Stihl MS390 decomp vs. non-decomp head?

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I have one with decompression....never use it......I use it ones just to see the different.
Suggestion: If you have that baby a part, I would change the bearings and the seals.
That would not be expensive and give you a pace of mind.
Good luck.

Truthfully, the head design question became a non-issue when I figured out I could re-use the original one. I wasn't expecting that but it's an added bonus.

I'll be doing crank bearings and seals as well (Like you said, good peace of mind). That actually re-opens a question I had asked earlier. Other than obvious runout, how do you check for bearing condition in general on these things? Especially since the rod bearing isn't serviceable... It would be a shame to put everything together just to have a rod bearing take a walk after a few hours of use....
 
Truthfully, the head design question became a non-issue when I figured out I could re-use the original one. I wasn't expecting that but it's an added bonus.

I'll be doing crank bearings and seals as well (Like you said, good peace of mind). That actually re-opens a question I had asked earlier. Other than obvious runout, how do you check for bearing condition in general on these things? Especially since the rod bearing isn't serviceable... It would be a shame to put everything together just to have a rod bearing take a walk after a few hours of use....
That's correct, you can do nothing with the road bearing. There is a roller bearing. Try to move the push road up and down and then side way (not sliding). If have any play, you most replace it.
Good luck.
 
Try to move the push road up and down and then side way (not sliding). If have any play, you most replace it.
Good luck.
I have a little bit of angular play (maybe 2mm or so measured at the small end). No up and down play. Since I haven't messed with a fresh bearing, I don't have any frame of reference for comparison. Is that little bit of angular play cause to replace the crank?
 
I have a little bit of angular play (maybe 2mm or so measured at the small end). No up and down play. Since I haven't messed with a fresh bearing, I don't have any frame of reference for comparison. Is that little bit of angular play cause to replace the crank?
If you would have 2 mm angular play, you would have up and down to....
If you don't have up and down, don't replace it.
 
If you want more compression shave a bit off the cylinder base or delete the base gasket. You want at least 0.020 clearance between piston and head TDC. Clean up the ports and do a muffler mod. Do the crank seals and impluse line, check the carb boot for cracks
 
If you would have 2 mm angular play, you would have up and down to....
If you don't have up and down, don't replace it.

Perhaps we're talking about different things then. By "up and down" I'm assuming you mean pulling and pushing along the axis of the con rod without rotating the crank. This does not result in any movement, clicking, etc. By angular play, I mean that I'm keeping the big end of the rod in a fixed position, holding the wristpin end, and moving laterally without letting the big end slide back and forth. In that case there is a small amount of side to side play. Definitely one but not the other.
 
Perhaps we're talking about different things then. By "up and down" I'm assuming you mean pulling and pushing along the axis of the con rod without rotating the crank. This does not result in any movement, clicking, etc. By angular play, I mean that I'm keeping the big end of the rod in a fixed position, holding the wristpin end, and moving laterally without letting the big end slide back and forth. In that case there is a small amount of side to side play. Definitely one but not the other.
I know from the first post what you was asking. That is normal how long you don't have up and down play.
 
I know from the first post what you was asking. That is normal how long you don't have up and down play.
Gotcha. I thought you were saying that if I had side to side play, then I'd definitely have up and down play as well; which kinda threw me off. Sorry for the misunderstanding on that one and thanks for the guidance.
 
Just in case anybody is interested, the Forester TS400 piston (I went ahead and ordered one), has the exact same deck height as the stock MS390 piston. That particular route for raising compression appears to be closed.

So much for a high compression alternative. With my altitude, it sure would be nice to bump the CR a bit, but I'm thinking it's not worth the effort at this point.
 
Just in case anybody is interested, the Forester TS400 piston (I went ahead and ordered one), has the exact same deck height as the stock MS390 piston. That particular route for raising compression appears to be closed.

So much for a high compression alternative. With my altitude, it sure would be nice to bump the CR a bit, but I'm thinking it's not worth the effort at this point.
That's a bummer
Thanks for posting back on the height

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If you want more compression shave a bit off the cylinder base or delete the base gasket. You want at least 0.020 clearance between piston and head TDC. Clean up the ports and do a muffler mod. Do the crank seals and impluse line, check the carb boot for cracks

This is a clamshell. No gasket to delete or base to shave. A popup can be welded but probably not worth it unless the op can weld aluminum and wants to play.


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A popup can be welded but probably not worth it unless the op can weld aluminum and wants to play.
My TIG is pretty old and jankety and it's been long enough since I welded AL that I'd need to practice before I could do a decent enough job. But my lathe works fine. That's kinda why I was asking about the TS350/360 pistons earlier. If one happened to be a little too tall but had enough meat in the crown, I could go the other way and just cut it to fit.... Turns out they have a 12mm wristpin and by the looks (never did get the measurement) it's located much lower. If I had one in hand I'd measure to see if an offset bushing could be used to locate the wristpin higher, but that's still getting into a lot more "play" than I'm interested in.

Since the TS400 piston has a slightly different skirt design, it's probably worth asking if there's any disadvantage to running it vs. the stock 390 piston?
 
Lots of variables in saws. What seems like it would hurt a saw sometimes helps and vise versa. Won't know til you try it.


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