Ms390 rebuild questions

Arborist Forum

Help Support Arborist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
I just happen to have an example handy...

I use Dirko. You can put one of these together with Dirko, bolt it in and run the saw right away with no problem. I still bolt it together outside the engine housing for the main reason of doing a leak check before committing to complete reassembly..., especially aftermarket kits or cylinders. The machining and casting is all over the map with the cheapy kits and cylinders and more will leak than not unless excessive sealant is used. Some say to always use the old OEM bottom pan but I've found using the pan that comes with the cylinder can often be a better match for the seal pockets. Regardless, it's nice to know you have a completely tight shortblock beforehand than finding out the hard way there's a leaky seal once the saw is back together AND you have that damned bar stud screwed back in! BTW, I just use Gorilla tape to block off the intake and exhaust ports for a quick vac/pressure test outside of the engine housing. Works fine up to around 5~7 psi and 12~15 inHg which is all that's necessary anyway. Intake and exhaust surfaces just need to be nice and clean for a good seal. I use a little alcohol. Another good reason for sealing outside the housing and letting it completely set up is for ease of getting the assembly back into the housing itself. It's just easier when you're not trying to keep the pan from sliding around while also trying to position the assembly.
That's exactly how I do it, even down to the little sleeves you put over the bolts for a spacer. Thought about using shorter bolts for the pan but I'd probably lose them before the next assembly..one thing I do a little different: on the intake I put the boot on it and stick a rubber cork in it, it will hold 7 lbs. easily..
 
I keep meaning to get an assortment of rubber stoppers for the purpose. I also do another vac/pressure test with blockoff plates once the intake components are assembled just to have everything covered.

Something else I forgot to mention about the cylinder kits (for anyone who hasn't worked with one) is the lack of threads in the cylinder base..., at least on most of the ones I've used. They're intended to be threaded by the self tapping hardware included with the kit. Not a big deal, but care needs to be taken to get a straight start for even threads. And by all means, perform this step before getting everything all gooped up with sealant and put together or it'll get messy real fast!
 
After reading this thread, I have to wonder how on earth that I upgraded several 290s to 390s by not removing any bar stud or the engine pan, and then bolting it all back into place. The engines I upgraded measured 150 psi compression, started, and they run today. It seems to me that if the engine pan is not damaged, it does not have to be removed and replaced before installing the new cylinder assembly. However, to each his own.
 
DSCF1401.JPG



DSCF1399.JPGNotice the correct bolts for the threads that are in the cylinder as they are different from the OEM cylinder
I let mine set for a while as I have several ready to go as I get saws to put them in. I have done quite a few of them with no problems as of yet. Make sure that they four stroke out of wood. These dam thinks will turn 15 grand if you are not careful
and that is the highway to heaven for a saw
I get the bolts off ebay
 
The Hyway kit comes with it's own bolts.

20191011_221024-756x1008.jpg

After reading this thread, I have to wonder how on earth that I upgraded several 290s to 390s by not removing any bar stud or the engine pan, and then bolting it all back into place. The engines I upgraded measured 150 psi compression, started, and they run today. It seems to me that if the engine pan is not damaged, it does not have to be removed and replaced before installing the new cylinder assembly. However, to each his own.

You just leave the pan in the case ? Where do you put sealer? I used Motoseal, and I put a THIN layer around the seals too. But I let them set up before I put the engine back in the case. I guess I'm just asking why you don't remove the pan. I'm not questioning your methods, just a question about your motive for methods.
 
After reading this thread, I have to wonder how on earth that I upgraded several 290s to 390s by not removing any bar stud or the engine pan, and then bolting it all back into place. The engines I upgraded measured 150 psi compression, started, and they run today. It seems to me that if the engine pan is not damaged, it does not have to be removed and replaced before installing the new cylinder assembly. However, to each his own.
I like to take all the parts out of it, including the pan so I can clean them all thoroughly but yeah, you don't have to take the pan out to do a rebuild.
 
The Hyway kit comes with it's own bolts.

View attachment 765368
You just leave the pan in the case ? Where do you put sealer? I used Motoseal, and I put a THIN layer around the seals too. But I let them set up before I put the engine back in the case. I guess I'm just asking why you don't remove the pan. I'm not questioning your methods, just a question about your motive for methods.
It just seems a lot of trouble to remove the pan. I apply the sealer on the pan and on the cylinder, then I drop the cylinder back in atop the pan, hold together, and secure the bolts. It's a little tricky, and your method might be easier in the long run, assuming the sealer holds tight when you remove the bolts. I cannot find much about it in the service manual.

Whenever I rebuild a 290, I always wish that I had four hands. I really don't think Stihl designed this engine to be rebuilt on a regular basis. Some guys have no trouble. I'm not one of them that truly enjoys it.
 
I never said the manual called for taking it out. Just EZer for me plus I work at a Stihl dealer and don't take chances on having a leak down the road.
 
Back
Top