Just to clarify my suggestion . I was suggesting fitting a standard oversize piston , not a rebore .
Having a quick look on ebay , there were a few pistons available . They were all 251-11631-00-99 .
Presuming yours is the common .96 size ( it will be stamped on the piston crown ) , the new one will be
about.0012 " bigger , so with a quick deglaze , will slot right in . The prices varied . but there were inexpensive ones there .

Ya, I guess I would have to pull the cylinder and check the rod bearing on the crank for play. I don't really want to do a full rebuild, but I definitely don't want a catastrophic failure. I know I can do the job. I'm new to motorcycles, but have enough mechanical experience to dive in if needed. I would have to outsource any machining of course.

As far as choosing the piston size, wouldn't I need to get the bore measured to determine what I need? I'm not sure how else you would check other than ring gap, but that would require me to purchase the rings first (I'd be guessing).

I'm on my phone so might be missing full posts but my take on what Swoop was saying is-- standard 66mm pistons come in 96 to 99 std size which is incremental increases still within 66mm or in other words 64.96 to 99 but still just a gnats whisker tighter if original has 96 on top. If you end up needing to reborn then sizes step up in 10thou sizes or 0.25mm sizes all the way to 1mm over. .25 .50 .75 1.0. If you go aftermarket then likely pistons will go in .50mm steps not .25mm steps. Happy to be corrected.

hi again,

from you exhaust port vid the piston actually doesnt look that bad, if you were to lift the top end, which is a doddle, you would know all you need to know and could work from there.

cheers
les.

Sounds sound to me.
Ringing is the fins ringing.

Piston slap is much different.

cliff

hi again,

the noise in the engine running vid did not sount like piston slap to me either, my first thought was end noise, the piston movement in the second vid absolutely should not occur, you can even here the tapping of a totally knackered bearing, this will let go sooner or later.
moisture from condensation when parked up for years rusts the bearings and on the first start up the bearing will chew itself to pieces. there should be zero rotational movement of the piston as in your second vid.

if you whip the top end off, please post the results, i will be eager to see inside!
cheers
les.
 

I just read the service manual again and realized I don't need to remove the engine to pull the cylinder off. This will be my next step and as mentioned I can actually see what's going on inside.

I really appreciate the input and I will definitely post back when I have more info.

On another note I checked the compression today and it was 130.

Sorry, this is just my opinion, but I’m going against the grain when I say just ride it and enjoy it. For the minimal amount of miles you will put on that bike you could probably go another 10 years just the way it is.

Now if you actually plan to put a few thousand miles a year on it then that’s a horse of a different color. But you would notice it getting worse before it implodes.

But that’s just me…..and I’m far from an expert on these bikes.

I’m with Schu on this one cell phones pick up more strange noises than you can hear plus your bike looks so good it doesn’t look like it was abused by your Dad 

If you do lift the cylinder & remove the piston, pop the rings off & see what feeler gauge blade it'll take on the skirt.
That'll sort your next move.
Also the rings probably have expanders behind them so get the piston on the bench before removing the rings as the expanders can break up, "not unheard of".
You don't want things like that dropping into the case.
Plus they get carboned into the groves just to add to the fun.