looks like they long stroked the "79IT like they did with the YZ in "79 one year only. So "79 is 82mm. YZ "79 piston is a 2X5 number so worth checking out & has the "Go Fast" big cut away on the inlet side but just might match up in other respects. I run one of these in my overbored RT2--80mm out to 82mm.
Pay to measure yours just to be sure to be sure using verniers across it best you can as difference between 82 & 85 should be easy to see.

Again, check with Enduronut if you haven't already.

I have a question since we are talking pistons MarkT or Enduronut would probably know I see pistons on eBay and remember 50yrs ago seeing Rocky pistons and just wondering who made them for Rocky, Art Brand, Hap Jones ? Just curious?

Message has already been sent. Thanks again.

This is how far I've been able to get the piston to move soaking in Kroil and then using a wooden dowel and rubber mallet to move. How close does this look to BDC? Will it move down any more? I guess I need to pull the cylinder studs this weekend and see if I can jack the cylinder up some and then pound the piston out more.

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nice work, not too bad IMO
can you get to the wrist pin?
my thought here is that if you could get pin out you can get piston/cylinder on your bench making removal easier without risk of messing up big end, etc. 

Have a look on my post #24 on this thread to see what I did with my GT550.

Incoming ….. The Brave Pill - Page 3 - Vintage Enduro Discussions (yamaha-enduros.com)

I haven't pulled the nuts off the studs to mount the cylinder to the cases yet. This weekend I'm also going to pull the left side case to see if the flywheel area is corroded.

Just pull the stud nuts & smack around the cylinder with a block of wood across multiple fins & hammer to free the gasket. Then turn the crank the oposite way--whichever 'that' is--& it'll lift the cylinder. Either way might do if at true BDC. Force the mag flywheel that is. Then block it & keep driving. What a damn shame--crying shame really & so easily avoided.
Look through the pics on this link of mine if it works. Had a very similar situation. My saying 'is' she'll be running by lunchtime--& nearly was haha.
yamaha-enduros.com/index.php/forums/the-...n-at-it-again#139199

Morely, you need to change your forum name to "2 stroke motor whisperer" you get motors running that just shouldn't!!!