MS 390 Reassembly

[cedarshark]

Good morning. I have a MS390 that locked up in the cut last fall. I finished the job with my 044 and set the 390 on the bench, afraid to take my first crack at messing with a major clamshell repair. I got tired of it staring at me and, on disassembly, I discovered a small ball bearing from the compression release lodged under the crank. The ball bearing pinged the top of the piston a couple of times and the bottom of the pan but did not get into the bearings or do any damage to the sides of the piston or cylinder. The bearings rotate smoothly.

1) I cannot locate the compression release pin. I guess the saw dispelled it out the top. I plan to plug the compression release with a bolt.

2) My plan is to replace the seals, clean the ring grooves and re-use all orig parts. The piston shows light wear wear but the skirts still show machining marks, so I believe it is good to go.

3) The engine has been removed from the saw and I would like to re-install the same way if possible. When the engine was removed, the Dirko sealant held the pan on the cylinder. Can I apply Dirko the pan, clamp the pan to the cylinder,let it dry, the reinstall in the case, or do the bolts that hold the engine to the case provide the necessary torque to seal the pan to the cylinder ? I guess (in summary)I am confused about the order of the engine reassembly back into the frame.

4) Is there something else that I could be missing? I can post pics of the damage to the piston top but it's not very dramatic !

Thanks for the help.

 

[machinisttx]

Put in some new caber rings while you have it apart. As far as assembly goes, apply the sealant to one side or both(not sure on the directions for dirko), then reassemble the engine and bolt it into the saw. If you have bolts that are the right length, you could do it outside the saw, but you'd risk disturbing the seal when taking the bolts out and then putting the engine into the saw.

 

[cedarshark]

Put in some new caber rings while you have it apart. As far as assembly goes, apply the sealant to one side or both(not sure on the directions for dirko), then reassemble the engine and bolt it into the saw. If you have bolts that are the right length, you could do it outside the saw, but you'd risk disturbing the seal when taking the bolts out and then putting the engine into the saw.

Good advice to replace the rings while saw is apart. Will do.

So the sealant(Dirko) gives you ample working time to assemble the engine outside the frame, get it into saw and torqued down ? The 4 bottom bolts don't appear to be a problem.....but that bar stud may take a while.

Does it make more sense to place the pan in the case, apply the sealant, then place piston, crank, bearings and seals down on the pan ? These clammies are weird !

 

[50blues]

I like to assemble it outside the saw. I put some oversized nuts, like 1/2 inch or so on the bolts to act as spacers so the bolts will fit. I let it sit overnight and then reassemble.

 

[cedarshark]

I like to assemble it outside the saw. I put some oversized nuts, like 1/2 inch or so on the bolts to act as spacers so the bolts will fit. I let it sit overnight and then reassemble.

This is how I would like to do it and it is good to hear someone else has used this technique. Do you torque the pan bolts to the specified torque while you let it sit overnight to spread the Dirko evenly and do you coat the seals where they contact the pan and cylinder ? Thanks.

 

[machinisttx]

I coated the seal bores on my 029S when I rebuilt it as a 039/390.

 

[cedarshark]

I read the OEM shop manual for seal installs on a 039 and they recommend a thin coat of sealer on the outside of seal, install the seal, wait 1 minute...then turn the crank several times. I did note when I tore the 390 down, it did not have any sealant on the outside of the seals.

 

[50blues]

I coat the seals. One more thing to stop a leak. I don't torque the pan bolts, just get them tight like when I took it apart. One thing, don't use to much sealant. I have taken saws apart with way to much glue used, it will make a mess inside.

 

[griffonks]

I don't use dirko in clamshells anymore. It sets up to fast. If I use hondabond, yamabond, or 3 bond (all pretty much the same) I have ample time to seal the engine and run in the mount bolts. Dirko is more difficult to spread and sets fast.

Clean the sealing surfaces and scuff them up a little. Use new seals and smear a little around the rim. Apply a thin coat to both sides. Practice the angles you use to put the engine in the case before you seal.

MWEBA has a video on YouTube where he shows how to seal an 1127 series saw.... A 390 I think. Its worth your time.

I bought 6 basket case Stihls from a dealer 3 weeks ago and I have been sealing clamshells ever since. It's not hard. Reassembling a 390 after the engine is in is more difficult.

Lastly good luck and plug that decomp hole while you are at it..... a decomp isn't necessary in that saw.


Sent from my SCH-R530C using Tapatalk

 

[cedarshark]

I coat the seals. One more thing to stop a leak. I don't torque the pan bolts, just get them tight like when I took it apart. One thing, don't use to much sealant. I have taken saws apart with way to much glue used, it will make a mess inside.

Thanks. I noted when I took the saw apart, that it took a lot more than 8 ft/lbs (service manual specs) to get the pan bolts out. Do you guys use loc-tite on any of the engine mounting bolts other than the bar stud ?

 

[cedarshark]

I don't use dirko in clamshells anymore. It sets up to fast. If I use hondabond, yamabond, or 3 bond (all pretty much the same) I have ample time to seal the engine and run in the mount bolts. Dirko is more difficult to spread and sets fast.

Clean the sealing surfaces and scuff them up a little. Use new seals and smear a little around the rim. Apply a thin coat to both sides. Practice the angles you use to put the engine in the case before you seal.

MWEBA has a video on YouTube where he shows how to seal an 1127 series saw.... A 390 I think. Its worth your time.

I bought 6 basket case Stihls from a dealer 3 weeks ago and I have been sealing clamshells ever since. It's not hard. Reassembling a 390 after the engine is in is more difficult.

Lastly good luck and plug that decomp hole while you are at it..... a decomp isn't necessary in that saw.


Sent from my SCH-R530C using Tapatalk

Thank You. I have ordered the seals,decomp valve plug and bar stud. Based on your advice, I would not use Dirko if I did not have a tube. As expensive as it is, I guess I better use it. I did note the very small bead on the pan when I tapped it off the cylinder. The pan mating surface was not covered with Dirko as I anticipated. Good advice on practicing the engine tilt when installing in the case. This is the reason why I wanted to pre-assemble outside the case, let the Dirko dry, then place the engine in the case. It took several "angles" to get that engine out of the case. You guys have been a huge help, as I was nervous about my first clamshell rebuild.

 

[griffonks]

I bought a stihl service manual for the 1127 series saw when I did the first straight gassed MS390 3 years ago. But when all was said and done, Fish (no longer here, now permanently banned for God knows why), grumpily helped me get the saw reassembled. So I owe, if you need help, PM me, I'll talk you through it, or write you through it.

There's a finished 039/MS390 (MS390 had a broken av mount in the case so I frankenstihled it) and a MS290/390 conversion in my shop. I can get pics of the manual or one of the saws if you need help. I am selling these saws.

 

[griffonks]

BTW I noticed you have an 041AV, I left a decent 041AV basket case in the dealer's shop that I bought the 6 saws from. If you want a parts saw for your 041 I could go back for it for you.

 

[blsnelling]

I personally don't like disturbing the pan bolts once they've been tightened. It's not hard to assemble and then tighten. No worries about breaking the seal that way.

 

[cedarshark]

I bought a stihl service manual for the 1127 series saw when I did the first straight gassed MS390 3 years ago. But when all was said and done, Fish (no longer here, now permanently banned for God knows why), grumpily helped me get the saw reassembled. So I owe, if you need help, PM me, I'll talk you through it, or write you through it.

There's a finished 039/MS390 (MS390 had a broken av mount in the case so I frankenstihled it) and a MS290/390 conversion in my shop. I can get pics of the manual or one of the saws if you need help. I am selling these saws.

That's a very kind offer and I appreciate it. I have the 039/390 service manual and it has been very helpful. I buy/rebuild/resell about 20-30 saws a year but have concentrated on 026/036/044 for the past several years. I avoided the clamshells but the 290 and 390 are very popular saws and sell well. I am sure once the 390 is up and running, I will have confidence in my ability to do it again. I can already see a distinct advantage to the clamshells...a bearing replacement will be much easier compared to splitting a case on the pro series saw. However, a top job on the 390 means complete disassembly of the saw while the pro series saw is very easy and quick. I guess you just have to pick your poison ! Thanks again for your help.

 

[cedarshark]

BTW I noticed you have an 041AV, I left a decent 041AV basket case in the dealer's shop that I bought the 6 saws from. If you want a parts saw for your 041 I could go back for it for you.

I sold 041 years ago but failed to update my profile. It was replaced with a 10mm wristpin 044....my favorite saw ! Thanks though !

 

[cedarshark]

I personally don't like disturbing the pan bolts once they've been tightened. It's not hard to assemble and then tighten. No worries about breaking the seal that way.

I value your advice & opinion immensely. It appears to me that engine must be assembled,sealant applied, angled 2-3 different ways to get it in the frame, then turned upside down to get the pan bolts in and torqued. So this is possible without smearing the Dirko ? It seems much easier with the pan sealant dry and somewhat "glueing" the pan to the cylinder. However, I see your point about breaking the seal.

 

[griffonks]

I agree with Brad, some people drill and tap the base and cylinder, assemble the clamshell and leave the tapped in screw's in when they mount the cylinder. That's how Bailey's is shipping ready to mount 390 blocks. However many of them are leaking, even though they were advertised as "assembled and tested" I got to do a 390 twice last fall....

For me, Hondabond and drop straight in is so much easier...

 

[cedarshark]

I was thinking a couple of tie wraps...get the engine in position...and then cut the tie wraps but you guys are probably right, just assemble & drop it in ...hold in place...turn upside down & start the bolts.

Couple questions: 1)The cylinder appears beveled at the bottom so it may compress the rings upon piston insertion. I do not own Stihl's "special ring clamp". Is a ring compressor need to insert the piston ? I have the cheap Bailey's plastic pinch type but it looks like the piston sets so deep in the cylinder that this type wont work. Many Thanks.

2)The pan has dimples in it where the ball bearing got squashed between the crank and pan. Do I need to smoothe the dimples with emery cloth or dremel tool ? They do not interfere with the crank rotation (now that the ball bearing is removed) but I had no idea that the crank clearance to the pan was so small. If a pic will clarify...let me know.

 

[cedarshark]

Update: The local Stihl shop had rings but no seals. The piston & crank are back in the cylinder without the aid of a ring compressor. Just has to gently finagle the rings past the intake ports(I think they are intake ports). Per comments from Griffonks and Brad, tried a dry run to get the engine back in the saw and realized if the coil was out, this would be a cake walk. I removed the coil, and the engine slid into place with virtually no rotation needed. Removing the coil gave me chance to clean and inspect the connections anyway. If I had the seals and bar stud, this saw would be running tomorrow night. Thanks for all the help.