Zeek, I'll weigh in a bit....for as much as my inexperience and relative noob'ish is worth. I originally watched this thread because in the new posts list it caught my eye as "346xp" and "rebuild". I agree that 7 pages in and the thread is still *fairly* on track is pretty good....although the 550xp debate is trying to derail it!
I've also gone through some of the same emotions & thought processes as others. Initially I was happy that you would be open to the idea of rebuilding and from your very first statements it felt like you knew exactly how to R&R the saw but were hesitant due to downtime/hassle/etc, but then based on a lot of your questions I started to think that I initially misjudged your capability/knowledge and felt maybe you should be setting this aside or putting your money towards a new saw. But then thunk more deeply about it and regarding your mechanical aptitude, willingness to learn and be vulnerable (ask those questions) and with the resources of people here, YouTube, and workshop manuals, along with statements that "everybody has to start somewhere" (including how I did....I was just way more of a lurker), reconsidered that you deserve the help and encouragement and we can all just hope you end up with a strong 346 again as well as new tools and confidence.
You ask about the 90psi compression just earlier today. You had two events of 90psi compression correct? One before upside-down-backwards-ring and then the second 90psi result after installing the ring incorrectly and running the saw. It could be your decompression valve is/was sticky or leaking, or even activated. It also could be that you had 90psi the first time simply from 11yrs of use and it truly was a worn top end.... THEN with a new ring that was installed completely wrong it still didn't seal correctly therefor you got the same result as a worn out top end. It's too bad you don't have a specific recollection of how the cylinder or piston was when you initially took it apart for just the re-ring. I would say all the scoring and damage we observe today is from the ring being installed incorrectly.
Now with all that being said, I think it's not worth fretting about why it was 90psi a week ago.... Move forward from that and focus on what you are planning to achieve with these new parts and help and guidance. At this point it doesn't matter why it was 90psi because unfortunately you don't have enough past information on which to make any assertions.... its really all just conjecture and hypothesis and guesswork. You'll never really know until you have the NEW P/C installed and can judge these results against what you previously have experienced. You don't need to actually run the saw to do a initial compression check....once the intake tract is all buttoned up, a little bit of oil through the spark plug hole and then just pull it over "dry" and you'll have a good idea where things are at. You should get 140'ish PSI at least and then it'll break in from there. Anyhow that's my thoughts on your 90psi dilemma.
On to other matters,
People talk about changing the seals.... just so you're aware that's quite a bit more disassembly. It's also easy to **** it up (unfortunately) and there are actually "special" tools in consideration of reinstalling the seals. You'll need to pop the flywheel off (this can be a bitchy task sometimes, you need to be reasonably gentle), and on the PTO side you'll need to remove the clutch (remember it's reverse thread!), and then the oil pump. So on either side of the case you are "digging down" about as deep as you can go. I have found sometimes when the oil pump is R&R'd you end up with subsequent bar oil leaks because you disturb the original seating of the pump with its suction hoses. Those hoses will be old and stiff and sometimes don't play nice after everything came apart. Often if I have to dismantle a well-used saw that far I'll just replace the oil hoses in order to get new soft flexible ones that will seal properly. That just adds to cost and time...how much of a pandora's box do you want to open here.
The advice about checking for leaks is good, and you should have that foundation of knowledge for where things are at as you reassemble. My suggestion would be to just get the top end bolted back, and then plug the intake boot and exhaust flange and do the pressure-vaccum test. Don't forget to make sure the impulse hole is blocked somehow too in whatever way you decide to plug the intake boot. This way you have maximum "visibility" around the cylinder base and metal intake-clamp with which to check for leaking (soapy water misted onto mating surface). *IF* you have leaks that you determine aren't originating from the base gasket or intake clamp, THEN you can decide if you want to disassemble the flywheel and clutch-oil pump. Don't just assume that because it's a old saw that the seals must be junky leaky and should be replaced. Is it great preventative maintenance and peace of mind to do them, yes.. just beware that there are perils in both removing and replacing the seals.
Oh and of course for pressure-vaccum test you'll need to own (or buy) a MityVac type automotive pressure-vac gun. There are some great YouTube videos for all of this. I would specifically search for videos from
TheChainsawGuy or
MattyO , they are good and informative, as well watch stuff from Walt
AFleetCommand . These would be my suggestions for getting video-based guidance.
Base Gasket: You don't need to use any additional sealant
As far as the decompression valve goes, I'd just consider eliminating it...Husky has a nice OEM block-off bolt and washer if you don't want to just wing it. A stock 346xp isn't that hard to pull through, on my saw I've never felt I needed decompression for starting. If you have a valve that you are confident in then by all means use it....just be aware that it can be a source of compression leak.
Overall I think you're definitely on the right track, and there are so many resources available to you that it's almost a overwhelming amount of direction and guidance. It's possible to spend hours & hours & hours reading, watching, and replying. Oh, I'm also going to suggest you grab the Husqvarna workshop manual. It will help with some of your teardown specs and procedure.
Husqvarna 346xp Workshop Manual
I don't know what else I was going to convey.....
Put the piston on
Put the ring on (Correctly! the sloped angle surfaces where the ring "pinches" need to compress AT and UNDERNEATH the pin on the piston)
Clamp the intake boot to the cylinder before you install the cylinder
Install the cylinder with base gasket....tighten everything down
Pressure-Vaccum test. Husqvarna says pressurize to 0.8bar and in 30seconds it shouldn't leak below 0.6bar. Perfectionists will tell you it shouldn't leak at all
IF you are satisfied THEN you can decide if you want to do preventative seal replacement maintenance
IF it fails pressure-vaccum not from base gasket, intake clamp, or where you plugged things off from THEN replace seals
Check Compression & decide if you have a good rebuild or need to chase more problems
Put everything else back together, run the saw, break it in, and hope for no further complications!
Oh and great you found that original little pin
Maybe somebody local with experience and tools could give you a hand....that would be the best. Godspeed and God Bless
Cheers,
-Rob
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. That being said, with the power of a 550mk2, if you just remove the amount of fuel it takes to offset the weight, it would still probably cut just as much wood as a 346 
