MS391 Top End rebuild project

[Patrick1903]

Hi everyone, I picked up a decent looking MS391 (PHO) on CL for $40, about 5 years old. The seller told me he accidentally straight gassed it. Thought he had put the oil in the 1 gallon can, then filled at the station, and the rest is history. He had bought it to mill with and it didn't seem abused and was pretty clean (no sawdust caked in cooling fins, no bluing from heat around clutch drum, etc). I ordered a Proline piston & cylinder, (didn't want to do the $300 right now for OEM, and it's a project really. I don't have too much of a need for this saw - but it could certainly be put into the farm saw rotation.) I've only done 1 top end before, on an 041 (I have a thread about my 041 issues, which I think are now resolved!).

After talking to some folks, they recommended I try to clean the cylinder up first before writing it off. I have never cleaned up a cylinder before, so I made an attempt at that tonight. I watched Mastermind Saw's videos on cleaning with sandpaper and scotch brite. I couldn't figure out a decent mandrel setup for my drill with emery cloth, so I just went straight to the scotch brite (I read green scotch brite was around 600 grit, so not sure skipping the 150-200 grit was a good idea).

So I'd like to hear if anyone has thoughts on:
1) Should I consider just replacing the piston? I think I could get a Metor cylinder for around $50. And maybe use the Piston/Cylinder kit that's in the mail as a backup in case the cylinder doesn't work out with a new piston?
2) It may be hard to tell from the pictures, but are there obvious show-stoppers with the cylinder? I think there is still some aluminum stuck in there - I was trying to figure out if it was the plating gouged out or if it was proud of the cylinder walls, so I took a letter opener and knocked off some aluminum. I can't tell if there is obvious pitting or wearing away of the coating. There are a few "grooves" but don't seem to catch my nail exactly. It's also hard for me to tell b/c can't get my finger into some of the top of the cylinder.

Pics attached.
New saw

Piston - not good. Disintegrated in part.


Cylinder - after I cleaned with scotch brite and drill.



Last pic - Cylinder before scotch brite and drill cleaning. Lighting is bad , flash only. The cleaned cylinder pics I was holding a light and taking pics. I think that was better.

 

[Patrick1903]

I got my Proline piston and cylinder kit yesterday. I see a nick inside the cylinder. It’s catches a fingernail. I’m guessing I should send it back?

It looks like the piston is from Hyway. Is that right based in the HW etched on the piston top. And the cursive H cast on the piston? See pic. No real identification markings on the cylinder, just the letters AA.

Says it’s nikasil.
https://sawzillaparts.com/proline-cylinder-piston-kit-for-stihl-ms391-49mm-1140-020-1204-nikasil/
It came with the G or E circlips, with the ears. It appears from other threads that replacing them with OEM is the better idea, so I’ll do that.

What about the wrist pin? Should I reuse the OEM one or replace with the new one?

Cylinder nick.

Hyway piston?

 

[Woodslasher]

I'd see which wrist pin fits best. I had a Proline piston that the supplied wrist pin wouldn't work in but my old one did the trick, so I wouldn't be surprised if the opposite could be true as well.

 

[Patrick1903]

Regarding the cylinder nick, does anyone have a thought on if that would create a problem? I’m leaning towards on trying to do an exchange but the couple week delay is frustrating and if if it not necessary than I would reconsider. It seems fairly close to the top of the cylinder, not sure if it would cause problem with rings, or if they would reach it. If it was above the rings at TDC could it still create a problem?

 

[Chevboy0167]

Sand the edges down around the nick with 400 grit and run it. One by itself should not cause any issues.

 

[Patrick1903]

Removing Dirko is a chore…definitely taking longer around the crankcase, due to the channel where some of the Dirko still sits. My biggest issue is removing it in the notches that fill with Dirko to hold the crank seals - assuming that’s the intent of those notches. I’m trying gently with a razor blade to separate it, but struggling getting it all. In general I’ve used alcohol per Dirko website, also used gas and adhesive remover (acetone based).

Any idea on best remover, and any ideas on best tools to scrape, especially those “notches”?

Thanks.

 

[Vintage Engine Repairs]

Removing Dirko is a chore…definitely taking longer around the crankcase, due to the channel where some of the Dirko still sits. My biggest issue is removing it in the notches that fill with Dirko to hold the crank seals - assuming that’s the intent of those notches. I’m trying gently with a razor blade to separate it, but struggling getting it all. In general I’ve used alcohol per Dirko website, also used gas and adhesive remover (acetone based).

Any idea on best remover, and any ideas on best tools to scrape, especially those “notches”?

Thanks.
View attachment 988085
View attachment 988084

You’ve done a great job so far mate, it can be quite tricky, old Dirko is incredibly tenacious!
My two favourites are either 600 grit scotchbrite (green) or my favourite is a brass wire wheel on a dremel.

 

[Patrick1903]

You’ve done a great job so far mate, it can be quite tricky, old Dirko is incredibly tenacious!
My two favourites are either 600 grit scotchbrite (green) or my favourite is a brass wire wheel on a dremel.

The green scotch brite was key, thanks Tom. I used a screwdriver to help pull the scour pad through those grooves (ensuring not to have metal on metal with screwdriver). And hand scrub the flat surfaces with the pad. I used wd40 to wet the pad a little, seemed like it worked. No idea if that’s useful otherwise as a cleaner for adhesive. I’ll wipe down the surface with acetone prior to reinstalling, to ensure the wd40 and anything else is off the mating surfaces. Great ideas, thanks again. And I watched your Dirko video, hopefully I won’t make too much of a mess on the install.

 

[Patrick1903]

OK - so ready to replace piston. I'm looking at the bearings and one feels better than the other. The flywheel sides seems to have some slight metal shavings in it, and it appears the cages (surrounding the ball bearings), occasionally appear slightly brighter like they were rubbed by something else - not sure if that's a good description...but I think they were worn slightly. The bearing turns still turns smoothly and doesn't have much play at all, when moved side-to-side, or up/down.

Can I try to flush out (penetrating oil) or blow out (air) the bearings to get any metal out?

First picture - I think shows a slightly damaged cage


Second picture - Cage that appears normal (like most of them on both bearings)

Last question for the day - is it normal for seals to not be a uniform around entire seal? Wondering if clamping pressure from the top and bottom end would cause a slightly different width (or maybe flat spot) and if that is usable still? I think you can see the difference here:

 

[Vintage Engine Repairs]

You did a great job mate, well done! You could even clean up the bearing pockets just a touch more too if you are so inclined. Yep, WD40 and the screwdriver on the inside of scotchbite carefully avoiding marring is good! In the unlikely event you have popsicle sticks they work wonders and are a bit safer.

On the install, have everything all ready, laid out in the order of installation, lots of space and go through a few times what you’ll do and how. You have about 7 minutes with Dirko, which is loads of time here, but time flys when you’re flustered if something won’t quite fit or go in and you’re against the clock on your first rebuild.

 

[Vintage Engine Repairs]

Regarding the bearings and seals, you may be able to flush and clean them and be fine, but honestly, you can walk into any bearing supplier and tell them the bearing you need and they’ll have it for a few dollars. I’d personally change them out as you’ve gotten this far and knowing there was galling in the top end, it’s understandable if some got into the bearings.

Same with seals, I bet the same shop would have the exact ones you need, I don’t think the 391 uses proprietary seals (though I could be mistaken).

Did you pressure or vacuum test the saw before disassembly to assess if they are still good? If not, grab a new pair - again your this deep it’s worth the few dollars. Request viton ones or at least let them know they’ll be in contact with fuel and warm temperatures yes that little nub can be repositioned to fit back in the same location between the two halves of the clam shell if you know they weren’t leaking before you removed them.

It sucks thinking you can’t just stick it back together right here and now, but patience will reward. It was something I had to learn and I’ve gotten better with with time haha. Do it right the first time

 

[Patrick1903]

OK thanks @Vintage Engine Repairs for the recommendations. I see a bearing/seal set from Proline for $30. I may go that route. I really need to look into replacing bearings as I've never done it. The service manual indicates heating them to 300F. I couldn't pull them off at first attempt. I will also see if there's a bearing supplier around.

Unfortunately the rings scraped the piston slightly, as I tried to work the first one down into the bottom groove - that's annoying since I had a pristine piston. Will this have an adverse effect on performance? I had used oil to lubricate prior to installing rings. See attached:

 

[Vintage Engine Repairs]

Oh no, that’s a shame, there won’t be any issues with the fine scratches, it’s just not ideal. The key here though is to remove any high spots / burrs. Very gently with some scotchbrite just brush over them a couple of times. Feel after with your finger to ensure no high spots or burrs remain and you’ll be good. If you had much more galling, say on a seizure, carefully filing away the high spots ensuring to keep the file pivoting to avoid flat spots is good too. Here it’s over kill for sure. 2-3 light brushes with 600 scotchbrite and you’ll be fine. Make sure you clean all internal surfaces (piston / cylinder) very well after using it though for obvious reasons.

I find I’m a perfectionist and I give myself a really hard time if I mess up even slightly. However, as important as it is to do a job to a high standard mistakes happen and also, these engines are quite hardy, you’ll be surprised how bad they can look internally and still run.

I have repaired seized pistons, having gently filed the mass of galling and used them in instances put back in the saw when the engine wasn’t of any value but I wanted it to run. Long story short, you’re fine, it’s not worth a second thought once you’ve given it a slight brush with scotchbrite. Just don’t do it again

 

[Patrick1903]

Long story short, you’re fine, it’s not worth a second thought once you’ve given it a slight brush with scotchbrite. Just don’t do it again

Not sure how I messed this up…I cant exactly remember how I did it; I think I should have put the open end of the ring into the groove, then slid the back (ie opposite the open end, down into the groove. I must have done opposite and the sharp open ends of the ring scratched when pulled down. I also tried needle nose
Pliers to pull the ring into place - none of it felt great. I was afraid I was pulling the ring too open and was afraid I’d stretch it out.

 

[Patrick1903]

So, regarding crank seals - the IPL shows two p/ns, which coincide with the service manual, depending on if the saw is "open" or "closed." The service manual shows the "open" seal to be larger. However, some of the aftermarket parts online show seals that satisfy both p/n's.

Stihl seals are $13/each from my local dealer. I'd love to replace everything OEM but I bought this saw for $40 and will be added to a fleet of saws that may not see a ton of use. IOW, I'm not intending to do a quick flip and it's not in the hands of a pro - mostly a rebuild project and occasional use saw. So the good deal on the acquisition quickly gets eaten up by the price of OEM parts.

Does #1 below indicate (with the 24.96 and 39.90 dimensions) that it's slightly smaller than the other seal?

#1 below - Closed saw seal - 9638 003 1581 15x25x5 (B) (24.96),(39.90)
#2 below - Open saw seal - 9639 003 1585 15x25x5



Sawzilla has these Proline seals - that say they replace both p/n's. How can they replace both if the p/ns are slightly diff't sizes?

https://sawzillaparts.com/proline-crankshaft-seal-set-for-stihl-ms270-ms271-ms280-ms291-ms311-ms391/

HL has - Non-Genuine Oil Seals Set for Stihl Replaces 9639-003-1585. I like the fact that it doesn't say it fits both p/n's.
https://www.hlsproparts.com/Stihl-017-018-019-021-023-025-oil-seals-p/h29025.htm

How important do you all think it is to have that exact p/n (ending - 1585) if I have the saw apart? Just curious about why they are different.

 

[Woodslasher]

One is meant to be pressed in while the engine is still assembled, one is meant to be "clamped" in when you reassemble the two halves of the engine.

 

[Vintage Engine Repairs]

Ahh the memories haha -

https://www.arboristsite.com/threads/two-different-oil-seals-on-ms-039-help-please.339547/

 

[Chevboy0167]

^^^^ Hey now... I pulled the same rookie mistake on 290 FB I did a cpl yrs ago.... it happens. I thought, man... these seals are cheaper than the others at $45 a pair from local dealer.

I learned on this site why that day. And continue to learn from the wealth of knowledge from all of you. Thank you again!

 

[Patrick1903]

One is meant to be pressed in while the engine is still assembled, one is meant to be "clamped" in when you reassemble the two halves of the engine.

So it’s odd that the Proline seals say they replace BOTH part numbers, right? When they are two distinct parts with two distinct part numbers?

The ones that I received match the OEM seals from the factory.

I haven’t made much progress lately but will work on getting the top end back together in the next couple weeks. Sooner I hope.

 

[Patrick1903]

Ahh the memories haha -

https://www.arboristsite.com/threads/two-different-oil-seals-on-ms-039-help-please.339547/

Funny…good read. Awesome manual from @HarleyT in there. Thanks for sharing the manual, and thanks Tom for reviving your thread.