'70 rebuild - top end

Well, I'm glad I yanked the head and cylinder off.

The head is out to get rebuilt, and should be back in the next couple weeks. Was told it was a real low mileage bike, and besides the valves, guides and seals, everything is in great shape.

I was also planning on bringing in the cylinder and pistons for review, so I went out today and popped it off. It was stuck on really good, so I rotated around the nuts to break the seal and loosen it enough for removal (thanks for that tip.)

Upon initial inspection on the left piston (compression on left cylinder was 0 before teardown) I saw that the top ring was fully compressed, had no gap and was cinched in tight. The middle ring was the same on one side, while all the "slack" was popped out on the other. There are no noticeable scars or gouges in the cylinder, so that's good I guess.

So, I'm assuming I'll need to replace the piston, but was curious what the cause of this may have been - perhaps just ran real hard (heat/pressure/friction)?

Thanks for helping the noob)

I'm also curious how much horizontal play is allowable on the con rods? They don't slide, just rock a little.

Commando pistons are a little prone to rings sticking in their grooves - seems there was no clearance there anywhere to prevent this, and if they have clearance then they are worn. ??
The current vogue for starting rings up dry would not help this situation either ??
Make sure all carbon is cleaned from the grooves before reinstalling.

There isn't much room for sideways, but its the up-and-down in a conrod that is all important. Should be no perceptible up and down...

Opethiselps.