Again... someone hasn't been watching my vids.
There is a reason that piston failed. Unless you diagnose it, it will undoubtedly fail again. If one of those crank seals is bad, or the case gasket is leaking....it'll cause a brandy new piston and cylinder to fail....again. That leads to piles of frustration and usually ya end up throwing the saw in the river. ...and it isn't the saw's fault. Its yours cause you didn't diagnose the issue properly.
In this case, its possible this saw had straight gas in its history. But, since this is likely an early 1990's chainsaw, the RUBBER seals are 20+years old.... rubber breaks down, so generally good policy to replace the rubber bits on saws (and cars) before its too late. imho, the age of the saw alone constitutes a full teardown. It isn't that hard to do. There are a few extra tools you may need, but compared to buying a new saw its pretty inexpensive.
I hate to sound like a broken record here, but one of the reasons I make the videos (and spend HOURS editing etc) is because there are many questions that are common in sawbuilding.
a question similar to : "Should i fail, will the missing replacement of the seals most likely cause piston cylinder to fail quickly? " comes up all the time because some genius undoubtedly buys a new piston and cylinder to put on an old saw and doesn't actually figure out why the thing failed in the first place. This is a sure fire way to frustration. it happens all the time. its silly for members of such forums to keep answering the same question over and over and over again, when there can be a reference of videos to watch first. i know my vids are quite lengthy, but the idea is that they are packed with information that I have acquired through reading these forums every single day for the last 4 years....and by my own hand as well. Once the basic and common questions are answered, here on the forum we can get down to the nitty gritty and start answering the oddball questions.... which incidentally ... kinda the whole point of such a forum....as opposed to answering common questions over and over again.
I used to get frustrated when teachers sent me to the library to look up something....that didn't exist. used to spent hours researching topics that had no answer. Now we have youtube....there are far more answers than ever, just have to spend a little bit of time. I don't have all the answers, but I've dedicated the last 2 years to answering most of the common questions on my channel. The best way for you to be respectful of me and my time is to spend a little bit...and tiny fraction of the time I've spent and check out the vids. Take notes. THEN come here with questions and anyone here will bend over backwards to help ya.