauge and will test the seals. Also look to the boot for cracks, etc.. Thanks for that reminder. I just thought, since I would buy a cylinder, piston set anyway, what would it take for me to learn to hone what is there (I see no scores...or feel no scratches or digs) in the cylinder. It is just discolored as was the piston. I just did not know if I honed the cylinder, how to order the rings for new piston.
OK..that pretty much sums it up for me. I will be rebuilding to resell since I bought a "lot" of stihl saws. This seemed like it was not repair for what is maybe the obvious. The end user did not know how ..or just was too busy to put on new P and C. I def do not want to put 12 hours in. Yes, I am retired, BUT, I'd rather spend my time outside splitting and cutting wood! Thx again!
SteveSr
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Stihl uses nikasil plated cylinders and the plating is only a few thou thick. honing is NOT recommended as you will likely eat into the plating and ruin the cylinder. Search for removing aluminum transfer without using acid on this site for alternatives.
The chrome plating in the cylinders is pretty thin, so any honing or cleanup should be minimal. I do like to run a ball hone through a cylinder (in the parts washer with the solvent running) just to clean them up for a better visual inspection.
If there is any aluminum transfer from the piston it is possible to remove it by careful sanding, the chrome plating is pretty hard and typically won't be affected by some careful application of emery cloth and scotch brite pads.
There won't be any "oversize" rings or pistons for a chrome bore cylinder. Oversize pistons and rings will only be applicable to saws made with cast iron or steel sleeves.
Mark
If there is any aluminum transfer from the piston it is possible to remove it by careful sanding, the chrome plating is pretty hard and typically won't be affected by some careful application of emery cloth and scotch brite pads.
There won't be any "oversize" rings or pistons for a chrome bore cylinder. Oversize pistons and rings will only be applicable to saws made with cast iron or steel sleeves.
Mark
THANKS SO MUCH. I am sold on just buying a P/C kit and installing. Appreciate your wisdom!
Thanks MARK! Really appreciate your educating me. I LOVE this site!!!
Post some pictures of inside the cylinder. It's really not too hard to clean up transfer if the plating is still intact, and OEM cylinders are better quality than aftermarket.
Maybe offer it (cylinder) to a member (ME) on this site so a 029 could be UPSIZED to a 039..... But my personal opinion would be to try cleaning it up and RUNNING IT. Good luck with your project..
Did you already use a flapper wheel on this (appears like it) and if you use this cylinder be sure to clean all of the grit out of the transfers and you know the drill. I like using a 1/8" welding (stick) rod flattened and slit on one end to accommodate emery cloth strips 120, then 180 then 320 grit then red Scotch Brite and a soapy hot water bath-rinse. They refer this as a split mandrel. I put mine in a variable speed Dremel (1/8" chuck) to remove transfer. Look up MasterMind on utube "Removing transfer without acid" for some tutorial . There are many ways to remove the aluminum transfer and you can find them here on AS .. I like the split mandrel 'cuz you can focus /pinpoint the abrasive where it is needed and not compromise the plating elsewhere.
Picture of the exhaust side???
I was just asking about this on the 10-10 thread. so Mark you do ball hone the MAC chrome cylinders?
This is the best way to hone a chrome bore cylinder imo.
SteveSr
Addicted to ArboristSite
You said that this was a MS391 yet you post photos of an 1125 cylinder from an 034, 036, MS360. So what is it you actually have?
Your next project is to figure out why it scored in the first place before you do it again.
OP was it scored on both sides of the piston or one?
The pics aren't that good, but it appears to me the plating has been worn away at TDC.
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