MS391 Top End rebuild project

[HarleyT]

Things are getting serious whenever I break out my
"blown seal" jokes....

 

[Patrick1903]

Regarding oiling the crank or the inside of the seals prior to install - the service manual does not indicate doing that. I have seen and heard of people doing that - what is the preferred method or should I just stick to the book, so long as I don't turn the lip of the seal when installing?? The book just says "Push the new oil seals, open side facing inwards, on to the crankshaft stubs"

 

[Vintage Engine Repairs]

Grease inside, silicone outside

 

[Patrick1903]

Grease inside, silicone outside

Ahh, I see that the manual, under the installation of the seals in the section on the closed engine seal installation, says "– Lubricate sealing lips of new oil seal with grease."

For clarification, are you recommending silicone around the outside of the seal, prior to seating into the space on the cylinder (and obviously prior to installing into the bottom of the saw)? What type of silicone do you use?

 

[Patrick1903]

Vacuum/Pressure testing - I was struggling with a way to come up with a way to block off the intake. I was trying to sandwich a piece of rubber between the cylinder and the intake manifold. That was trickier than what I would like, primarily because of the two "inserts" on the cylinder side of the manifold that go into the intake on the cylinder, doesn't allow me to "sandwhich" the piece of rubber against it and the cylinder. Then I decided to cut a flat metal piece to use to hold the rubber piece against the cylinder intake, that took a while to cut the holes to match the screw locations, and cut it to size. I finished that and ran a test and it was building up vacuum/pressure but not holding it.

I have a feeling that my blockages aren't sufficient. I'm wondering if my rubber pieces are soft enough - I think they are, but not sure. I bought replacement gasket material in the plumbing section at Home Depot. Haven't tried it yet, but may try to cut new gaskets to at least eliminate the rubber as the issue. I am using the same material to block the exhaust.

This is where is gets frustrating...I have to #1) Build my test materials, and 2) Test my test materials. The testing is taking more time than the top swap.

This is also why special tools are created, I suppose. I will post in the classifieds to see if someone will sell me the Stihl tool (Stihl p/n 1140 890 1200) that plugs into the manifold and see if that is a feasabile route.

 

[Patrick1903]

I tried a couple different combos of homemade blocking devices for intake. My first attempt blocking directly against cylinder worked best. But still leaked. Other attempts were more porous (trying to block in between intake and carb). So I threw in the towel and figured I’d reassemble the saw and do the carb spray/brake cleaner test. It failed. The saw idled initially but it idled fast. I turned the idle screw all the way down and it would come down but occasionally idle up. Then took the clutch drum off sprayed carb spray there. Seemed ok. Then did the same thing on flywheel side with top cover off. Idled down and another spray killed it. I tightened down the cylinder screws. They were tight. Retried and saw died again when sprayed. Reacts quickly to the carb spray when I spray at the bottom left cylinder screw area. Wonder if it’s the cylinder base or the oil seal on that side?

So…would it be best to try to add pressure and spray soapy water? Or would that not show anything if it was the crank seal?

Thanks!

 

[Vintage Engine Repairs]

Good to hear from you mate sadly this is what you can expect from these knock off products. Oem seals are generally best, but as I mentioned I have had great luck from a bearing / seal supplier. It’s their bread and butter, if they leaked the business wouldn’t last.

Do remind me, did you use Dirko HT Red?

 

[Patrick1903]

Good to hear from you mate sadly this is what you can expect from these knock off products. Oem seals are generally best, but as I mentioned I have had great luck from a bearing / seal supplier. It’s their bread and butter, if they leaked the business wouldn’t last.

Do remind me, did you use Dirko HT Red?

Hi Tom, yes, I used Dirko HT Red in the channel on the mating surface of the lower half of the crankcase. Per the SM instructions. This is where i am second guessing - should I have used a small bead of Dirko all the way around the carrier for the oil seal? Since the book didn’t say to do that, I didn’t, and read other postings saying that the compression of the clamshell would seal around outside of seals adequately.

I truly do not know if it is the seal or an air leak at the cylinder base.

 

[HarleyT]

We used to have a good hardware store that had a variety of corks and stoppers, which I found helpful most of the time.

 

[Patrick1903]

We used to have a good hardware store that had a variety of corks and stoppers, which I found helpful most of the time.
View attachment 995381

I had some stoppers that came with my mityvac pressure/vacuum tester. I was happy with the lower plug, and thought I had impulse covered with a small plastic insert, but the bend in the top hole didn’t seem to allow my rubber stopper to make a good seal. I also tried a piece of plastic bag into those holes and then sandwiched with the carb screwed down, but that didn’t seem too good.

I think my initial attempt taking it back to the cylinder was the best, as far as being able to build some pressure/vacuum. That’s why I am curious to see what would happen with the stihl testing tool, if I can borrow one.

Question, can I verify a seal is leaking with soap and water on vacuum? Or will that only show leaking on pressure (which I understand seals will hold pressure but fail on vacuum)?

Or would next step be to try to test pressure, looks for leaks around cylinder base with soapy water. Then if that shows ok, assume it is seal leak?

My thought is - if cylinder base seems sealed, do I go about replacing seal using the “closed engine” replacement seal and method for this clamshell saw, or just open the saw back up and replace both seals with the “open engine” version seals?

 

[Patrick1903]

Quick update - A generous member leant me the vacuum/pressure test plugging tool for the intake. I installed that (and coveredthe exhaust) and saw zero vacuum first. So switch to pressure. Nothing at all. I can hear air escaping. Sprayed around seals and cylinder base. No bubbles. Sprayed around intake manifold where it is fit against cylinder. Big bubbles on left side of the intake. Also bubbles on right side of intake manifold at the cylinder. It appears to not be sealing against the cylinder on both sides. The face of the manifold and of the cylinder seem clean and no raised edges. The small rubber seals (like gaskets) seem fine. The only think I wonder about is if the "face" of the manifold, where connecting to cylinder, is not flat enough to make a tight connection. There are only 3 screws holding this into the cylinder. Two on top, one on bottom. It seems the middle portion (and moreso on the left side specifically) could be bowed out, away from the cylinder. Is that possible? If you recall - this saw had been straight gassed, so presumably got quite hot. Could that have distorted the intake face?

I will try to source a new one (or at least look at it first at the dealer) to see if a new one has a "flatter" face and would make a better seal.

Also, am I screwing this manifold onto the cylinder correctly? THe boot is in the way of the screws, so I am slightly twisting the boot just enough to get my t-handle onto the torx heads.

See pics below:
1) Manifold face - is it curved, or should it be flatter?
2) Picture of potential gap b/w manifold and cylinder

 

[Patrick1903]

It seems the middle portion (and moreso on the left side specifically) could be bowed out, away from the cylinder. Is that possible? If you recall - this saw had been straight gassed, so presumably got quite hot. Could that have distorted the intake face?

I was thinking about it...I wonder if my initial attempts at a homemade intake blocking tool may have attributed to this manifold bending (if that's what's happening here). Initially my plan was to block off the intake/impulse at the cylinder, and use the mandifold to apply pressure to the rubber and metal plate I made. With the location of the 3 screws on the manifold, and the rubber "ears" on the manifold face (that normally insert into the intake port on the cylinder, but would have not inserted and instead pressed against the flat plate I created), perhaps when the screws were tightened down it pulled the face of the manifold out of it's normal shape (i.e. pressure on top and bottom of manifold, but not middle). Just a thought.

 

[Patrick1903]

I called my Stihl dealer and they quoted me $60+ for the 3 screw manifold. I decided to just pry the manifold mount back to flatten it out, with a vise and crescent wrench. It sealed up much better, but still had some air leaking out. After second attempt at adjusting it, I was able to get it to seal up. Does anyone ever put anything on the gasket to make it seal better against the cylinder intake?

After sorting out the manifold leaks, discovered my blocking on the exhaust was leaking. I removed the heat sheild and just went directly against the cylinder with the rubber blocking plate, and tightened down with muffler. Worked like a charm. I found really small bubbles from decomp and spark plug, but a little more tightening down and they were good. Vacuum had a really slow leak - much less than allowable per the service manual. So I think I'm ok. Pressure held for 5 minutes.

Biggest concern is now I have to take off the manifold to install the air guide shroud piece, then reinstall manifold. I guess I could test after reinstalling manifold again, but I've already reinstalled muffler. Argh... I suppose if I don't bend the manifold and make sure it's tight, it should be fine.

 

[Chevboy0167]

Permatex Motoseal wouldn't hurt on the intake side of things I would think...

https://www.amazon.com/Permatex-291...&sprefix=permatex+motoseal,aps,79&sr=8-2&th=1

 

[Patrick1903]

Permatex Motoseal wouldn't hurt on the intake side of things I would think...

https://www.amazon.com/Permatex-291...&sprefix=permatex+motoseal,aps,79&sr=8-2&th=1

Everything was reassembled and the saw seems to be running well so far. I put a 20” bar on it. Ran almost one tank through and it seems strong, good torque. I’m going to get some more time on it and pull the exhaust and see how things look on the new piston. Hoping things work out with the original cylinder on it, but only time will tell. It’s a good experiment/test.